[lttng-tools.git] / src / bin / lttng-sessiond / ust-app.cpp

/*
 * Copyright (C) 2011 EfficiOS Inc.
 * Copyright (C) 2016 Jérémie Galarneau <jeremie.galarneau@efficios.com>
 *
 * SPDX-License-Identifier: GPL-2.0-only
 *
 */

#define _LGPL_SOURCE

#include "buffer-registry.hpp"
#include "condition-internal.hpp"
#include "event-notifier-error-accounting.hpp"
#include "event.hpp"
#include "fd-limit.hpp"
#include "field.hpp"
#include "health-sessiond.hpp"
#include "lttng-sessiond.hpp"
#include "lttng-ust-ctl.hpp"
#include "lttng-ust-error.hpp"
#include "notification-thread-commands.hpp"
#include "session.hpp"
#include "ust-app.hpp"
#include "ust-consumer.hpp"
#include "ust-field-convert.hpp"
#include "utils.hpp"

#include <common/bytecode/bytecode.hpp>
#include <common/common.hpp>
#include <common/compat/errno.hpp>
#include <common/exception.hpp>
#include <common/format.hpp>
#include <common/hashtable/utils.hpp>
#include <common/make-unique.hpp>
#include <common/pthread-lock.hpp>
#include <common/sessiond-comm/sessiond-comm.hpp>
#include <common/urcu.hpp>

#include <lttng/condition/condition.h>
#include <lttng/condition/event-rule-matches-internal.hpp>
#include <lttng/condition/event-rule-matches.h>
#include <lttng/event-rule/event-rule-internal.hpp>
#include <lttng/event-rule/event-rule.h>
#include <lttng/event-rule/user-tracepoint.h>
#include <lttng/trigger/trigger-internal.hpp>

#include <errno.h>
#include <fcntl.h>
#include <inttypes.h>
#include <pthread.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include <urcu/compiler.h>
#include <vector>

namespace lsu = lttng::sessiond::ust;
namespace lst = lttng::sessiond::trace;

struct lttng_ht *ust_app_ht;
struct lttng_ht *ust_app_ht_by_sock;
struct lttng_ht *ust_app_ht_by_notify_sock;

static int ust_app_flush_app_session(ust_app& app, ust_app_session& ua_sess);

/* Next available channel key. Access under next_channel_key_lock. */
static uint64_t _next_channel_key;
static pthread_mutex_t next_channel_key_lock = PTHREAD_MUTEX_INITIALIZER;

/* Next available session ID. Access under next_session_id_lock. */
static uint64_t _next_session_id;
static pthread_mutex_t next_session_id_lock = PTHREAD_MUTEX_INITIALIZER;

namespace {

/*
 * Return the session registry according to the buffer type of the given
 * session.
 *
 * A registry per UID object MUST exists before calling this function or else
 * it LTTNG_ASSERT() if not found. RCU read side lock must be acquired.
 */
static lsu::registry_session *get_session_registry(const struct ust_app_session *ua_sess)
{
        lsu::registry_session *registry = nullptr;

        LTTNG_ASSERT(ua_sess);

        switch (ua_sess->buffer_type) {
        case LTTNG_BUFFER_PER_PID:
        {
                struct buffer_reg_pid *reg_pid = buffer_reg_pid_find(ua_sess->id);
                if (!reg_pid) {
                        goto error;
                }
                registry = reg_pid->registry->reg.ust;
                break;
        }
        case LTTNG_BUFFER_PER_UID:
        {
                struct buffer_reg_uid *reg_uid =
                        buffer_reg_uid_find(ua_sess->tracing_id,
                                            ua_sess->bits_per_long,
                                            lttng_credentials_get_uid(&ua_sess->real_credentials));
                if (!reg_uid) {
                        goto error;
                }
                registry = reg_uid->registry->reg.ust;
                break;
        }
        default:
                abort();
        };

error:
        return registry;
}

lsu::registry_session::locked_ref get_locked_session_registry(const struct ust_app_session *ua_sess)
{
        auto session = get_session_registry(ua_sess);
        if (session) {
                pthread_mutex_lock(&session->_lock);
        }

        return lsu::registry_session::locked_ref{ session };
}
} /* namespace */

/*
 * Return the incremented value of next_channel_key.
 */
static uint64_t get_next_channel_key()
{
        uint64_t ret;

        pthread_mutex_lock(&next_channel_key_lock);
        ret = ++_next_channel_key;
        pthread_mutex_unlock(&next_channel_key_lock);
        return ret;
}

/*
 * Return the atomically incremented value of next_session_id.
 */
static uint64_t get_next_session_id()
{
        uint64_t ret;

        pthread_mutex_lock(&next_session_id_lock);
        ret = ++_next_session_id;
        pthread_mutex_unlock(&next_session_id_lock);
        return ret;
}

static void copy_channel_attr_to_ustctl(struct lttng_ust_ctl_consumer_channel_attr *attr,
                                        struct lttng_ust_abi_channel_attr *uattr)
{
        /* Copy event attributes since the layout is different. */
        attr->subbuf_size = uattr->subbuf_size;
        attr->num_subbuf = uattr->num_subbuf;
        attr->overwrite = uattr->overwrite;
        attr->switch_timer_interval = uattr->switch_timer_interval;
        attr->read_timer_interval = uattr->read_timer_interval;
        attr->output = (lttng_ust_abi_output) uattr->output;
        attr->blocking_timeout = uattr->u.s.blocking_timeout;
}

/*
 * Match function for the hash table lookup.
 *
 * It matches an ust app event based on three attributes which are the event
 * name, the filter bytecode and the loglevel.
 */
static int ht_match_ust_app_event(struct cds_lfht_node *node, const void *_key)
{
        struct ust_app_event *event;
        const struct ust_app_ht_key *key;

        LTTNG_ASSERT(node);
        LTTNG_ASSERT(_key);

        event = caa_container_of(node, struct ust_app_event, node.node);
        key = (ust_app_ht_key *) _key;

        /* Match the 4 elements of the key: name, filter, loglevel, exclusions */

        /* Event name */
        if (strncmp(event->attr.name, key->name, sizeof(event->attr.name)) != 0) {
                goto no_match;
        }

        /* Event loglevel. */
        if (!loglevels_match(event->attr.loglevel_type,
                             event->attr.loglevel,
                             key->loglevel_type,
                             key->loglevel_value,
                             LTTNG_UST_ABI_LOGLEVEL_ALL)) {
                goto no_match;
        }

        /* One of the filters is NULL, fail. */
        if ((key->filter && !event->filter) || (!key->filter && event->filter)) {
                goto no_match;
        }

        if (key->filter && event->filter) {
                /* Both filters exists, check length followed by the bytecode. */
                if (event->filter->len != key->filter->len ||
                    memcmp(event->filter->data, key->filter->data, event->filter->len) != 0) {
                        goto no_match;
                }
        }

        /* One of the exclusions is NULL, fail. */
        if ((key->exclusion && !event->exclusion) || (!key->exclusion && event->exclusion)) {
                goto no_match;
        }

        if (key->exclusion && event->exclusion) {
                /* Both exclusions exists, check count followed by the names. */
                if (event->exclusion->count != key->exclusion->count ||
                    memcmp(event->exclusion->names,
                           key->exclusion->names,
                           event->exclusion->count * LTTNG_UST_ABI_SYM_NAME_LEN) != 0) {
                        goto no_match;
                }
        }

        /* Match. */
        return 1;

no_match:
        return 0;
}

/*
 * Unique add of an ust app event in the given ht. This uses the custom
 * ht_match_ust_app_event match function and the event name as hash.
 */
static void add_unique_ust_app_event(struct ust_app_channel *ua_chan, struct ust_app_event *event)
{
        struct cds_lfht_node *node_ptr;
        struct ust_app_ht_key key;
        struct lttng_ht *ht;

        LTTNG_ASSERT(ua_chan);
        LTTNG_ASSERT(ua_chan->events);
        LTTNG_ASSERT(event);

        ht = ua_chan->events;
        key.name = event->attr.name;
        key.filter = event->filter;
        key.loglevel_type = (lttng_ust_abi_loglevel_type) event->attr.loglevel_type;
        key.loglevel_value = event->attr.loglevel;
        key.exclusion = event->exclusion;

        node_ptr = cds_lfht_add_unique(ht->ht,
                                       ht->hash_fct(event->node.key, lttng_ht_seed),
                                       ht_match_ust_app_event,
                                       &key,
                                       &event->node.node);
        LTTNG_ASSERT(node_ptr == &event->node.node);
}

/*
 * Close the notify socket from the given RCU head object. This MUST be called
 * through a call_rcu().
 */
static void close_notify_sock_rcu(struct rcu_head *head)
{
        int ret;
        struct ust_app_notify_sock_obj *obj =
                lttng::utils::container_of(head, &ust_app_notify_sock_obj::head);

        /* Must have a valid fd here. */
        LTTNG_ASSERT(obj->fd >= 0);

        ret = close(obj->fd);
        if (ret) {
                ERR("close notify sock %d RCU", obj->fd);
        }
        lttng_fd_put(LTTNG_FD_APPS, 1);

        free(obj);
}

/*
 * Delete ust context safely. RCU read lock must be held before calling
 * this function.
 */
static void delete_ust_app_ctx(int sock, struct ust_app_ctx *ua_ctx, struct ust_app *app)
{
        int ret;

        LTTNG_ASSERT(ua_ctx);
        ASSERT_RCU_READ_LOCKED();

        if (ua_ctx->obj) {
                pthread_mutex_lock(&app->sock_lock);
                ret = lttng_ust_ctl_release_object(sock, ua_ctx->obj);
                pthread_mutex_unlock(&app->sock_lock);
                if (ret < 0) {
                        if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                                DBG3("UST app release ctx failed. Application is dead: pid = %d, sock = %d",
                                     app->pid,
                                     app->sock);
                        } else if (ret == -EAGAIN) {
                                WARN("UST app release ctx failed. Communication time out: pid = %d, sock = %d",
                                     app->pid,
                                     app->sock);
                        } else {
                                ERR("UST app release ctx obj handle %d failed with ret %d: pid = %d, sock = %d",
                                    ua_ctx->obj->handle,
                                    ret,
                                    app->pid,
                                    app->sock);
                        }
                }
                free(ua_ctx->obj);
        }

        if (ua_ctx->ctx.ctx == LTTNG_UST_ABI_CONTEXT_APP_CONTEXT) {
                free(ua_ctx->ctx.u.app_ctx.provider_name);
                free(ua_ctx->ctx.u.app_ctx.ctx_name);
        }

        free(ua_ctx);
}

/*
 * Delete ust app event safely. RCU read lock must be held before calling
 * this function.
 */
static void delete_ust_app_event(int sock, struct ust_app_event *ua_event, struct ust_app *app)
{
        int ret;

        LTTNG_ASSERT(ua_event);
        ASSERT_RCU_READ_LOCKED();

        free(ua_event->filter);
        if (ua_event->exclusion != nullptr)
                free(ua_event->exclusion);
        if (ua_event->obj != nullptr) {
                pthread_mutex_lock(&app->sock_lock);
                ret = lttng_ust_ctl_release_object(sock, ua_event->obj);
                pthread_mutex_unlock(&app->sock_lock);
                if (ret < 0) {
                        if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                                DBG3("UST app release event failed. Application is dead: pid = %d, sock = %d",
                                     app->pid,
                                     app->sock);
                        } else if (ret == -EAGAIN) {
                                WARN("UST app release event failed. Communication time out: pid = %d, sock = %d",
                                     app->pid,
                                     app->sock);
                        } else {
                                ERR("UST app release event obj failed with ret %d: pid = %d, sock = %d",
                                    ret,
                                    app->pid,
                                    app->sock);
                        }
                }
                free(ua_event->obj);
        }
        free(ua_event);
}

/*
 * Delayed reclaim of a ust_app_event_notifier_rule object. This MUST be called
 * through a call_rcu().
 */
static void free_ust_app_event_notifier_rule_rcu(struct rcu_head *head)
{
        struct ust_app_event_notifier_rule *obj =
                lttng::utils::container_of(head, &ust_app_event_notifier_rule::rcu_head);

        free(obj);
}

/*
 * Delete ust app event notifier rule safely.
 */
static void delete_ust_app_event_notifier_rule(
        int sock, struct ust_app_event_notifier_rule *ua_event_notifier_rule, struct ust_app *app)
{
        int ret;

        LTTNG_ASSERT(ua_event_notifier_rule);

        if (ua_event_notifier_rule->exclusion != nullptr) {
                free(ua_event_notifier_rule->exclusion);
        }

        if (ua_event_notifier_rule->obj != nullptr) {
                pthread_mutex_lock(&app->sock_lock);
                ret = lttng_ust_ctl_release_object(sock, ua_event_notifier_rule->obj);
                pthread_mutex_unlock(&app->sock_lock);
                if (ret < 0) {
                        if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                                DBG3("UST app release event notifier failed. Application is dead: pid = %d, sock = %d",
                                     app->pid,
                                     app->sock);
                        } else if (ret == -EAGAIN) {
                                WARN("UST app release event notifier failed. Communication time out: pid = %d, sock = %d",
                                     app->pid,
                                     app->sock);
                        } else {
                                ERR("UST app release event notifier failed with ret %d: pid = %d, sock = %d",
                                    ret,
                                    app->pid,
                                    app->sock);
                        }
                }

                free(ua_event_notifier_rule->obj);
        }

        lttng_trigger_put(ua_event_notifier_rule->trigger);
        call_rcu(&ua_event_notifier_rule->rcu_head, free_ust_app_event_notifier_rule_rcu);
}

/*
 * Release ust data object of the given stream.
 *
 * Return 0 on success or else a negative value.
 */
static int release_ust_app_stream(int sock, struct ust_app_stream *stream, struct ust_app *app)
{
        int ret = 0;

        LTTNG_ASSERT(stream);

        if (stream->obj) {
                pthread_mutex_lock(&app->sock_lock);
                ret = lttng_ust_ctl_release_object(sock, stream->obj);
                pthread_mutex_unlock(&app->sock_lock);
                if (ret < 0) {
                        if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                                DBG3("UST app release stream failed. Application is dead: pid = %d, sock = %d",
                                     app->pid,
                                     app->sock);
                        } else if (ret == -EAGAIN) {
                                WARN("UST app release stream failed. Communication time out: pid = %d, sock = %d",
                                     app->pid,
                                     app->sock);
                        } else {
                                ERR("UST app release stream obj failed with ret %d: pid = %d, sock = %d",
                                    ret,
                                    app->pid,
                                    app->sock);
                        }
                }
                lttng_fd_put(LTTNG_FD_APPS, 2);
                free(stream->obj);
        }

        return ret;
}

/*
 * Delete ust app stream safely. RCU read lock must be held before calling
 * this function.
 */
static void delete_ust_app_stream(int sock, struct ust_app_stream *stream, struct ust_app *app)
{
        LTTNG_ASSERT(stream);
        ASSERT_RCU_READ_LOCKED();

        (void) release_ust_app_stream(sock, stream, app);
        free(stream);
}

static void delete_ust_app_channel_rcu(struct rcu_head *head)
{
        struct ust_app_channel *ua_chan =
                lttng::utils::container_of(head, &ust_app_channel::rcu_head);

        lttng_ht_destroy(ua_chan->ctx);
        lttng_ht_destroy(ua_chan->events);
        free(ua_chan);
}

/*
 * Extract the lost packet or discarded events counter when the channel is
 * being deleted and store the value in the parent channel so we can
 * access it from lttng list and at stop/destroy.
 *
 * The session list lock must be held by the caller.
 */
static void save_per_pid_lost_discarded_counters(struct ust_app_channel *ua_chan)
{
        uint64_t discarded = 0, lost = 0;
        struct ltt_ust_channel *uchan;

        if (ua_chan->attr.type != LTTNG_UST_ABI_CHAN_PER_CPU) {
                return;
        }

        lttng::urcu::read_lock_guard read_lock;

        ltt_session::ref session;
        try {
                session = ltt_session::find_session(ua_chan->session->tracing_id);
        } catch (const lttng::sessiond::exceptions::session_not_found_error& ex) {
                DBG_FMT("Failed to save per-pid lost/discarded counters: {}, location='{}'",
                        ex.what(),
                        ex.source_location);
        }

        if (!session || !session->ust_session) {
                /*
                 * Not finding the session is not an error because there are
                 * multiple ways the channels can be torn down.
                 *
                 * 1) The session daemon can initiate the destruction of the
                 *    ust app session after receiving a destroy command or
                 *    during its shutdown/teardown.
                 * 2) The application, since we are in per-pid tracing, is
                 *    unregistering and tearing down its ust app session.
                 *
                 * Both paths are protected by the session list lock which
                 * ensures that the accounting of lost packets and discarded
                 * events is done exactly once. The session is then unpublished
                 * from the session list, resulting in this condition.
                 */
                return;
        }

        if (ua_chan->attr.overwrite) {
                consumer_get_lost_packets(ua_chan->session->tracing_id,
                                          ua_chan->key,
                                          session->ust_session->consumer,
                                          &lost);
        } else {
                consumer_get_discarded_events(ua_chan->session->tracing_id,
                                              ua_chan->key,
                                              session->ust_session->consumer,
                                              &discarded);
        }
        uchan = trace_ust_find_channel_by_name(session->ust_session->domain_global.channels,
                                               ua_chan->name);
        if (!uchan) {
                ERR("Missing UST channel to store discarded counters");
                return;
        }

        uchan->per_pid_closed_app_discarded += discarded;
        uchan->per_pid_closed_app_lost += lost;
}

/*
 * Delete ust app channel safely. RCU read lock must be held before calling
 * this function.
 *
 * The session list lock must be held by the caller.
 */
static void delete_ust_app_channel(int sock,
                                   struct ust_app_channel *ua_chan,
                                   struct ust_app *app,
                                   const lsu::registry_session::locked_ref& locked_registry)
{
        int ret;
        struct lttng_ht_iter iter;
        struct ust_app_event *ua_event;
        struct ust_app_ctx *ua_ctx;
        struct ust_app_stream *stream, *stmp;

        LTTNG_ASSERT(ua_chan);
        ASSERT_RCU_READ_LOCKED();

        DBG3("UST app deleting channel %s", ua_chan->name);

        /* Wipe stream */
        cds_list_for_each_entry_safe (stream, stmp, &ua_chan->streams.head, list) {
                cds_list_del(&stream->list);
                delete_ust_app_stream(sock, stream, app);
        }

        /* Wipe context */
        cds_lfht_for_each_entry (ua_chan->ctx->ht, &iter.iter, ua_ctx, node.node) {
                cds_list_del(&ua_ctx->list);
                ret = lttng_ht_del(ua_chan->ctx, &iter);
                LTTNG_ASSERT(!ret);
                delete_ust_app_ctx(sock, ua_ctx, app);
        }

        /* Wipe events */
        cds_lfht_for_each_entry (ua_chan->events->ht, &iter.iter, ua_event, node.node) {
                ret = lttng_ht_del(ua_chan->events, &iter);
                LTTNG_ASSERT(!ret);
                delete_ust_app_event(sock, ua_event, app);
        }

        if (ua_chan->session->buffer_type == LTTNG_BUFFER_PER_PID) {
                /* Wipe and free registry from session registry. */
                if (locked_registry) {
                        try {
                                locked_registry->remove_channel(ua_chan->key, sock >= 0);
                        } catch (const std::exception& ex) {
                                DBG("Could not find channel for removal: %s", ex.what());
                        }
                }

                /*
                 * A negative socket can be used by the caller when
                 * cleaning-up a ua_chan in an error path. Skip the
                 * accounting in this case.
                 */
                if (sock >= 0) {
                        save_per_pid_lost_discarded_counters(ua_chan);
                }
        }

        if (ua_chan->obj != nullptr) {
                /* Remove channel from application UST object descriptor. */
                iter.iter.node = &ua_chan->ust_objd_node.node;
                ret = lttng_ht_del(app->ust_objd, &iter);
                LTTNG_ASSERT(!ret);
                pthread_mutex_lock(&app->sock_lock);
                ret = lttng_ust_ctl_release_object(sock, ua_chan->obj);
                pthread_mutex_unlock(&app->sock_lock);
                if (ret < 0) {
                        if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                                DBG3("UST app channel %s release failed. Application is dead: pid = %d, sock = %d",
                                     ua_chan->name,
                                     app->pid,
                                     app->sock);
                        } else if (ret == -EAGAIN) {
                                WARN("UST app channel %s release failed. Communication time out: pid = %d, sock = %d",
                                     ua_chan->name,
                                     app->pid,
                                     app->sock);
                        } else {
                                ERR("UST app channel %s release failed with ret %d: pid = %d, sock = %d",
                                    ua_chan->name,
                                    ret,
                                    app->pid,
                                    app->sock);
                        }
                }
                lttng_fd_put(LTTNG_FD_APPS, 1);
                free(ua_chan->obj);
        }
        call_rcu(&ua_chan->rcu_head, delete_ust_app_channel_rcu);
}

int ust_app_register_done(struct ust_app *app)
{
        int ret;

        pthread_mutex_lock(&app->sock_lock);
        ret = lttng_ust_ctl_register_done(app->sock);
        pthread_mutex_unlock(&app->sock_lock);
        return ret;
}

int ust_app_release_object(struct ust_app *app, struct lttng_ust_abi_object_data *data)
{
        int ret, sock;

        if (app) {
                pthread_mutex_lock(&app->sock_lock);
                sock = app->sock;
        } else {
                sock = -1;
        }
        ret = lttng_ust_ctl_release_object(sock, data);
        if (app) {
                pthread_mutex_unlock(&app->sock_lock);
        }
        return ret;
}

/*
 * Push metadata to consumer socket.
 *
 * RCU read-side lock must be held to guarantee existence of socket.
 * Must be called with the ust app session lock held.
 * Must be called with the registry lock held.
 *
 * On success, return the len of metadata pushed or else a negative value.
 * Returning a -EPIPE return value means we could not send the metadata,
 * but it can be caused by recoverable errors (e.g. the application has
 * terminated concurrently).
 */
ssize_t ust_app_push_metadata(const lsu::registry_session::locked_ref& locked_registry,
                              struct consumer_socket *socket,
                              int send_zero_data)
{
        int ret;
        char *metadata_str = nullptr;
        size_t len, offset, new_metadata_len_sent;
        ssize_t ret_val;
        uint64_t metadata_key, metadata_version;

        LTTNG_ASSERT(locked_registry);
        LTTNG_ASSERT(socket);
        ASSERT_RCU_READ_LOCKED();

        metadata_key = locked_registry->_metadata_key;

        /*
         * Means that no metadata was assigned to the session. This can
         * happens if no start has been done previously.
         */
        if (!metadata_key) {
                return 0;
        }

        offset = locked_registry->_metadata_len_sent;
        len = locked_registry->_metadata_len - locked_registry->_metadata_len_sent;
        new_metadata_len_sent = locked_registry->_metadata_len;
        metadata_version = locked_registry->_metadata_version;
        if (len == 0) {
                DBG3("No metadata to push for metadata key %" PRIu64,
                     locked_registry->_metadata_key);
                ret_val = len;
                if (send_zero_data) {
                        DBG("No metadata to push");
                        goto push_data;
                }
                goto end;
        }

        /* Allocate only what we have to send. */
        metadata_str = calloc<char>(len);
        if (!metadata_str) {
                PERROR("zmalloc ust app metadata string");
                ret_val = -ENOMEM;
                goto error;
        }
        /* Copy what we haven't sent out. */
        memcpy(metadata_str, locked_registry->_metadata + offset, len);

push_data:
        pthread_mutex_unlock(&locked_registry->_lock);
        /*
         * We need to unlock the registry while we push metadata to
         * break a circular dependency between the consumerd metadata
         * lock and the sessiond registry lock. Indeed, pushing metadata
         * to the consumerd awaits that it gets pushed all the way to
         * relayd, but doing so requires grabbing the metadata lock. If
         * a concurrent metadata request is being performed by
         * consumerd, this can try to grab the registry lock on the
         * sessiond while holding the metadata lock on the consumer
         * daemon. Those push and pull schemes are performed on two
         * different bidirectionnal communication sockets.
         */
        ret = consumer_push_metadata(
                socket, metadata_key, metadata_str, len, offset, metadata_version);
        pthread_mutex_lock(&locked_registry->_lock);
        if (ret < 0) {
                /*
                 * There is an acceptable race here between the registry
                 * metadata key assignment and the creation on the
                 * consumer. The session daemon can concurrently push
                 * metadata for this registry while being created on the
                 * consumer since the metadata key of the registry is
                 * assigned *before* it is setup to avoid the consumer
                 * to ask for metadata that could possibly be not found
                 * in the session daemon.
                 *
                 * The metadata will get pushed either by the session
                 * being stopped or the consumer requesting metadata if
                 * that race is triggered.
                 */
                if (ret == -LTTCOMM_CONSUMERD_CHANNEL_FAIL) {
                        ret = 0;
                } else {
                        ERR("Error pushing metadata to consumer");
                }
                ret_val = ret;
                goto error_push;
        } else {
                /*
                 * Metadata may have been concurrently pushed, since
                 * we're not holding the registry lock while pushing to
                 * consumer.  This is handled by the fact that we send
                 * the metadata content, size, and the offset at which
                 * that metadata belongs. This may arrive out of order
                 * on the consumer side, and the consumer is able to
                 * deal with overlapping fragments. The consumer
                 * supports overlapping fragments, which must be
                 * contiguous starting from offset 0. We keep the
                 * largest metadata_len_sent value of the concurrent
                 * send.
                 */
                locked_registry->_metadata_len_sent =
                        std::max(locked_registry->_metadata_len_sent, new_metadata_len_sent);
        }
        free(metadata_str);
        return len;

end:
error:
        if (ret_val) {
                /*
                 * On error, flag the registry that the metadata is
                 * closed. We were unable to push anything and this
                 * means that either the consumer is not responding or
                 * the metadata cache has been destroyed on the
                 * consumer.
                 */
                locked_registry->_metadata_closed = true;
        }
error_push:
        free(metadata_str);
        return ret_val;
}

/*
 * For a given application and session, push metadata to consumer.
 * Either sock or consumer is required : if sock is NULL, the default
 * socket to send the metadata is retrieved from consumer, if sock
 * is not NULL we use it to send the metadata.
 * RCU read-side lock must be held while calling this function,
 * therefore ensuring existence of registry. It also ensures existence
 * of socket throughout this function.
 *
 * Return 0 on success else a negative error.
 * Returning a -EPIPE return value means we could not send the metadata,
 * but it can be caused by recoverable errors (e.g. the application has
 * terminated concurrently).
 */
static int push_metadata(const lsu::registry_session::locked_ref& locked_registry,
                         struct consumer_output *consumer)
{
        int ret_val;
        ssize_t ret;
        struct consumer_socket *socket;

        LTTNG_ASSERT(locked_registry);
        LTTNG_ASSERT(consumer);
        ASSERT_RCU_READ_LOCKED();

        if (locked_registry->_metadata_closed) {
                ret_val = -EPIPE;
                goto error;
        }

        /* Get consumer socket to use to push the metadata.*/
        socket = consumer_find_socket_by_bitness(locked_registry->abi.bits_per_long, consumer);
        if (!socket) {
                ret_val = -1;
                goto error;
        }

        ret = ust_app_push_metadata(locked_registry, socket, 0);
        if (ret < 0) {
                ret_val = ret;
                goto error;
        }
        return 0;

error:
        return ret_val;
}

/*
 * Send to the consumer a close metadata command for the given session. Once
 * done, the metadata channel is deleted and the session metadata pointer is
 * nullified. The session lock MUST be held unless the application is
 * in the destroy path.
 *
 * Do not hold the registry lock while communicating with the consumerd, because
 * doing so causes inter-process deadlocks between consumerd and sessiond with
 * the metadata request notification.
 *
 * Return 0 on success else a negative value.
 */
static int close_metadata(uint64_t metadata_key,
                          unsigned int consumer_bitness,
                          struct consumer_output *consumer)
{
        int ret;
        struct consumer_socket *socket;
        lttng::urcu::read_lock_guard read_lock_guard;

        LTTNG_ASSERT(consumer);

        /* Get consumer socket to use to push the metadata. */
        socket = consumer_find_socket_by_bitness(consumer_bitness, consumer);
        if (!socket) {
                ret = -1;
                goto end;
        }

        ret = consumer_close_metadata(socket, metadata_key);
        if (ret < 0) {
                goto end;
        }

end:
        return ret;
}

static void delete_ust_app_session_rcu(struct rcu_head *head)
{
        struct ust_app_session *ua_sess =
                lttng::utils::container_of(head, &ust_app_session::rcu_head);

        lttng_ht_destroy(ua_sess->channels);
        free(ua_sess);
}

/*
 * Delete ust app session safely. RCU read lock must be held before calling
 * this function.
 *
 * The session list lock must be held by the caller.
 */
static void delete_ust_app_session(int sock, struct ust_app_session *ua_sess, struct ust_app *app)
{
        int ret;
        struct lttng_ht_iter iter;
        struct ust_app_channel *ua_chan;

        LTTNG_ASSERT(ua_sess);
        ASSERT_RCU_READ_LOCKED();

        pthread_mutex_lock(&ua_sess->lock);

        LTTNG_ASSERT(!ua_sess->deleted);
        ua_sess->deleted = true;

        auto locked_registry = get_locked_session_registry(ua_sess);
        /* Registry can be null on error path during initialization. */
        if (locked_registry) {
                /* Push metadata for application before freeing the application. */
                (void) push_metadata(locked_registry, ua_sess->consumer);
        }

        cds_lfht_for_each_entry (ua_sess->channels->ht, &iter.iter, ua_chan, node.node) {
                ret = lttng_ht_del(ua_sess->channels, &iter);
                LTTNG_ASSERT(!ret);
                delete_ust_app_channel(sock, ua_chan, app, locked_registry);
        }

        if (locked_registry) {
                /*
                 * Don't ask to close metadata for global per UID buffers. Close
                 * metadata only on destroy trace session in this case. Also, the
                 * previous push metadata could have flag the metadata registry to
                 * close so don't send a close command if closed.
                 */
                if (ua_sess->buffer_type != LTTNG_BUFFER_PER_UID) {
                        const auto metadata_key = locked_registry->_metadata_key;
                        const auto consumer_bitness = locked_registry->abi.bits_per_long;

                        if (!locked_registry->_metadata_closed && metadata_key != 0) {
                                locked_registry->_metadata_closed = true;
                        }

                        /* Release lock before communication, see comments in close_metadata(). */
                        locked_registry.reset();
                        (void) close_metadata(metadata_key, consumer_bitness, ua_sess->consumer);
                }
        }

        /* In case of per PID, the registry is kept in the session. */
        if (ua_sess->buffer_type == LTTNG_BUFFER_PER_PID) {
                struct buffer_reg_pid *reg_pid = buffer_reg_pid_find(ua_sess->id);
                if (reg_pid) {
                        /*
                         * Registry can be null on error path during
                         * initialization.
                         */
                        buffer_reg_pid_remove(reg_pid);
                        buffer_reg_pid_destroy(reg_pid);
                }
        }

        if (ua_sess->handle != -1) {
                pthread_mutex_lock(&app->sock_lock);
                ret = lttng_ust_ctl_release_handle(sock, ua_sess->handle);
                pthread_mutex_unlock(&app->sock_lock);
                if (ret < 0) {
                        if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                                DBG3("UST app release session handle failed. Application is dead: pid = %d, sock = %d",
                                     app->pid,
                                     app->sock);
                        } else if (ret == -EAGAIN) {
                                WARN("UST app release session handle failed. Communication time out: pid = %d, sock = %d",
                                     app->pid,
                                     app->sock);
                        } else {
                                ERR("UST app release session handle failed with ret %d: pid = %d, sock = %d",
                                    ret,
                                    app->pid,
                                    app->sock);
                        }
                }

                /* Remove session from application UST object descriptor. */
                iter.iter.node = &ua_sess->ust_objd_node.node;
                ret = lttng_ht_del(app->ust_sessions_objd, &iter);
                LTTNG_ASSERT(!ret);
        }

        pthread_mutex_unlock(&ua_sess->lock);

        consumer_output_put(ua_sess->consumer);

        call_rcu(&ua_sess->rcu_head, delete_ust_app_session_rcu);
}

/*
 * Delete a traceable application structure from the global list. Never call
 * this function outside of a call_rcu call.
 */
static void delete_ust_app(struct ust_app *app)
{
        int ret, sock;
        struct ust_app_session *ua_sess, *tmp_ua_sess;
        struct lttng_ht_iter iter;
        struct ust_app_event_notifier_rule *event_notifier_rule;
        bool event_notifier_write_fd_is_open;

        /*
         * The session list lock must be held during this function to guarantee
         * the existence of ua_sess.
         */
        const auto list_lock = lttng::sessiond::lock_session_list();
        /* Delete ust app sessions info */
        sock = app->sock;
        app->sock = -1;

        /* Wipe sessions */
        cds_list_for_each_entry_safe (ua_sess, tmp_ua_sess, &app->teardown_head, teardown_node) {
                /* Free every object in the session and the session. */
                lttng::urcu::read_lock_guard read_lock;
                delete_ust_app_session(sock, ua_sess, app);
        }

        /* Remove the event notifier rules associated with this app. */
        {
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (app->token_to_event_notifier_rule_ht->ht,
                                         &iter.iter,
                                         event_notifier_rule,
                                         node.node) {
                        ret = lttng_ht_del(app->token_to_event_notifier_rule_ht, &iter);
                        LTTNG_ASSERT(!ret);

                        delete_ust_app_event_notifier_rule(app->sock, event_notifier_rule, app);
                }
        }

        lttng_ht_destroy(app->sessions);
        lttng_ht_destroy(app->ust_sessions_objd);
        lttng_ht_destroy(app->ust_objd);
        lttng_ht_destroy(app->token_to_event_notifier_rule_ht);

        /*
         * This could be NULL if the event notifier setup failed (e.g the app
         * was killed or the tracer does not support this feature).
         */
        if (app->event_notifier_group.object) {
                enum lttng_error_code ret_code;
                enum event_notifier_error_accounting_status status;

                const int event_notifier_read_fd =
                        lttng_pipe_get_readfd(app->event_notifier_group.event_pipe);

                ret_code = notification_thread_command_remove_tracer_event_source(
                        the_notification_thread_handle, event_notifier_read_fd);
                if (ret_code != LTTNG_OK) {
                        ERR("Failed to remove application tracer event source from notification thread");
                }

                status = event_notifier_error_accounting_unregister_app(app);
                if (status != EVENT_NOTIFIER_ERROR_ACCOUNTING_STATUS_OK) {
                        ERR("Error unregistering app from event notifier error accounting");
                }

                lttng_ust_ctl_release_object(sock, app->event_notifier_group.object);
                free(app->event_notifier_group.object);
        }

        event_notifier_write_fd_is_open =
                lttng_pipe_is_write_open(app->event_notifier_group.event_pipe);
        lttng_pipe_destroy(app->event_notifier_group.event_pipe);
        /*
         * Release the file descriptors reserved for the event notifier pipe.
         * The app could be destroyed before the write end of the pipe could be
         * passed to the application (and closed). In that case, both file
         * descriptors must be released.
         */
        lttng_fd_put(LTTNG_FD_APPS, event_notifier_write_fd_is_open ? 2 : 1);

        /*
         * Wait until we have deleted the application from the sock hash table
         * before closing this socket, otherwise an application could re-use the
         * socket ID and race with the teardown, using the same hash table entry.
         *
         * It's OK to leave the close in call_rcu. We want it to stay unique for
         * all RCU readers that could run concurrently with unregister app,
         * therefore we _need_ to only close that socket after a grace period. So
         * it should stay in this RCU callback.
         *
         * This close() is a very important step of the synchronization model so
         * every modification to this function must be carefully reviewed.
         */
        ret = close(sock);
        if (ret) {
                PERROR("close");
        }
        lttng_fd_put(LTTNG_FD_APPS, 1);

        DBG2("UST app pid %d deleted", app->pid);
        free(app);
}

/*
 * URCU intermediate call to delete an UST app.
 */
static void delete_ust_app_rcu(struct rcu_head *head)
{
        struct lttng_ht_node_ulong *node =
                lttng::utils::container_of(head, &lttng_ht_node_ulong::head);
        struct ust_app *app = lttng::utils::container_of(node, &ust_app::pid_n);

        DBG3("Call RCU deleting app PID %d", app->pid);
        delete_ust_app(app);
}

/*
 * Delete the session from the application ht and delete the data structure by
 * freeing every object inside and releasing them.
 *
 * The session list lock must be held by the caller.
 */
static void destroy_app_session(struct ust_app *app, struct ust_app_session *ua_sess)
{
        int ret;
        struct lttng_ht_iter iter;

        LTTNG_ASSERT(app);
        LTTNG_ASSERT(ua_sess);

        iter.iter.node = &ua_sess->node.node;
        ret = lttng_ht_del(app->sessions, &iter);
        if (ret) {
                /* Already scheduled for teardown. */
                goto end;
        }

        /* Once deleted, free the data structure. */
        delete_ust_app_session(app->sock, ua_sess, app);

end:
        return;
}

/*
 * Alloc new UST app session.
 */
static struct ust_app_session *alloc_ust_app_session()
{
        struct ust_app_session *ua_sess;

        /* Init most of the default value by allocating and zeroing */
        ua_sess = zmalloc<ust_app_session>();
        if (ua_sess == nullptr) {
                PERROR("malloc");
                goto error_free;
        }

        ua_sess->handle = -1;
        ua_sess->channels = lttng_ht_new(0, LTTNG_HT_TYPE_STRING);
        ua_sess->metadata_attr.type = LTTNG_UST_ABI_CHAN_METADATA;
        pthread_mutex_init(&ua_sess->lock, nullptr);

        return ua_sess;

error_free:
        return nullptr;
}

/*
 * Alloc new UST app channel.
 */
static struct ust_app_channel *alloc_ust_app_channel(const char *name,
                                                     struct ust_app_session *ua_sess,
                                                     struct lttng_ust_abi_channel_attr *attr)
{
        struct ust_app_channel *ua_chan;

        /* Init most of the default value by allocating and zeroing */
        ua_chan = zmalloc<ust_app_channel>();
        if (ua_chan == nullptr) {
                PERROR("malloc");
                goto error;
        }

        /* Setup channel name */
        strncpy(ua_chan->name, name, sizeof(ua_chan->name));
        ua_chan->name[sizeof(ua_chan->name) - 1] = '\0';

        ua_chan->enabled = true;
        ua_chan->handle = -1;
        ua_chan->session = ua_sess;
        ua_chan->key = get_next_channel_key();
        ua_chan->ctx = lttng_ht_new(0, LTTNG_HT_TYPE_ULONG);
        ua_chan->events = lttng_ht_new(0, LTTNG_HT_TYPE_STRING);
        lttng_ht_node_init_str(&ua_chan->node, ua_chan->name);

        CDS_INIT_LIST_HEAD(&ua_chan->streams.head);
        CDS_INIT_LIST_HEAD(&ua_chan->ctx_list);

        /* Copy attributes */
        if (attr) {
                /* Translate from lttng_ust_channel to lttng_ust_ctl_consumer_channel_attr. */
                ua_chan->attr.subbuf_size = attr->subbuf_size;
                ua_chan->attr.num_subbuf = attr->num_subbuf;
                ua_chan->attr.overwrite = attr->overwrite;
                ua_chan->attr.switch_timer_interval = attr->switch_timer_interval;
                ua_chan->attr.read_timer_interval = attr->read_timer_interval;
                ua_chan->attr.output = (lttng_ust_abi_output) attr->output;
                ua_chan->attr.blocking_timeout = attr->u.s.blocking_timeout;
        }
        /* By default, the channel is a per cpu channel. */
        ua_chan->attr.type = LTTNG_UST_ABI_CHAN_PER_CPU;

        DBG3("UST app channel %s allocated", ua_chan->name);

        return ua_chan;

error:
        return nullptr;
}

/*
 * Allocate and initialize a UST app stream.
 *
 * Return newly allocated stream pointer or NULL on error.
 */
struct ust_app_stream *ust_app_alloc_stream()
{
        struct ust_app_stream *stream = nullptr;

        stream = zmalloc<ust_app_stream>();
        if (stream == nullptr) {
                PERROR("zmalloc ust app stream");
                goto error;
        }

        /* Zero could be a valid value for a handle so flag it to -1. */
        stream->handle = -1;

error:
        return stream;
}

/*
 * Alloc new UST app event.
 */
static struct ust_app_event *alloc_ust_app_event(char *name, struct lttng_ust_abi_event *attr)
{
        struct ust_app_event *ua_event;

        /* Init most of the default value by allocating and zeroing */
        ua_event = zmalloc<ust_app_event>();
        if (ua_event == nullptr) {
                PERROR("Failed to allocate ust_app_event structure");
                goto error;
        }

        ua_event->enabled = true;
        strncpy(ua_event->name, name, sizeof(ua_event->name));
        ua_event->name[sizeof(ua_event->name) - 1] = '\0';
        lttng_ht_node_init_str(&ua_event->node, ua_event->name);

        /* Copy attributes */
        if (attr) {
                memcpy(&ua_event->attr, attr, sizeof(ua_event->attr));
        }

        DBG3("UST app event %s allocated", ua_event->name);

        return ua_event;

error:
        return nullptr;
}

/*
 * Allocate a new UST app event notifier rule.
 */
static struct ust_app_event_notifier_rule *
alloc_ust_app_event_notifier_rule(struct lttng_trigger *trigger)
{
        enum lttng_event_rule_generate_exclusions_status generate_exclusion_status;
        enum lttng_condition_status cond_status;
        struct ust_app_event_notifier_rule *ua_event_notifier_rule;
        struct lttng_condition *condition = nullptr;
        const struct lttng_event_rule *event_rule = nullptr;

        ua_event_notifier_rule = zmalloc<ust_app_event_notifier_rule>();
        if (ua_event_notifier_rule == nullptr) {
                PERROR("Failed to allocate ust_app_event_notifier_rule structure");
                goto error;
        }

        ua_event_notifier_rule->enabled = true;
        ua_event_notifier_rule->token = lttng_trigger_get_tracer_token(trigger);
        lttng_ht_node_init_u64(&ua_event_notifier_rule->node, ua_event_notifier_rule->token);

        condition = lttng_trigger_get_condition(trigger);
        LTTNG_ASSERT(condition);
        LTTNG_ASSERT(lttng_condition_get_type(condition) ==
                     LTTNG_CONDITION_TYPE_EVENT_RULE_MATCHES);

        cond_status = lttng_condition_event_rule_matches_get_rule(condition, &event_rule);
        LTTNG_ASSERT(cond_status == LTTNG_CONDITION_STATUS_OK);
        LTTNG_ASSERT(event_rule);

        ua_event_notifier_rule->error_counter_index =
                lttng_condition_event_rule_matches_get_error_counter_index(condition);
        /* Acquire the event notifier's reference to the trigger. */
        lttng_trigger_get(trigger);

        ua_event_notifier_rule->trigger = trigger;
        ua_event_notifier_rule->filter = lttng_event_rule_get_filter_bytecode(event_rule);
        generate_exclusion_status = lttng_event_rule_generate_exclusions(
                event_rule, &ua_event_notifier_rule->exclusion);
        switch (generate_exclusion_status) {
        case LTTNG_EVENT_RULE_GENERATE_EXCLUSIONS_STATUS_OK:
        case LTTNG_EVENT_RULE_GENERATE_EXCLUSIONS_STATUS_NONE:
                break;
        default:
                /* Error occurred. */
                ERR("Failed to generate exclusions from trigger while allocating an event notifier rule");
                goto error_put_trigger;
        }

        DBG3("UST app event notifier rule allocated: token = %" PRIu64,
             ua_event_notifier_rule->token);

        return ua_event_notifier_rule;

error_put_trigger:
        lttng_trigger_put(trigger);
error:
        free(ua_event_notifier_rule);
        return nullptr;
}

/*
 * Alloc new UST app context.
 */
static struct ust_app_ctx *alloc_ust_app_ctx(struct lttng_ust_context_attr *uctx)
{
        struct ust_app_ctx *ua_ctx;

        ua_ctx = zmalloc<ust_app_ctx>();
        if (ua_ctx == nullptr) {
                goto error;
        }

        CDS_INIT_LIST_HEAD(&ua_ctx->list);

        if (uctx) {
                memcpy(&ua_ctx->ctx, uctx, sizeof(ua_ctx->ctx));
                if (uctx->ctx == LTTNG_UST_ABI_CONTEXT_APP_CONTEXT) {
                        char *provider_name = nullptr, *ctx_name = nullptr;

                        provider_name = strdup(uctx->u.app_ctx.provider_name);
                        ctx_name = strdup(uctx->u.app_ctx.ctx_name);
                        if (!provider_name || !ctx_name) {
                                free(provider_name);
                                free(ctx_name);
                                goto error;
                        }

                        ua_ctx->ctx.u.app_ctx.provider_name = provider_name;
                        ua_ctx->ctx.u.app_ctx.ctx_name = ctx_name;
                }
        }

        DBG3("UST app context %d allocated", ua_ctx->ctx.ctx);
        return ua_ctx;
error:
        free(ua_ctx);
        return nullptr;
}

/*
 * Create a liblttng-ust filter bytecode from given bytecode.
 *
 * Return allocated filter or NULL on error.
 */
static struct lttng_ust_abi_filter_bytecode *
create_ust_filter_bytecode_from_bytecode(const struct lttng_bytecode *orig_f)
{
        struct lttng_ust_abi_filter_bytecode *filter = nullptr;

        /* Copy filter bytecode. */
        filter = zmalloc<lttng_ust_abi_filter_bytecode>(sizeof(*filter) + orig_f->len);
        if (!filter) {
                PERROR("Failed to allocate lttng_ust_filter_bytecode: bytecode len = %" PRIu32
                       " bytes",
                       orig_f->len);
                goto error;
        }

        LTTNG_ASSERT(sizeof(struct lttng_bytecode) == sizeof(struct lttng_ust_abi_filter_bytecode));
        memcpy(filter, orig_f, sizeof(*filter) + orig_f->len);
error:
        return filter;
}

/*
 * Create a liblttng-ust capture bytecode from given bytecode.
 *
 * Return allocated filter or NULL on error.
 */
static struct lttng_ust_abi_capture_bytecode *
create_ust_capture_bytecode_from_bytecode(const struct lttng_bytecode *orig_f)
{
        struct lttng_ust_abi_capture_bytecode *capture = nullptr;

        /* Copy capture bytecode. */
        capture = zmalloc<lttng_ust_abi_capture_bytecode>(sizeof(*capture) + orig_f->len);
        if (!capture) {
                PERROR("Failed to allocate lttng_ust_abi_capture_bytecode: bytecode len = %" PRIu32
                       " bytes",
                       orig_f->len);
                goto error;
        }

        LTTNG_ASSERT(sizeof(struct lttng_bytecode) ==
                     sizeof(struct lttng_ust_abi_capture_bytecode));
        memcpy(capture, orig_f, sizeof(*capture) + orig_f->len);
error:
        return capture;
}

/*
 * Find an ust_app using the sock and return it. RCU read side lock must be
 * held before calling this helper function.
 */
struct ust_app *ust_app_find_by_sock(int sock)
{
        struct lttng_ht_node_ulong *node;
        struct lttng_ht_iter iter;

        ASSERT_RCU_READ_LOCKED();

        lttng_ht_lookup(ust_app_ht_by_sock, (void *) ((unsigned long) sock), &iter);
        node = lttng_ht_iter_get_node_ulong(&iter);
        if (node == nullptr) {
                DBG2("UST app find by sock %d not found", sock);
                goto error;
        }

        return lttng::utils::container_of(node, &ust_app::sock_n);

error:
        return nullptr;
}

/*
 * Find an ust_app using the notify sock and return it. RCU read side lock must
 * be held before calling this helper function.
 */
static struct ust_app *find_app_by_notify_sock(int sock)
{
        struct lttng_ht_node_ulong *node;
        struct lttng_ht_iter iter;

        ASSERT_RCU_READ_LOCKED();

        lttng_ht_lookup(ust_app_ht_by_notify_sock, (void *) ((unsigned long) sock), &iter);
        node = lttng_ht_iter_get_node_ulong(&iter);
        if (node == nullptr) {
                DBG2("UST app find by notify sock %d not found", sock);
                goto error;
        }

        return lttng::utils::container_of(node, &ust_app::notify_sock_n);

error:
        return nullptr;
}

/*
 * Lookup for an ust app event based on event name, filter bytecode and the
 * event loglevel.
 *
 * Return an ust_app_event object or NULL on error.
 */
static struct ust_app_event *find_ust_app_event(struct lttng_ht *ht,
                                                const char *name,
                                                const struct lttng_bytecode *filter,
                                                lttng_ust_abi_loglevel_type loglevel_type,
                                                int loglevel_value,
                                                const struct lttng_event_exclusion *exclusion)
{
        struct lttng_ht_iter iter;
        struct lttng_ht_node_str *node;
        struct ust_app_event *event = nullptr;
        struct ust_app_ht_key key;

        LTTNG_ASSERT(name);
        LTTNG_ASSERT(ht);

        /* Setup key for event lookup. */
        key.name = name;
        key.filter = filter;
        key.loglevel_type = loglevel_type;
        key.loglevel_value = loglevel_value;
        /* lttng_event_exclusion and lttng_ust_event_exclusion structures are similar */
        key.exclusion = exclusion;

        /* Lookup using the event name as hash and a custom match fct. */
        cds_lfht_lookup(ht->ht,
                        ht->hash_fct((void *) name, lttng_ht_seed),
                        ht_match_ust_app_event,
                        &key,
                        &iter.iter);
        node = lttng_ht_iter_get_node_str(&iter);
        if (node == nullptr) {
                goto end;
        }

        event = lttng::utils::container_of(node, &ust_app_event::node);

end:
        return event;
}

/*
 * Look-up an event notifier rule based on its token id.
 *
 * Must be called with the RCU read lock held.
 * Return an ust_app_event_notifier_rule object or NULL on error.
 */
static struct ust_app_event_notifier_rule *find_ust_app_event_notifier_rule(struct lttng_ht *ht,
                                                                            uint64_t token)
{
        struct lttng_ht_iter iter;
        struct lttng_ht_node_u64 *node;
        struct ust_app_event_notifier_rule *event_notifier_rule = nullptr;

        LTTNG_ASSERT(ht);
        ASSERT_RCU_READ_LOCKED();

        lttng_ht_lookup(ht, &token, &iter);
        node = lttng_ht_iter_get_node_u64(&iter);
        if (node == nullptr) {
                DBG2("UST app event notifier rule token not found: token = %" PRIu64, token);
                goto end;
        }

        event_notifier_rule = lttng::utils::container_of(node, &ust_app_event_notifier_rule::node);
end:
        return event_notifier_rule;
}

/*
 * Create the channel context on the tracer.
 *
 * Called with UST app session lock held.
 */
static int create_ust_channel_context(struct ust_app_channel *ua_chan,
                                      struct ust_app_ctx *ua_ctx,
                                      struct ust_app *app)
{
        int ret;

        health_code_update();

        pthread_mutex_lock(&app->sock_lock);
        ret = lttng_ust_ctl_add_context(app->sock, &ua_ctx->ctx, ua_chan->obj, &ua_ctx->obj);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        ret = 0;
                        DBG3("UST app create channel context failed. Application is dead: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else if (ret == -EAGAIN) {
                        ret = 0;
                        WARN("UST app create channel context failed. Communication time out: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else {
                        ERR("UST app create channel context failed with ret %d: pid = %d, sock = %d",
                            ret,
                            app->pid,
                            app->sock);
                }
                goto error;
        }

        ua_ctx->handle = ua_ctx->obj->handle;

        DBG2("UST app context handle %d created successfully for channel %s",
             ua_ctx->handle,
             ua_chan->name);

error:
        health_code_update();
        return ret;
}

/*
 * Set the filter on the tracer.
 */
static int set_ust_object_filter(struct ust_app *app,
                                 const struct lttng_bytecode *bytecode,
                                 struct lttng_ust_abi_object_data *ust_object)
{
        int ret;
        struct lttng_ust_abi_filter_bytecode *ust_bytecode = nullptr;

        health_code_update();

        ust_bytecode = create_ust_filter_bytecode_from_bytecode(bytecode);
        if (!ust_bytecode) {
                ret = -LTTNG_ERR_NOMEM;
                goto error;
        }
        pthread_mutex_lock(&app->sock_lock);
        ret = lttng_ust_ctl_set_filter(app->sock, ust_bytecode, ust_object);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        ret = 0;
                        DBG3("UST app  set filter failed. Application is dead: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else if (ret == -EAGAIN) {
                        ret = 0;
                        WARN("UST app  set filter failed. Communication time out: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else {
                        ERR("UST app  set filter failed with ret %d: pid = %d, sock = %d, object = %p",
                            ret,
                            app->pid,
                            app->sock,
                            ust_object);
                }
                goto error;
        }

        DBG2("UST filter successfully set: object = %p", ust_object);

error:
        health_code_update();
        free(ust_bytecode);
        return ret;
}

/*
 * Set a capture bytecode for the passed object.
 * The sequence number enforces the ordering at runtime and on reception of
 * the captured payloads.
 */
static int set_ust_capture(struct ust_app *app,
                           const struct lttng_bytecode *bytecode,
                           unsigned int capture_seqnum,
                           struct lttng_ust_abi_object_data *ust_object)
{
        int ret;
        struct lttng_ust_abi_capture_bytecode *ust_bytecode = nullptr;

        health_code_update();

        ust_bytecode = create_ust_capture_bytecode_from_bytecode(bytecode);
        if (!ust_bytecode) {
                ret = -LTTNG_ERR_NOMEM;
                goto error;
        }

        /*
         * Set the sequence number to ensure the capture of fields is ordered.
         */
        ust_bytecode->seqnum = capture_seqnum;

        pthread_mutex_lock(&app->sock_lock);
        ret = lttng_ust_ctl_set_capture(app->sock, ust_bytecode, ust_object);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        ret = 0;
                        DBG3("UST app set capture failed. Application is dead: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else if (ret == -EAGAIN) {
                        ret = 0;
                        DBG3("UST app set capture failed. Communication timeout: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else {
                        ERR("UST app event set capture failed with ret %d: pid = %d, sock = %d",
                            ret,
                            app->pid,
                            app->sock);
                }

                goto error;
        }

        DBG2("UST capture successfully set: object = %p", ust_object);

error:
        health_code_update();
        free(ust_bytecode);
        return ret;
}

static struct lttng_ust_abi_event_exclusion *
create_ust_exclusion_from_exclusion(const struct lttng_event_exclusion *exclusion)
{
        struct lttng_ust_abi_event_exclusion *ust_exclusion = nullptr;
        size_t exclusion_alloc_size = sizeof(struct lttng_ust_abi_event_exclusion) +
                LTTNG_UST_ABI_SYM_NAME_LEN * exclusion->count;

        ust_exclusion = zmalloc<lttng_ust_abi_event_exclusion>(exclusion_alloc_size);
        if (!ust_exclusion) {
                PERROR("malloc");
                goto end;
        }

        LTTNG_ASSERT(sizeof(struct lttng_event_exclusion) ==
                     sizeof(struct lttng_ust_abi_event_exclusion));
        memcpy(ust_exclusion, exclusion, exclusion_alloc_size);
end:
        return ust_exclusion;
}

/*
 * Set event exclusions on the tracer.
 */
static int set_ust_object_exclusions(struct ust_app *app,
                                     const struct lttng_event_exclusion *exclusions,
                                     struct lttng_ust_abi_object_data *ust_object)
{
        int ret;
        struct lttng_ust_abi_event_exclusion *ust_exclusions = nullptr;

        LTTNG_ASSERT(exclusions && exclusions->count > 0);

        health_code_update();

        ust_exclusions = create_ust_exclusion_from_exclusion(exclusions);
        if (!ust_exclusions) {
                ret = -LTTNG_ERR_NOMEM;
                goto error;
        }
        pthread_mutex_lock(&app->sock_lock);
        ret = lttng_ust_ctl_set_exclusion(app->sock, ust_exclusions, ust_object);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        ret = 0;
                        DBG3("UST app event exclusion failed. Application is dead: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else if (ret == -EAGAIN) {
                        ret = 0;
                        WARN("UST app event exclusion failed. Communication time out(pid: %d, sock = %d",
                             app->pid,
                             app->sock);
                } else {
                        ERR("UST app event exclusions failed with ret %d: pid = %d, sock = %d, object = %p",
                            ret,
                            app->pid,
                            app->sock,
                            ust_object);
                }
                goto error;
        }

        DBG2("UST exclusions set successfully for object %p", ust_object);

error:
        health_code_update();
        free(ust_exclusions);
        return ret;
}

/*
 * Disable the specified event on to UST tracer for the UST session.
 */
static int disable_ust_object(struct ust_app *app, struct lttng_ust_abi_object_data *object)
{
        int ret;

        health_code_update();

        pthread_mutex_lock(&app->sock_lock);
        ret = lttng_ust_ctl_disable(app->sock, object);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        ret = 0;
                        DBG3("UST app disable object failed. Application is dead: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else if (ret == -EAGAIN) {
                        ret = 0;
                        WARN("UST app disable object failed. Communication time out: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else {
                        ERR("UST app disable object failed with ret %d: pid = %d, sock = %d, object = %p",
                            ret,
                            app->pid,
                            app->sock,
                            object);
                }
                goto error;
        }

        DBG2("UST app object %p disabled successfully for app: pid = %d", object, app->pid);

error:
        health_code_update();
        return ret;
}

/*
 * Disable the specified channel on to UST tracer for the UST session.
 */
static int disable_ust_channel(struct ust_app *app,
                               struct ust_app_session *ua_sess,
                               struct ust_app_channel *ua_chan)
{
        int ret;

        health_code_update();

        pthread_mutex_lock(&app->sock_lock);
        ret = lttng_ust_ctl_disable(app->sock, ua_chan->obj);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        ret = 0;
                        DBG3("UST app disable channel failed. Application is dead: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else if (ret == -EAGAIN) {
                        ret = 0;
                        WARN("UST app disable channel failed. Communication time out: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else {
                        ERR("UST app channel %s disable failed, session handle %d, with ret %d: pid = %d, sock = %d",
                            ua_chan->name,
                            ua_sess->handle,
                            ret,
                            app->pid,
                            app->sock);
                }
                goto error;
        }

        DBG2("UST app channel %s disabled successfully for app: pid = %d", ua_chan->name, app->pid);

error:
        health_code_update();
        return ret;
}

/*
 * Enable the specified channel on to UST tracer for the UST session.
 */
static int enable_ust_channel(struct ust_app *app,
                              struct ust_app_session *ua_sess,
                              struct ust_app_channel *ua_chan)
{
        int ret;

        health_code_update();

        pthread_mutex_lock(&app->sock_lock);
        ret = lttng_ust_ctl_enable(app->sock, ua_chan->obj);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        ret = 0;
                        DBG3("UST app channel %s enable failed. Application is dead: pid = %d, sock = %d",
                             ua_chan->name,
                             app->pid,
                             app->sock);
                } else if (ret == -EAGAIN) {
                        ret = 0;
                        WARN("UST app channel %s enable failed. Communication time out: pid = %d, sock = %d",
                             ua_chan->name,
                             app->pid,
                             app->sock);
                } else {
                        ERR("UST app channel %s enable failed, session handle %d, with ret %d: pid = %d, sock = %d",
                            ua_chan->name,
                            ua_sess->handle,
                            ret,
                            app->pid,
                            app->sock);
                }
                goto error;
        }

        ua_chan->enabled = true;

        DBG2("UST app channel %s enabled successfully for app: pid = %d", ua_chan->name, app->pid);

error:
        health_code_update();
        return ret;
}

/*
 * Enable the specified event on to UST tracer for the UST session.
 */
static int enable_ust_object(struct ust_app *app, struct lttng_ust_abi_object_data *ust_object)
{
        int ret;

        health_code_update();

        pthread_mutex_lock(&app->sock_lock);
        ret = lttng_ust_ctl_enable(app->sock, ust_object);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        ret = 0;
                        DBG3("UST app enable object failed. Application is dead: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else if (ret == -EAGAIN) {
                        ret = 0;
                        WARN("UST app enable object failed. Communication time out: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else {
                        ERR("UST app enable object failed with ret %d: pid = %d, sock = %d, object = %p",
                            ret,
                            app->pid,
                            app->sock,
                            ust_object);
                }
                goto error;
        }

        DBG2("UST app object %p enabled successfully for app: pid = %d", ust_object, app->pid);

error:
        health_code_update();
        return ret;
}

/*
 * Send channel and stream buffer to application.
 *
 * Return 0 on success. On error, a negative value is returned.
 */
static int send_channel_pid_to_ust(struct ust_app *app,
                                   struct ust_app_session *ua_sess,
                                   struct ust_app_channel *ua_chan)
{
        int ret;
        struct ust_app_stream *stream, *stmp;

        LTTNG_ASSERT(app);
        LTTNG_ASSERT(ua_sess);
        LTTNG_ASSERT(ua_chan);

        health_code_update();

        DBG("UST app sending channel %s to UST app sock %d", ua_chan->name, app->sock);

        /* Send channel to the application. */
        ret = ust_consumer_send_channel_to_ust(app, ua_sess, ua_chan);
        if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                ret = -ENOTCONN; /* Caused by app exiting. */
                goto error;
        } else if (ret == -EAGAIN) {
                /* Caused by timeout. */
                WARN("Communication with application %d timed out on send_channel for channel \"%s\" of session \"%" PRIu64
                     "\".",
                     app->pid,
                     ua_chan->name,
                     ua_sess->tracing_id);
                /* Treat this the same way as an application that is exiting. */
                ret = -ENOTCONN;
                goto error;
        } else if (ret < 0) {
                goto error;
        }

        health_code_update();

        /* Send all streams to application. */
        cds_list_for_each_entry_safe (stream, stmp, &ua_chan->streams.head, list) {
                ret = ust_consumer_send_stream_to_ust(app, ua_chan, stream);
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        ret = -ENOTCONN; /* Caused by app exiting. */
                        goto error;
                } else if (ret == -EAGAIN) {
                        /* Caused by timeout. */
                        WARN("Communication with application %d timed out on send_stream for stream \"%s\" of channel \"%s\" of session \"%" PRIu64
                             "\".",
                             app->pid,
                             stream->name,
                             ua_chan->name,
                             ua_sess->tracing_id);
                        /*
                         * Treat this the same way as an application that is
                         * exiting.
                         */
                        ret = -ENOTCONN;
                } else if (ret < 0) {
                        goto error;
                }
                /* We don't need the stream anymore once sent to the tracer. */
                cds_list_del(&stream->list);
                delete_ust_app_stream(-1, stream, app);
        }

error:
        health_code_update();
        return ret;
}

/*
 * Create the specified event onto the UST tracer for a UST session.
 *
 * Should be called with session mutex held.
 */
static int create_ust_event(struct ust_app *app,
                            struct ust_app_channel *ua_chan,
                            struct ust_app_event *ua_event)
{
        int ret = 0;

        health_code_update();

        /* Create UST event on tracer */
        pthread_mutex_lock(&app->sock_lock);
        ret = lttng_ust_ctl_create_event(app->sock, &ua_event->attr, ua_chan->obj, &ua_event->obj);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        ret = 0;
                        DBG3("UST app create event failed. Application is dead: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else if (ret == -EAGAIN) {
                        ret = 0;
                        WARN("UST app create event failed. Communication time out: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else {
                        ERR("UST app create event '%s' failed with ret %d: pid = %d, sock = %d",
                            ua_event->attr.name,
                            ret,
                            app->pid,
                            app->sock);
                }
                goto error;
        }

        ua_event->handle = ua_event->obj->handle;

        DBG2("UST app event %s created successfully for pid:%d object = %p",
             ua_event->attr.name,
             app->pid,
             ua_event->obj);

        health_code_update();

        /* Set filter if one is present. */
        if (ua_event->filter) {
                ret = set_ust_object_filter(app, ua_event->filter, ua_event->obj);
                if (ret < 0) {
                        goto error;
                }
        }

        /* Set exclusions for the event */
        if (ua_event->exclusion) {
                ret = set_ust_object_exclusions(app, ua_event->exclusion, ua_event->obj);
                if (ret < 0) {
                        goto error;
                }
        }

        /* If event not enabled, disable it on the tracer */
        if (ua_event->enabled) {
                /*
                 * We now need to explicitly enable the event, since it
                 * is now disabled at creation.
                 */
                ret = enable_ust_object(app, ua_event->obj);
                if (ret < 0) {
                        /*
                         * If we hit an EPERM, something is wrong with our enable call. If
                         * we get an EEXIST, there is a problem on the tracer side since we
                         * just created it.
                         */
                        switch (ret) {
                        case -LTTNG_UST_ERR_PERM:
                                /* Code flow problem */
                                abort();
                        case -LTTNG_UST_ERR_EXIST:
                                /* It's OK for our use case. */
                                ret = 0;
                                break;
                        default:
                                break;
                        }
                        goto error;
                }
        }

error:
        health_code_update();
        return ret;
}

static int
init_ust_event_notifier_from_event_rule(const struct lttng_event_rule *rule,
                                        struct lttng_ust_abi_event_notifier *event_notifier)
{
        enum lttng_event_rule_status status;
        enum lttng_ust_abi_loglevel_type ust_loglevel_type = LTTNG_UST_ABI_LOGLEVEL_ALL;
        int loglevel = -1, ret = 0;
        const char *pattern;

        memset(event_notifier, 0, sizeof(*event_notifier));

        if (lttng_event_rule_targets_agent_domain(rule)) {
                /*
                 * Special event for agents
                 * The actual meat of the event is in the filter that will be
                 * attached later on.
                 * Set the default values for the agent event.
                 */
                pattern = event_get_default_agent_ust_name(lttng_event_rule_get_domain_type(rule));
                loglevel = 0;
                ust_loglevel_type = LTTNG_UST_ABI_LOGLEVEL_ALL;
        } else {
                const struct lttng_log_level_rule *log_level_rule;

                LTTNG_ASSERT(lttng_event_rule_get_type(rule) ==
                             LTTNG_EVENT_RULE_TYPE_USER_TRACEPOINT);

                status = lttng_event_rule_user_tracepoint_get_name_pattern(rule, &pattern);
                if (status != LTTNG_EVENT_RULE_STATUS_OK) {
                        /* At this point, this is a fatal error. */
                        abort();
                }

                status = lttng_event_rule_user_tracepoint_get_log_level_rule(rule, &log_level_rule);
                if (status == LTTNG_EVENT_RULE_STATUS_UNSET) {
                        ust_loglevel_type = LTTNG_UST_ABI_LOGLEVEL_ALL;
                } else if (status == LTTNG_EVENT_RULE_STATUS_OK) {
                        enum lttng_log_level_rule_status llr_status;

                        switch (lttng_log_level_rule_get_type(log_level_rule)) {
                        case LTTNG_LOG_LEVEL_RULE_TYPE_EXACTLY:
                                ust_loglevel_type = LTTNG_UST_ABI_LOGLEVEL_SINGLE;
                                llr_status = lttng_log_level_rule_exactly_get_level(log_level_rule,
                                                                                    &loglevel);
                                break;
                        case LTTNG_LOG_LEVEL_RULE_TYPE_AT_LEAST_AS_SEVERE_AS:
                                ust_loglevel_type = LTTNG_UST_ABI_LOGLEVEL_RANGE;
                                llr_status = lttng_log_level_rule_at_least_as_severe_as_get_level(
                                        log_level_rule, &loglevel);
                                break;
                        default:
                                abort();
                        }

                        LTTNG_ASSERT(llr_status == LTTNG_LOG_LEVEL_RULE_STATUS_OK);
                } else {
                        /* At this point this is a fatal error. */
                        abort();
                }
        }

        event_notifier->event.instrumentation = LTTNG_UST_ABI_TRACEPOINT;
        ret = lttng_strncpy(
                event_notifier->event.name, pattern, sizeof(event_notifier->event.name));
        if (ret) {
                ERR("Failed to copy event rule pattern to notifier: pattern = '%s' ", pattern);
                goto end;
        }

        event_notifier->event.loglevel_type = ust_loglevel_type;
        event_notifier->event.loglevel = loglevel;
end:
        return ret;
}

/*
 * Create the specified event notifier against the user space tracer of a
 * given application.
 */
static int create_ust_event_notifier(struct ust_app *app,
                                     struct ust_app_event_notifier_rule *ua_event_notifier_rule)
{
        int ret = 0;
        enum lttng_condition_status condition_status;
        const struct lttng_condition *condition = nullptr;
        struct lttng_ust_abi_event_notifier event_notifier;
        const struct lttng_event_rule *event_rule = nullptr;
        unsigned int capture_bytecode_count = 0, i;
        enum lttng_condition_status cond_status;
        enum lttng_event_rule_type event_rule_type;

        health_code_update();
        LTTNG_ASSERT(app->event_notifier_group.object);

        condition = lttng_trigger_get_const_condition(ua_event_notifier_rule->trigger);
        LTTNG_ASSERT(condition);
        LTTNG_ASSERT(lttng_condition_get_type(condition) ==
                     LTTNG_CONDITION_TYPE_EVENT_RULE_MATCHES);

        condition_status = lttng_condition_event_rule_matches_get_rule(condition, &event_rule);
        LTTNG_ASSERT(condition_status == LTTNG_CONDITION_STATUS_OK);

        LTTNG_ASSERT(event_rule);

        event_rule_type = lttng_event_rule_get_type(event_rule);
        LTTNG_ASSERT(event_rule_type == LTTNG_EVENT_RULE_TYPE_USER_TRACEPOINT ||
                     event_rule_type == LTTNG_EVENT_RULE_TYPE_JUL_LOGGING ||
                     event_rule_type == LTTNG_EVENT_RULE_TYPE_LOG4J_LOGGING ||
                     event_rule_type == LTTNG_EVENT_RULE_TYPE_PYTHON_LOGGING);

        init_ust_event_notifier_from_event_rule(event_rule, &event_notifier);
        event_notifier.event.token = ua_event_notifier_rule->token;
        event_notifier.error_counter_index = ua_event_notifier_rule->error_counter_index;

        /* Create UST event notifier against the tracer. */
        pthread_mutex_lock(&app->sock_lock);
        ret = lttng_ust_ctl_create_event_notifier(app->sock,
                                                  &event_notifier,
                                                  app->event_notifier_group.object,
                                                  &ua_event_notifier_rule->obj);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        ret = 0;
                        DBG3("UST app create event notifier failed. Application is dead: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else if (ret == -EAGAIN) {
                        ret = 0;
                        WARN("UST app create event notifier failed. Communication time out: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else {
                        ERR("UST app create event notifier '%s' failed with ret %d: pid = %d, sock = %d",
                            event_notifier.event.name,
                            ret,
                            app->pid,
                            app->sock);
                }
                goto error;
        }

        ua_event_notifier_rule->handle = ua_event_notifier_rule->obj->handle;

        DBG2("UST app event notifier %s created successfully: app = '%s': pid = %d, object = %p",
             event_notifier.event.name,
             app->name,
             app->pid,
             ua_event_notifier_rule->obj);

        health_code_update();

        /* Set filter if one is present. */
        if (ua_event_notifier_rule->filter) {
                ret = set_ust_object_filter(
                        app, ua_event_notifier_rule->filter, ua_event_notifier_rule->obj);
                if (ret < 0) {
                        goto error;
                }
        }

        /* Set exclusions for the event. */
        if (ua_event_notifier_rule->exclusion) {
                ret = set_ust_object_exclusions(
                        app, ua_event_notifier_rule->exclusion, ua_event_notifier_rule->obj);
                if (ret < 0) {
                        goto error;
                }
        }

        /* Set the capture bytecodes. */
        cond_status = lttng_condition_event_rule_matches_get_capture_descriptor_count(
                condition, &capture_bytecode_count);
        LTTNG_ASSERT(cond_status == LTTNG_CONDITION_STATUS_OK);

        for (i = 0; i < capture_bytecode_count; i++) {
                const struct lttng_bytecode *capture_bytecode =
                        lttng_condition_event_rule_matches_get_capture_bytecode_at_index(condition,
                                                                                         i);

                ret = set_ust_capture(app, capture_bytecode, i, ua_event_notifier_rule->obj);
                if (ret < 0) {
                        goto error;
                }
        }

        /*
         * We now need to explicitly enable the event, since it
         * is disabled at creation.
         */
        ret = enable_ust_object(app, ua_event_notifier_rule->obj);
        if (ret < 0) {
                /*
                 * If we hit an EPERM, something is wrong with our enable call.
                 * If we get an EEXIST, there is a problem on the tracer side
                 * since we just created it.
                 */
                switch (ret) {
                case -LTTNG_UST_ERR_PERM:
                        /* Code flow problem. */
                        abort();
                case -LTTNG_UST_ERR_EXIST:
                        /* It's OK for our use case. */
                        ret = 0;
                        break;
                default:
                        break;
                }

                goto error;
        }

        ua_event_notifier_rule->enabled = true;

error:
        health_code_update();
        return ret;
}

/*
 * Copy data between an UST app event and a LTT event.
 */
static void shadow_copy_event(struct ust_app_event *ua_event, struct ltt_ust_event *uevent)
{
        size_t exclusion_alloc_size;

        strncpy(ua_event->name, uevent->attr.name, sizeof(ua_event->name));
        ua_event->name[sizeof(ua_event->name) - 1] = '\0';

        ua_event->enabled = uevent->enabled;

        /* Copy event attributes */
        memcpy(&ua_event->attr, &uevent->attr, sizeof(ua_event->attr));

        /* Copy filter bytecode */
        if (uevent->filter) {
                ua_event->filter = lttng_bytecode_copy(uevent->filter);
                /* Filter might be NULL here in case of ENONEM. */
        }

        /* Copy exclusion data */
        if (uevent->exclusion) {
                exclusion_alloc_size = sizeof(struct lttng_event_exclusion) +
                        LTTNG_UST_ABI_SYM_NAME_LEN * uevent->exclusion->count;
                ua_event->exclusion = zmalloc<lttng_event_exclusion>(exclusion_alloc_size);
                if (ua_event->exclusion == nullptr) {
                        PERROR("malloc");
                } else {
                        memcpy(ua_event->exclusion, uevent->exclusion, exclusion_alloc_size);
                }
        }
}

/*
 * Copy data between an UST app channel and a LTT channel.
 */
static void shadow_copy_channel(struct ust_app_channel *ua_chan, struct ltt_ust_channel *uchan)
{
        DBG2("UST app shadow copy of channel %s started", ua_chan->name);

        strncpy(ua_chan->name, uchan->name, sizeof(ua_chan->name));
        ua_chan->name[sizeof(ua_chan->name) - 1] = '\0';

        ua_chan->tracefile_size = uchan->tracefile_size;
        ua_chan->tracefile_count = uchan->tracefile_count;

        /* Copy event attributes since the layout is different. */
        ua_chan->attr.subbuf_size = uchan->attr.subbuf_size;
        ua_chan->attr.num_subbuf = uchan->attr.num_subbuf;
        ua_chan->attr.overwrite = uchan->attr.overwrite;
        ua_chan->attr.switch_timer_interval = uchan->attr.switch_timer_interval;
        ua_chan->attr.read_timer_interval = uchan->attr.read_timer_interval;
        ua_chan->monitor_timer_interval = uchan->monitor_timer_interval;
        ua_chan->attr.output = (lttng_ust_abi_output) uchan->attr.output;
        ua_chan->attr.blocking_timeout = uchan->attr.u.s.blocking_timeout;

        /*
         * Note that the attribute channel type is not set since the channel on the
         * tracing registry side does not have this information.
         */

        ua_chan->enabled = uchan->enabled;
        ua_chan->tracing_channel_id = uchan->id;

        DBG3("UST app shadow copy of channel %s done", ua_chan->name);
}

/*
 * Copy data between a UST app session and a regular LTT session.
 */
static void shadow_copy_session(struct ust_app_session *ua_sess,
                                struct ltt_ust_session *usess,
                                struct ust_app *app)
{
        struct tm *timeinfo;
        char datetime[16];
        int ret;
        char tmp_shm_path[PATH_MAX];

        timeinfo = localtime(&app->registration_time);
        strftime(datetime, sizeof(datetime), "%Y%m%d-%H%M%S", timeinfo);

        DBG2("Shadow copy of session handle %d", ua_sess->handle);

        ua_sess->tracing_id = usess->id;
        ua_sess->id = get_next_session_id();
        LTTNG_OPTIONAL_SET(&ua_sess->real_credentials.uid, app->uid);
        LTTNG_OPTIONAL_SET(&ua_sess->real_credentials.gid, app->gid);
        LTTNG_OPTIONAL_SET(&ua_sess->effective_credentials.uid, usess->uid);
        LTTNG_OPTIONAL_SET(&ua_sess->effective_credentials.gid, usess->gid);
        ua_sess->buffer_type = usess->buffer_type;
        ua_sess->bits_per_long = app->abi.bits_per_long;

        /* There is only one consumer object per session possible. */
        consumer_output_get(usess->consumer);
        ua_sess->consumer = usess->consumer;

        ua_sess->output_traces = usess->output_traces;
        ua_sess->live_timer_interval = usess->live_timer_interval;
        copy_channel_attr_to_ustctl(&ua_sess->metadata_attr, &usess->metadata_attr);

        switch (ua_sess->buffer_type) {
        case LTTNG_BUFFER_PER_PID:
                ret = snprintf(ua_sess->path,
                               sizeof(ua_sess->path),
                               DEFAULT_UST_TRACE_PID_PATH "/%s-%d-%s",
                               app->name,
                               app->pid,
                               datetime);
                break;
        case LTTNG_BUFFER_PER_UID:
                ret = snprintf(ua_sess->path,
                               sizeof(ua_sess->path),
                               DEFAULT_UST_TRACE_UID_PATH,
                               lttng_credentials_get_uid(&ua_sess->real_credentials),
                               app->abi.bits_per_long);
                break;
        default:
                abort();
                goto error;
        }
        if (ret < 0) {
                PERROR("asprintf UST shadow copy session");
                abort();
                goto error;
        }

        strncpy(ua_sess->root_shm_path, usess->root_shm_path, sizeof(ua_sess->root_shm_path));
        ua_sess->root_shm_path[sizeof(ua_sess->root_shm_path) - 1] = '\0';
        strncpy(ua_sess->shm_path, usess->shm_path, sizeof(ua_sess->shm_path));
        ua_sess->shm_path[sizeof(ua_sess->shm_path) - 1] = '\0';
        if (ua_sess->shm_path[0]) {
                switch (ua_sess->buffer_type) {
                case LTTNG_BUFFER_PER_PID:
                        ret = snprintf(tmp_shm_path,
                                       sizeof(tmp_shm_path),
                                       "/" DEFAULT_UST_TRACE_PID_PATH "/%s-%d-%s",
                                       app->name,
                                       app->pid,
                                       datetime);
                        break;
                case LTTNG_BUFFER_PER_UID:
                        ret = snprintf(tmp_shm_path,
                                       sizeof(tmp_shm_path),
                                       "/" DEFAULT_UST_TRACE_UID_PATH,
                                       app->uid,
                                       app->abi.bits_per_long);
                        break;
                default:
                        abort();
                        goto error;
                }
                if (ret < 0) {
                        PERROR("sprintf UST shadow copy session");
                        abort();
                        goto error;
                }
                strncat(ua_sess->shm_path,
                        tmp_shm_path,
                        sizeof(ua_sess->shm_path) - strlen(ua_sess->shm_path) - 1);
                ua_sess->shm_path[sizeof(ua_sess->shm_path) - 1] = '\0';
        }
        return;

error:
        consumer_output_put(ua_sess->consumer);
}

/*
 * Lookup sesison wrapper.
 */
static void __lookup_session_by_app(const struct ltt_ust_session *usess,
                                    struct ust_app *app,
                                    struct lttng_ht_iter *iter)
{
        /* Get right UST app session from app */
        lttng_ht_lookup(app->sessions, &usess->id, iter);
}

/*
 * Return ust app session from the app session hashtable using the UST session
 * id.
 */
static struct ust_app_session *lookup_session_by_app(const struct ltt_ust_session *usess,
                                                     struct ust_app *app)
{
        struct lttng_ht_iter iter;
        struct lttng_ht_node_u64 *node;

        __lookup_session_by_app(usess, app, &iter);
        node = lttng_ht_iter_get_node_u64(&iter);
        if (node == nullptr) {
                goto error;
        }

        return lttng::utils::container_of(node, &ust_app_session::node);

error:
        return nullptr;
}

/*
 * Setup buffer registry per PID for the given session and application. If none
 * is found, a new one is created, added to the global registry and
 * initialized. If regp is valid, it's set with the newly created object.
 *
 * Return 0 on success or else a negative value.
 */
static int setup_buffer_reg_pid(struct ust_app_session *ua_sess,
                                struct ust_app *app,
                                struct buffer_reg_pid **regp)
{
        int ret = 0;
        struct buffer_reg_pid *reg_pid;

        LTTNG_ASSERT(ua_sess);
        LTTNG_ASSERT(app);

        lttng::urcu::read_lock_guard read_lock;

        reg_pid = buffer_reg_pid_find(ua_sess->id);
        if (!reg_pid) {
                /*
                 * This is the create channel path meaning that if there is NO
                 * registry available, we have to create one for this session.
                 */
                ret = buffer_reg_pid_create(
                        ua_sess->id, &reg_pid, ua_sess->root_shm_path, ua_sess->shm_path);
                if (ret < 0) {
                        goto error;
                }
        } else {
                goto end;
        }

        /* Initialize registry. */
        reg_pid->registry->reg.ust = ust_registry_session_per_pid_create(
                app,
                app->abi,
                app->version.major,
                app->version.minor,
                reg_pid->root_shm_path,
                reg_pid->shm_path,
                lttng_credentials_get_uid(&ua_sess->effective_credentials),
                lttng_credentials_get_gid(&ua_sess->effective_credentials),
                ua_sess->tracing_id);
        if (!reg_pid->registry->reg.ust) {
                /*
                 * reg_pid->registry->reg.ust is NULL upon error, so we need to
                 * destroy the buffer registry, because it is always expected
                 * that if the buffer registry can be found, its ust registry is
                 * non-NULL.
                 */
                buffer_reg_pid_destroy(reg_pid);
                goto error;
        }

        buffer_reg_pid_add(reg_pid);

        DBG3("UST app buffer registry per PID created successfully");

end:
        if (regp) {
                *regp = reg_pid;
        }
error:
        return ret;
}

/*
 * Setup buffer registry per UID for the given session and application. If none
 * is found, a new one is created, added to the global registry and
 * initialized. If regp is valid, it's set with the newly created object.
 *
 * Return 0 on success or else a negative value.
 */
static int setup_buffer_reg_uid(struct ltt_ust_session *usess,
                                struct ust_app_session *ua_sess,
                                struct ust_app *app,
                                struct buffer_reg_uid **regp)
{
        int ret = 0;
        struct buffer_reg_uid *reg_uid;

        LTTNG_ASSERT(usess);
        LTTNG_ASSERT(app);

        lttng::urcu::read_lock_guard read_lock;

        reg_uid = buffer_reg_uid_find(usess->id, app->abi.bits_per_long, app->uid);
        if (!reg_uid) {
                /*
                 * This is the create channel path meaning that if there is NO
                 * registry available, we have to create one for this session.
                 */
                ret = buffer_reg_uid_create(usess->id,
                                            app->abi.bits_per_long,
                                            app->uid,
                                            LTTNG_DOMAIN_UST,
                                            &reg_uid,
                                            ua_sess->root_shm_path,
                                            ua_sess->shm_path);
                if (ret < 0) {
                        goto error;
                }
        } else {
                goto end;
        }

        /* Initialize registry. */
        reg_uid->registry->reg.ust = ust_registry_session_per_uid_create(app->abi,
                                                                         app->version.major,
                                                                         app->version.minor,
                                                                         reg_uid->root_shm_path,
                                                                         reg_uid->shm_path,
                                                                         usess->uid,
                                                                         usess->gid,
                                                                         ua_sess->tracing_id,
                                                                         app->uid);
        if (!reg_uid->registry->reg.ust) {
                /*
                 * reg_uid->registry->reg.ust is NULL upon error, so we need to
                 * destroy the buffer registry, because it is always expected
                 * that if the buffer registry can be found, its ust registry is
                 * non-NULL.
                 */
                buffer_reg_uid_destroy(reg_uid, nullptr);
                goto error;
        }

        /* Add node to teardown list of the session. */
        cds_list_add(&reg_uid->lnode, &usess->buffer_reg_uid_list);

        buffer_reg_uid_add(reg_uid);

        DBG3("UST app buffer registry per UID created successfully");
end:
        if (regp) {
                *regp = reg_uid;
        }
error:
        return ret;
}

/*
 * Create a session on the tracer side for the given app.
 *
 * On success, ua_sess_ptr is populated with the session pointer or else left
 * untouched. If the session was created, is_created is set to 1. On error,
 * it's left untouched. Note that ua_sess_ptr is mandatory but is_created can
 * be NULL.
 *
 * Returns 0 on success or else a negative code which is either -ENOMEM or
 * -ENOTCONN which is the default code if the lttng_ust_ctl_create_session fails.
 */
static int find_or_create_ust_app_session(struct ltt_ust_session *usess,
                                          struct ust_app *app,
                                          struct ust_app_session **ua_sess_ptr,
                                          int *is_created)
{
        int ret, created = 0;
        struct ust_app_session *ua_sess;

        LTTNG_ASSERT(usess);
        LTTNG_ASSERT(app);
        LTTNG_ASSERT(ua_sess_ptr);

        health_code_update();

        ua_sess = lookup_session_by_app(usess, app);
        if (ua_sess == nullptr) {
                DBG2("UST app pid: %d session id %" PRIu64 " not found, creating it",
                     app->pid,
                     usess->id);
                ua_sess = alloc_ust_app_session();
                if (ua_sess == nullptr) {
                        /* Only malloc can failed so something is really wrong */
                        ret = -ENOMEM;
                        goto error;
                }
                shadow_copy_session(ua_sess, usess, app);
                created = 1;
        }

        switch (usess->buffer_type) {
        case LTTNG_BUFFER_PER_PID:
                /* Init local registry. */
                ret = setup_buffer_reg_pid(ua_sess, app, nullptr);
                if (ret < 0) {
                        delete_ust_app_session(-1, ua_sess, app);
                        goto error;
                }
                break;
        case LTTNG_BUFFER_PER_UID:
                /* Look for a global registry. If none exists, create one. */
                ret = setup_buffer_reg_uid(usess, ua_sess, app, nullptr);
                if (ret < 0) {
                        delete_ust_app_session(-1, ua_sess, app);
                        goto error;
                }
                break;
        default:
                abort();
                ret = -EINVAL;
                goto error;
        }

        health_code_update();

        if (ua_sess->handle == -1) {
                pthread_mutex_lock(&app->sock_lock);
                ret = lttng_ust_ctl_create_session(app->sock);
                pthread_mutex_unlock(&app->sock_lock);
                if (ret < 0) {
                        if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                                DBG("UST app creating session failed. Application is dead: pid = %d, sock = %d",
                                    app->pid,
                                    app->sock);
                                ret = 0;
                        } else if (ret == -EAGAIN) {
                                DBG("UST app creating session failed. Communication time out: pid = %d, sock = %d",
                                    app->pid,
                                    app->sock);
                                ret = 0;
                        } else {
                                ERR("UST app creating session failed with ret %d: pid = %d, sock =%d",
                                    ret,
                                    app->pid,
                                    app->sock);
                        }
                        delete_ust_app_session(-1, ua_sess, app);
                        if (ret != -ENOMEM) {
                                /*
                                 * Tracer is probably gone or got an internal error so let's
                                 * behave like it will soon unregister or not usable.
                                 */
                                ret = -ENOTCONN;
                        }
                        goto error;
                }

                ua_sess->handle = ret;

                /* Add ust app session to app's HT */
                lttng_ht_node_init_u64(&ua_sess->node, ua_sess->tracing_id);
                lttng_ht_add_unique_u64(app->sessions, &ua_sess->node);
                lttng_ht_node_init_ulong(&ua_sess->ust_objd_node, ua_sess->handle);
                lttng_ht_add_unique_ulong(app->ust_sessions_objd, &ua_sess->ust_objd_node);

                DBG2("UST app session created successfully with handle %d", ret);
        }

        *ua_sess_ptr = ua_sess;
        if (is_created) {
                *is_created = created;
        }

        /* Everything went well. */
        ret = 0;

error:
        health_code_update();
        return ret;
}

/*
 * Match function for a hash table lookup of ust_app_ctx.
 *
 * It matches an ust app context based on the context type and, in the case
 * of perf counters, their name.
 */
static int ht_match_ust_app_ctx(struct cds_lfht_node *node, const void *_key)
{
        struct ust_app_ctx *ctx;
        const struct lttng_ust_context_attr *key;

        LTTNG_ASSERT(node);
        LTTNG_ASSERT(_key);

        ctx = caa_container_of(node, struct ust_app_ctx, node.node);
        key = (lttng_ust_context_attr *) _key;

        /* Context type */
        if (ctx->ctx.ctx != key->ctx) {
                goto no_match;
        }

        switch (key->ctx) {
        case LTTNG_UST_ABI_CONTEXT_PERF_THREAD_COUNTER:
                if (strncmp(key->u.perf_counter.name,
                            ctx->ctx.u.perf_counter.name,
                            sizeof(key->u.perf_counter.name)) != 0) {
                        goto no_match;
                }
                break;
        case LTTNG_UST_ABI_CONTEXT_APP_CONTEXT:
                if (strcmp(key->u.app_ctx.provider_name, ctx->ctx.u.app_ctx.provider_name) != 0 ||
                    strcmp(key->u.app_ctx.ctx_name, ctx->ctx.u.app_ctx.ctx_name) != 0) {
                        goto no_match;
                }
                break;
        default:
                break;
        }

        /* Match. */
        return 1;

no_match:
        return 0;
}

/*
 * Lookup for an ust app context from an lttng_ust_context.
 *
 * Must be called while holding RCU read side lock.
 * Return an ust_app_ctx object or NULL on error.
 */
static struct ust_app_ctx *find_ust_app_context(struct lttng_ht *ht,
                                                struct lttng_ust_context_attr *uctx)
{
        struct lttng_ht_iter iter;
        struct lttng_ht_node_ulong *node;
        struct ust_app_ctx *app_ctx = nullptr;

        LTTNG_ASSERT(uctx);
        LTTNG_ASSERT(ht);
        ASSERT_RCU_READ_LOCKED();

        /* Lookup using the lttng_ust_context_type and a custom match fct. */
        cds_lfht_lookup(ht->ht,
                        ht->hash_fct((void *) uctx->ctx, lttng_ht_seed),
                        ht_match_ust_app_ctx,
                        uctx,
                        &iter.iter);
        node = lttng_ht_iter_get_node_ulong(&iter);
        if (!node) {
                goto end;
        }

        app_ctx = lttng::utils::container_of(node, &ust_app_ctx::node);

end:
        return app_ctx;
}

/*
 * Create a context for the channel on the tracer.
 *
 * Called with UST app session lock held and a RCU read side lock.
 */
static int create_ust_app_channel_context(struct ust_app_channel *ua_chan,
                                          struct lttng_ust_context_attr *uctx,
                                          struct ust_app *app)
{
        int ret = 0;
        struct ust_app_ctx *ua_ctx;

        ASSERT_RCU_READ_LOCKED();

        DBG2("UST app adding context to channel %s", ua_chan->name);

        ua_ctx = find_ust_app_context(ua_chan->ctx, uctx);
        if (ua_ctx) {
                ret = -EEXIST;
                goto error;
        }

        ua_ctx = alloc_ust_app_ctx(uctx);
        if (ua_ctx == nullptr) {
                /* malloc failed */
                ret = -ENOMEM;
                goto error;
        }

        lttng_ht_node_init_ulong(&ua_ctx->node, (unsigned long) ua_ctx->ctx.ctx);
        lttng_ht_add_ulong(ua_chan->ctx, &ua_ctx->node);
        cds_list_add_tail(&ua_ctx->list, &ua_chan->ctx_list);

        ret = create_ust_channel_context(ua_chan, ua_ctx, app);
        if (ret < 0) {
                goto error;
        }

error:
        return ret;
}

/*
 * Enable on the tracer side a ust app event for the session and channel.
 *
 * Called with UST app session lock held.
 */
static int enable_ust_app_event(struct ust_app_event *ua_event, struct ust_app *app)
{
        int ret;

        ret = enable_ust_object(app, ua_event->obj);
        if (ret < 0) {
                goto error;
        }

        ua_event->enabled = true;

error:
        return ret;
}

/*
 * Disable on the tracer side a ust app event for the session and channel.
 */
static int disable_ust_app_event(struct ust_app_event *ua_event, struct ust_app *app)
{
        int ret;

        ret = disable_ust_object(app, ua_event->obj);
        if (ret < 0) {
                goto error;
        }

        ua_event->enabled = false;

error:
        return ret;
}

/*
 * Lookup ust app channel for session and disable it on the tracer side.
 */
static int disable_ust_app_channel(struct ust_app_session *ua_sess,
                                   struct ust_app_channel *ua_chan,
                                   struct ust_app *app)
{
        int ret;

        ret = disable_ust_channel(app, ua_sess, ua_chan);
        if (ret < 0) {
                goto error;
        }

        ua_chan->enabled = false;

error:
        return ret;
}

/*
 * Lookup ust app channel for session and enable it on the tracer side. This
 * MUST be called with a RCU read side lock acquired.
 */
static int enable_ust_app_channel(struct ust_app_session *ua_sess,
                                  struct ltt_ust_channel *uchan,
                                  struct ust_app *app)
{
        int ret = 0;
        struct lttng_ht_iter iter;
        struct lttng_ht_node_str *ua_chan_node;
        struct ust_app_channel *ua_chan;

        ASSERT_RCU_READ_LOCKED();

        lttng_ht_lookup(ua_sess->channels, (void *) uchan->name, &iter);
        ua_chan_node = lttng_ht_iter_get_node_str(&iter);
        if (ua_chan_node == nullptr) {
                DBG2("Unable to find channel %s in ust session id %" PRIu64,
                     uchan->name,
                     ua_sess->tracing_id);
                goto error;
        }

        ua_chan = lttng::utils::container_of(ua_chan_node, &ust_app_channel::node);

        ret = enable_ust_channel(app, ua_sess, ua_chan);
        if (ret < 0) {
                goto error;
        }

error:
        return ret;
}

/*
 * Ask the consumer to create a channel and get it if successful.
 *
 * Called with UST app session lock held.
 *
 * Return 0 on success or else a negative value.
 */
static int do_consumer_create_channel(struct ltt_ust_session *usess,
                                      struct ust_app_session *ua_sess,
                                      struct ust_app_channel *ua_chan,
                                      int bitness,
                                      lsu::registry_session *registry)
{
        int ret;
        unsigned int nb_fd = 0;
        struct consumer_socket *socket;

        LTTNG_ASSERT(usess);
        LTTNG_ASSERT(ua_sess);
        LTTNG_ASSERT(ua_chan);
        LTTNG_ASSERT(registry);

        lttng::urcu::read_lock_guard read_lock;
        health_code_update();

        /* Get the right consumer socket for the application. */
        socket = consumer_find_socket_by_bitness(bitness, usess->consumer);
        if (!socket) {
                ret = -EINVAL;
                goto error;
        }

        health_code_update();

        /* Need one fd for the channel. */
        ret = lttng_fd_get(LTTNG_FD_APPS, 1);
        if (ret < 0) {
                ERR("Exhausted number of available FD upon create channel");
                goto error;
        }

        /*
         * Ask consumer to create channel. The consumer will return the number of
         * stream we have to expect.
         */
        ret = ust_consumer_ask_channel(
                ua_sess, ua_chan, usess->consumer, socket, registry, usess->current_trace_chunk);
        if (ret < 0) {
                goto error_ask;
        }

        /*
         * Compute the number of fd needed before receiving them. It must be 2 per
         * stream (2 being the default value here).
         */
        nb_fd = DEFAULT_UST_STREAM_FD_NUM * ua_chan->expected_stream_count;

        /* Reserve the amount of file descriptor we need. */
        ret = lttng_fd_get(LTTNG_FD_APPS, nb_fd);
        if (ret < 0) {
                ERR("Exhausted number of available FD upon create channel");
                goto error_fd_get_stream;
        }

        health_code_update();

        /*
         * Now get the channel from the consumer. This call will populate the stream
         * list of that channel and set the ust objects.
         */
        if (usess->consumer->enabled) {
                ret = ust_consumer_get_channel(socket, ua_chan);
                if (ret < 0) {
                        goto error_destroy;
                }
        }

        return 0;

error_destroy:
        lttng_fd_put(LTTNG_FD_APPS, nb_fd);
error_fd_get_stream:
        /*
         * Initiate a destroy channel on the consumer since we had an error
         * handling it on our side. The return value is of no importance since we
         * already have a ret value set by the previous error that we need to
         * return.
         */
        (void) ust_consumer_destroy_channel(socket, ua_chan);
error_ask:
        lttng_fd_put(LTTNG_FD_APPS, 1);
error:
        health_code_update();
        return ret;
}

/*
 * Duplicate the ust data object of the ust app stream and save it in the
 * buffer registry stream.
 *
 * Return 0 on success or else a negative value.
 */
static int duplicate_stream_object(struct buffer_reg_stream *reg_stream,
                                   struct ust_app_stream *stream)
{
        int ret;

        LTTNG_ASSERT(reg_stream);
        LTTNG_ASSERT(stream);

        /* Duplicating a stream requires 2 new fds. Reserve them. */
        ret = lttng_fd_get(LTTNG_FD_APPS, 2);
        if (ret < 0) {
                ERR("Exhausted number of available FD upon duplicate stream");
                goto error;
        }

        /* Duplicate object for stream once the original is in the registry. */
        ret = lttng_ust_ctl_duplicate_ust_object_data(&stream->obj, reg_stream->obj.ust);
        if (ret < 0) {
                ERR("Duplicate stream obj from %p to %p failed with ret %d",
                    reg_stream->obj.ust,
                    stream->obj,
                    ret);
                lttng_fd_put(LTTNG_FD_APPS, 2);
                goto error;
        }
        stream->handle = stream->obj->handle;

error:
        return ret;
}

/*
 * Duplicate the ust data object of the ust app. channel and save it in the
 * buffer registry channel.
 *
 * Return 0 on success or else a negative value.
 */
static int duplicate_channel_object(struct buffer_reg_channel *buf_reg_chan,
                                    struct ust_app_channel *ua_chan)
{
        int ret;

        LTTNG_ASSERT(buf_reg_chan);
        LTTNG_ASSERT(ua_chan);

        /* Duplicating a channel requires 1 new fd. Reserve it. */
        ret = lttng_fd_get(LTTNG_FD_APPS, 1);
        if (ret < 0) {
                ERR("Exhausted number of available FD upon duplicate channel");
                goto error_fd_get;
        }

        /* Duplicate object for stream once the original is in the registry. */
        ret = lttng_ust_ctl_duplicate_ust_object_data(&ua_chan->obj, buf_reg_chan->obj.ust);
        if (ret < 0) {
                ERR("Duplicate channel obj from %p to %p failed with ret: %d",
                    buf_reg_chan->obj.ust,
                    ua_chan->obj,
                    ret);
                goto error;
        }
        ua_chan->handle = ua_chan->obj->handle;

        return 0;

error:
        lttng_fd_put(LTTNG_FD_APPS, 1);
error_fd_get:
        return ret;
}

/*
 * For a given channel buffer registry, setup all streams of the given ust
 * application channel.
 *
 * Return 0 on success or else a negative value.
 */
static int setup_buffer_reg_streams(struct buffer_reg_channel *buf_reg_chan,
                                    struct ust_app_channel *ua_chan,
                                    struct ust_app *app)
{
        int ret = 0;
        struct ust_app_stream *stream, *stmp;

        LTTNG_ASSERT(buf_reg_chan);
        LTTNG_ASSERT(ua_chan);

        DBG2("UST app setup buffer registry stream");

        /* Send all streams to application. */
        cds_list_for_each_entry_safe (stream, stmp, &ua_chan->streams.head, list) {
                struct buffer_reg_stream *reg_stream;

                ret = buffer_reg_stream_create(&reg_stream);
                if (ret < 0) {
                        goto error;
                }

                /*
                 * Keep original pointer and nullify it in the stream so the delete
                 * stream call does not release the object.
                 */
                reg_stream->obj.ust = stream->obj;
                stream->obj = nullptr;
                buffer_reg_stream_add(reg_stream, buf_reg_chan);

                /* We don't need the streams anymore. */
                cds_list_del(&stream->list);
                delete_ust_app_stream(-1, stream, app);
        }

error:
        return ret;
}

/*
 * Create a buffer registry channel for the given session registry and
 * application channel object. If regp pointer is valid, it's set with the
 * created object. Important, the created object is NOT added to the session
 * registry hash table.
 *
 * Return 0 on success else a negative value.
 */
static int create_buffer_reg_channel(struct buffer_reg_session *reg_sess,
                                     struct ust_app_channel *ua_chan,
                                     struct buffer_reg_channel **regp)
{
        int ret;
        struct buffer_reg_channel *buf_reg_chan = nullptr;

        LTTNG_ASSERT(reg_sess);
        LTTNG_ASSERT(ua_chan);

        DBG2("UST app creating buffer registry channel for %s", ua_chan->name);

        /* Create buffer registry channel. */
        ret = buffer_reg_channel_create(ua_chan->tracing_channel_id, &buf_reg_chan);
        if (ret < 0) {
                goto error_create;
        }
        LTTNG_ASSERT(buf_reg_chan);
        buf_reg_chan->consumer_key = ua_chan->key;
        buf_reg_chan->subbuf_size = ua_chan->attr.subbuf_size;
        buf_reg_chan->num_subbuf = ua_chan->attr.num_subbuf;

        /* Create and add a channel registry to session. */
        try {
                reg_sess->reg.ust->add_channel(ua_chan->tracing_channel_id);
        } catch (const std::exception& ex) {
                ERR("Failed to add a channel registry to userspace registry session: %s",
                    ex.what());
                ret = -1;
                goto error;
        }

        buffer_reg_channel_add(reg_sess, buf_reg_chan);

        if (regp) {
                *regp = buf_reg_chan;
        }

        return 0;

error:
        /* Safe because the registry channel object was not added to any HT. */
        buffer_reg_channel_destroy(buf_reg_chan, LTTNG_DOMAIN_UST);
error_create:
        return ret;
}

/*
 * Setup buffer registry channel for the given session registry and application
 * channel object. If regp pointer is valid, it's set with the created object.
 *
 * Return 0 on success else a negative value.
 */
static int setup_buffer_reg_channel(struct buffer_reg_session *reg_sess,
                                    struct ust_app_channel *ua_chan,
                                    struct buffer_reg_channel *buf_reg_chan,
                                    struct ust_app *app)
{
        int ret;

        LTTNG_ASSERT(reg_sess);
        LTTNG_ASSERT(buf_reg_chan);
        LTTNG_ASSERT(ua_chan);
        LTTNG_ASSERT(ua_chan->obj);

        DBG2("UST app setup buffer registry channel for %s", ua_chan->name);

        /* Setup all streams for the registry. */
        ret = setup_buffer_reg_streams(buf_reg_chan, ua_chan, app);
        if (ret < 0) {
                goto error;
        }

        buf_reg_chan->obj.ust = ua_chan->obj;
        ua_chan->obj = nullptr;

        return 0;

error:
        buffer_reg_channel_remove(reg_sess, buf_reg_chan);
        buffer_reg_channel_destroy(buf_reg_chan, LTTNG_DOMAIN_UST);
        return ret;
}

/*
 * Send buffer registry channel to the application.
 *
 * Return 0 on success else a negative value.
 */
static int send_channel_uid_to_ust(struct buffer_reg_channel *buf_reg_chan,
                                   struct ust_app *app,
                                   struct ust_app_session *ua_sess,
                                   struct ust_app_channel *ua_chan)
{
        int ret;
        struct buffer_reg_stream *reg_stream;

        LTTNG_ASSERT(buf_reg_chan);
        LTTNG_ASSERT(app);
        LTTNG_ASSERT(ua_sess);
        LTTNG_ASSERT(ua_chan);

        DBG("UST app sending buffer registry channel to ust sock %d", app->sock);

        ret = duplicate_channel_object(buf_reg_chan, ua_chan);
        if (ret < 0) {
                goto error;
        }

        /* Send channel to the application. */
        ret = ust_consumer_send_channel_to_ust(app, ua_sess, ua_chan);
        if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                ret = -ENOTCONN; /* Caused by app exiting. */
                goto error;
        } else if (ret == -EAGAIN) {
                /* Caused by timeout. */
                WARN("Communication with application %d timed out on send_channel for channel \"%s\" of session \"%" PRIu64
                     "\".",
                     app->pid,
                     ua_chan->name,
                     ua_sess->tracing_id);
                /* Treat this the same way as an application that is exiting. */
                ret = -ENOTCONN;
                goto error;
        } else if (ret < 0) {
                goto error;
        }

        health_code_update();

        /* Send all streams to application. */
        pthread_mutex_lock(&buf_reg_chan->stream_list_lock);
        cds_list_for_each_entry (reg_stream, &buf_reg_chan->streams, lnode) {
                struct ust_app_stream stream = {};

                ret = duplicate_stream_object(reg_stream, &stream);
                if (ret < 0) {
                        goto error_stream_unlock;
                }

                ret = ust_consumer_send_stream_to_ust(app, ua_chan, &stream);
                if (ret < 0) {
                        if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                                ret = -ENOTCONN; /* Caused by app exiting. */
                        } else if (ret == -EAGAIN) {
                                /*
                                 * Caused by timeout.
                                 * Treat this the same way as an application
                                 * that is exiting.
                                 */
                                WARN("Communication with application %d timed out on send_stream for stream of channel \"%s\" of session \"%" PRIu64
                                     "\".",
                                     app->pid,
                                     ua_chan->name,
                                     ua_sess->tracing_id);
                                ret = -ENOTCONN;
                        }
                        (void) release_ust_app_stream(-1, &stream, app);
                        goto error_stream_unlock;
                }

                /*
                 * The return value is not important here. This function will output an
                 * error if needed.
                 */
                (void) release_ust_app_stream(-1, &stream, app);
        }

error_stream_unlock:
        pthread_mutex_unlock(&buf_reg_chan->stream_list_lock);
error:
        return ret;
}

/*
 * Create and send to the application the created buffers with per UID buffers.
 *
 * This MUST be called with a RCU read side lock acquired.
 * The session list lock and the session's lock must be acquired.
 *
 * Return 0 on success else a negative value.
 */
static int create_channel_per_uid(struct ust_app *app,
                                  struct ltt_ust_session *usess,
                                  struct ust_app_session *ua_sess,
                                  struct ust_app_channel *ua_chan)
{
        int ret;
        struct buffer_reg_uid *reg_uid;
        struct buffer_reg_channel *buf_reg_chan;
        ltt_session::ref session;
        enum lttng_error_code notification_ret;

        LTTNG_ASSERT(app);
        LTTNG_ASSERT(usess);
        LTTNG_ASSERT(ua_sess);
        LTTNG_ASSERT(ua_chan);
        ASSERT_RCU_READ_LOCKED();

        DBG("UST app creating channel %s with per UID buffers", ua_chan->name);

        reg_uid = buffer_reg_uid_find(usess->id, app->abi.bits_per_long, app->uid);
        /*
         * The session creation handles the creation of this global registry
         * object. If none can be find, there is a code flow problem or a
         * teardown race.
         */
        LTTNG_ASSERT(reg_uid);

        buf_reg_chan = buffer_reg_channel_find(ua_chan->tracing_channel_id, reg_uid);
        if (buf_reg_chan) {
                goto send_channel;
        }

        /* Create the buffer registry channel object. */
        ret = create_buffer_reg_channel(reg_uid->registry, ua_chan, &buf_reg_chan);
        if (ret < 0) {
                ERR("Error creating the UST channel \"%s\" registry instance", ua_chan->name);
                goto error;
        }

        /* Guaranteed to exist; will not throw. */
        session = ltt_session::find_session(ua_sess->tracing_id);
        ASSERT_LOCKED(session->_lock);
        ASSERT_SESSION_LIST_LOCKED();

        /*
         * Create the buffers on the consumer side. This call populates the
         * ust app channel object with all streams and data object.
         */
        ret = do_consumer_create_channel(
                usess, ua_sess, ua_chan, app->abi.bits_per_long, reg_uid->registry->reg.ust);
        if (ret < 0) {
                ERR("Error creating UST channel \"%s\" on the consumer daemon", ua_chan->name);

                /*
                 * Let's remove the previously created buffer registry channel so
                 * it's not visible anymore in the session registry.
                 */
                auto locked_registry = reg_uid->registry->reg.ust->lock();
                try {
                        locked_registry->remove_channel(ua_chan->tracing_channel_id, false);
                } catch (const std::exception& ex) {
                        DBG("Could not find channel for removal: %s", ex.what());
                }
                buffer_reg_channel_remove(reg_uid->registry, buf_reg_chan);
                buffer_reg_channel_destroy(buf_reg_chan, LTTNG_DOMAIN_UST);
                goto error;
        }

        /*
         * Setup the streams and add it to the session registry.
         */
        ret = setup_buffer_reg_channel(reg_uid->registry, ua_chan, buf_reg_chan, app);
        if (ret < 0) {
                ERR("Error setting up UST channel \"%s\"", ua_chan->name);
                goto error;
        }

        {
                auto locked_registry = reg_uid->registry->reg.ust->lock();
                auto& ust_reg_chan = locked_registry->channel(ua_chan->tracing_channel_id);

                ust_reg_chan._consumer_key = ua_chan->key;
        }

        /* Notify the notification subsystem of the channel's creation. */
        notification_ret = notification_thread_command_add_channel(
                the_notification_thread_handle,
                session->id,
                ua_chan->name,
                ua_chan->key,
                LTTNG_DOMAIN_UST,
                ua_chan->attr.subbuf_size * ua_chan->attr.num_subbuf);
        if (notification_ret != LTTNG_OK) {
                ret = -(int) notification_ret;
                ERR("Failed to add channel to notification thread");
                goto error;
        }

send_channel:
        /* Send buffers to the application. */
        ret = send_channel_uid_to_ust(buf_reg_chan, app, ua_sess, ua_chan);
        if (ret < 0) {
                if (ret != -ENOTCONN) {
                        ERR("Error sending channel to application");
                }
                goto error;
        }

error:
        return ret;
}

/*
 * Create and send to the application the created buffers with per PID buffers.
 *
 * Called with UST app session lock held.
 * The session list lock and the session's lock must be acquired.
 *
 * Return 0 on success else a negative value.
 */
static int create_channel_per_pid(struct ust_app *app,
                                  struct ltt_ust_session *usess,
                                  struct ust_app_session *ua_sess,
                                  struct ust_app_channel *ua_chan)
{
        int ret;
        lsu::registry_session *registry;
        enum lttng_error_code cmd_ret;
        ltt_session::ref session;
        uint64_t chan_reg_key;

        LTTNG_ASSERT(app);
        LTTNG_ASSERT(usess);
        LTTNG_ASSERT(ua_sess);
        LTTNG_ASSERT(ua_chan);

        DBG("UST app creating channel %s with per PID buffers", ua_chan->name);

        lttng::urcu::read_lock_guard read_lock;

        registry = get_session_registry(ua_sess);
        /* The UST app session lock is held, registry shall not be null. */
        LTTNG_ASSERT(registry);

        /* Create and add a new channel registry to session. */
        try {
                registry->add_channel(ua_chan->key);
        } catch (const std::exception& ex) {
                ERR("Error creating the UST channel \"%s\" registry instance: %s",
                    ua_chan->name,
                    ex.what());
                ret = -1;
                goto error;
        }

        /* Guaranteed to exist; will not throw. */
        session = ltt_session::find_session(ua_sess->tracing_id);
        ASSERT_LOCKED(session->_lock);
        ASSERT_SESSION_LIST_LOCKED();

        /* Create and get channel on the consumer side. */
        ret = do_consumer_create_channel(usess, ua_sess, ua_chan, app->abi.bits_per_long, registry);
        if (ret < 0) {
                ERR("Error creating UST channel \"%s\" on the consumer daemon", ua_chan->name);
                goto error_remove_from_registry;
        }

        ret = send_channel_pid_to_ust(app, ua_sess, ua_chan);
        if (ret < 0) {
                if (ret != -ENOTCONN) {
                        ERR("Error sending channel to application");
                }
                goto error_remove_from_registry;
        }

        chan_reg_key = ua_chan->key;
        {
                auto locked_registry = registry->lock();

                auto& ust_reg_chan = locked_registry->channel(chan_reg_key);
                ust_reg_chan._consumer_key = ua_chan->key;
        }

        cmd_ret = notification_thread_command_add_channel(the_notification_thread_handle,
                                                          session->id,
                                                          ua_chan->name,
                                                          ua_chan->key,
                                                          LTTNG_DOMAIN_UST,
                                                          ua_chan->attr.subbuf_size *
                                                                  ua_chan->attr.num_subbuf);
        if (cmd_ret != LTTNG_OK) {
                ret = -(int) cmd_ret;
                ERR("Failed to add channel to notification thread");
                goto error_remove_from_registry;
        }

error_remove_from_registry:
        if (ret) {
                try {
                        auto locked_registry = registry->lock();
                        locked_registry->remove_channel(ua_chan->key, false);
                } catch (const std::exception& ex) {
                        DBG("Could not find channel for removal: %s", ex.what());
                }
        }
error:
        return ret;
}

/*
 * From an already allocated ust app channel, create the channel buffers if
 * needed and send them to the application. This MUST be called with a RCU read
 * side lock acquired.
 *
 * Called with UST app session lock held.
 *
 * Return 0 on success or else a negative value. Returns -ENOTCONN if
 * the application exited concurrently.
 */
static int ust_app_channel_send(struct ust_app *app,
                                struct ltt_ust_session *usess,
                                struct ust_app_session *ua_sess,
                                struct ust_app_channel *ua_chan)
{
        int ret;

        LTTNG_ASSERT(app);
        LTTNG_ASSERT(usess);
        LTTNG_ASSERT(usess->active);
        LTTNG_ASSERT(ua_sess);
        LTTNG_ASSERT(ua_chan);
        ASSERT_RCU_READ_LOCKED();

        /* Handle buffer type before sending the channel to the application. */
        switch (usess->buffer_type) {
        case LTTNG_BUFFER_PER_UID:
        {
                ret = create_channel_per_uid(app, usess, ua_sess, ua_chan);
                if (ret < 0) {
                        goto error;
                }
                break;
        }
        case LTTNG_BUFFER_PER_PID:
        {
                ret = create_channel_per_pid(app, usess, ua_sess, ua_chan);
                if (ret < 0) {
                        goto error;
                }
                break;
        }
        default:
                abort();
                ret = -EINVAL;
                goto error;
        }

        /* Initialize ust objd object using the received handle and add it. */
        lttng_ht_node_init_ulong(&ua_chan->ust_objd_node, ua_chan->handle);
        lttng_ht_add_unique_ulong(app->ust_objd, &ua_chan->ust_objd_node);

        /* If channel is not enabled, disable it on the tracer */
        if (!ua_chan->enabled) {
                ret = disable_ust_channel(app, ua_sess, ua_chan);
                if (ret < 0) {
                        goto error;
                }
        }

error:
        return ret;
}

/*
 * Create UST app channel and return it through ua_chanp if not NULL.
 *
 * Called with UST app session lock and RCU read-side lock held.
 *
 * Return 0 on success or else a negative value.
 */
static int ust_app_channel_allocate(struct ust_app_session *ua_sess,
                                    struct ltt_ust_channel *uchan,
                                    enum lttng_ust_abi_chan_type type,
                                    struct ltt_ust_session *usess __attribute__((unused)),
                                    struct ust_app_channel **ua_chanp)
{
        int ret = 0;
        struct lttng_ht_iter iter;
        struct lttng_ht_node_str *ua_chan_node;
        struct ust_app_channel *ua_chan;

        ASSERT_RCU_READ_LOCKED();

        /* Lookup channel in the ust app session */
        lttng_ht_lookup(ua_sess->channels, (void *) uchan->name, &iter);
        ua_chan_node = lttng_ht_iter_get_node_str(&iter);
        if (ua_chan_node != nullptr) {
                ua_chan = lttng::utils::container_of(ua_chan_node, &ust_app_channel::node);
                goto end;
        }

        ua_chan = alloc_ust_app_channel(uchan->name, ua_sess, &uchan->attr);
        if (ua_chan == nullptr) {
                /* Only malloc can fail here */
                ret = -ENOMEM;
                goto error;
        }
        shadow_copy_channel(ua_chan, uchan);

        /* Set channel type. */
        ua_chan->attr.type = type;

        /* Only add the channel if successful on the tracer side. */
        lttng_ht_add_unique_str(ua_sess->channels, &ua_chan->node);
end:
        if (ua_chanp) {
                *ua_chanp = ua_chan;
        }

        /* Everything went well. */
        return 0;

error:
        return ret;
}

/*
 * Create UST app event and create it on the tracer side.
 *
 * Must be called with the RCU read side lock held.
 * Called with ust app session mutex held.
 */
static int create_ust_app_event(struct ust_app_channel *ua_chan,
                                struct ltt_ust_event *uevent,
                                struct ust_app *app)
{
        int ret = 0;
        struct ust_app_event *ua_event;

        ASSERT_RCU_READ_LOCKED();

        ua_event = alloc_ust_app_event(uevent->attr.name, &uevent->attr);
        if (ua_event == nullptr) {
                /* Only failure mode of alloc_ust_app_event(). */
                ret = -ENOMEM;
                goto end;
        }
        shadow_copy_event(ua_event, uevent);

        /* Create it on the tracer side */
        ret = create_ust_event(app, ua_chan, ua_event);
        if (ret < 0) {
                /*
                 * Not found previously means that it does not exist on the
                 * tracer. If the application reports that the event existed,
                 * it means there is a bug in the sessiond or lttng-ust
                 * (or corruption, etc.)
                 */
                if (ret == -LTTNG_UST_ERR_EXIST) {
                        ERR("Tracer for application reported that an event being created already existed: "
                            "event_name = \"%s\", pid = %d, ppid = %d, uid = %d, gid = %d",
                            uevent->attr.name,
                            app->pid,
                            app->ppid,
                            app->uid,
                            app->gid);
                }
                goto error;
        }

        add_unique_ust_app_event(ua_chan, ua_event);

        DBG2("UST app create event completed: app = '%s' pid = %d", app->name, app->pid);

end:
        return ret;

error:
        /* Valid. Calling here is already in a read side lock */
        delete_ust_app_event(-1, ua_event, app);
        return ret;
}

/*
 * Create UST app event notifier rule and create it on the tracer side.
 *
 * Must be called with the RCU read side lock held.
 * Called with ust app session mutex held.
 */
static int create_ust_app_event_notifier_rule(struct lttng_trigger *trigger, struct ust_app *app)
{
        int ret = 0;
        struct ust_app_event_notifier_rule *ua_event_notifier_rule;

        ASSERT_RCU_READ_LOCKED();

        ua_event_notifier_rule = alloc_ust_app_event_notifier_rule(trigger);
        if (ua_event_notifier_rule == nullptr) {
                ret = -ENOMEM;
                goto end;
        }

        /* Create it on the tracer side. */
        ret = create_ust_event_notifier(app, ua_event_notifier_rule);
        if (ret < 0) {
                /*
                 * Not found previously means that it does not exist on the
                 * tracer. If the application reports that the event existed,
                 * it means there is a bug in the sessiond or lttng-ust
                 * (or corruption, etc.)
                 */
                if (ret == -LTTNG_UST_ERR_EXIST) {
                        ERR("Tracer for application reported that an event notifier being created already exists: "
                            "token = \"%" PRIu64 "\", pid = %d, ppid = %d, uid = %d, gid = %d",
                            lttng_trigger_get_tracer_token(trigger),
                            app->pid,
                            app->ppid,
                            app->uid,
                            app->gid);
                }
                goto error;
        }

        lttng_ht_add_unique_u64(app->token_to_event_notifier_rule_ht,
                                &ua_event_notifier_rule->node);

        DBG2("UST app create token event rule completed: app = '%s', pid = %d, token = %" PRIu64,
             app->name,
             app->pid,
             lttng_trigger_get_tracer_token(trigger));

        goto end;

error:
        /* The RCU read side lock is already being held by the caller. */
        delete_ust_app_event_notifier_rule(-1, ua_event_notifier_rule, app);
end:
        return ret;
}

/*
 * Create UST metadata and open it on the tracer side.
 *
 * Called with UST app session lock held and RCU read side lock.
 */
static int create_ust_app_metadata(struct ust_app_session *ua_sess,
                                   struct ust_app *app,
                                   struct consumer_output *consumer)
{
        int ret = 0;
        struct ust_app_channel *metadata;
        struct consumer_socket *socket;
        ltt_session::ref session;

        LTTNG_ASSERT(ua_sess);
        LTTNG_ASSERT(app);
        LTTNG_ASSERT(consumer);
        ASSERT_RCU_READ_LOCKED();

        auto locked_registry = get_locked_session_registry(ua_sess);
        /* The UST app session is held registry shall not be null. */
        LTTNG_ASSERT(locked_registry);

        /* Metadata already exists for this registry or it was closed previously */
        if (locked_registry->_metadata_key || locked_registry->_metadata_closed) {
                ret = 0;
                goto error;
        }

        /* Allocate UST metadata */
        metadata = alloc_ust_app_channel(DEFAULT_METADATA_NAME, ua_sess, nullptr);
        if (!metadata) {
                /* malloc() failed */
                ret = -ENOMEM;
                goto error;
        }

        memcpy(&metadata->attr, &ua_sess->metadata_attr, sizeof(metadata->attr));

        /* Need one fd for the channel. */
        ret = lttng_fd_get(LTTNG_FD_APPS, 1);
        if (ret < 0) {
                ERR("Exhausted number of available FD upon create metadata");
                goto error;
        }

        /* Get the right consumer socket for the application. */
        socket = consumer_find_socket_by_bitness(app->abi.bits_per_long, consumer);
        if (!socket) {
                ret = -EINVAL;
                goto error_consumer;
        }

        /*
         * Keep metadata key so we can identify it on the consumer side. Assign it
         * to the registry *before* we ask the consumer so we avoid the race of the
         * consumer requesting the metadata and the ask_channel call on our side
         * did not returned yet.
         */
        locked_registry->_metadata_key = metadata->key;

        /* Guaranteed to exist; will not throw. */
        session = ltt_session::find_session(ua_sess->tracing_id);
        ASSERT_LOCKED(session->_lock);
        ASSERT_SESSION_LIST_LOCKED();

        /*
         * Ask the metadata channel creation to the consumer. The metadata object
         * will be created by the consumer and kept their. However, the stream is
         * never added or monitored until we do a first push metadata to the
         * consumer.
         */
        ret = ust_consumer_ask_channel(ua_sess,
                                       metadata,
                                       consumer,
                                       socket,
                                       locked_registry.get(),
                                       session->current_trace_chunk);
        if (ret < 0) {
                /* Nullify the metadata key so we don't try to close it later on. */
                locked_registry->_metadata_key = 0;
                goto error_consumer;
        }

        /*
         * The setup command will make the metadata stream be sent to the relayd,
         * if applicable, and the thread managing the metadatas. This is important
         * because after this point, if an error occurs, the only way the stream
         * can be deleted is to be monitored in the consumer.
         */
        ret = consumer_setup_metadata(socket, metadata->key);
        if (ret < 0) {
                /* Nullify the metadata key so we don't try to close it later on. */
                locked_registry->_metadata_key = 0;
                goto error_consumer;
        }

        DBG2("UST metadata with key %" PRIu64 " created for app pid %d", metadata->key, app->pid);

error_consumer:
        lttng_fd_put(LTTNG_FD_APPS, 1);
        delete_ust_app_channel(-1, metadata, app, locked_registry);
error:
        return ret;
}

/*
 * Return ust app pointer or NULL if not found. RCU read side lock MUST be
 * acquired before calling this function.
 */
struct ust_app *ust_app_find_by_pid(pid_t pid)
{
        struct ust_app *app = nullptr;
        struct lttng_ht_node_ulong *node;
        struct lttng_ht_iter iter;

        lttng_ht_lookup(ust_app_ht, (void *) ((unsigned long) pid), &iter);
        node = lttng_ht_iter_get_node_ulong(&iter);
        if (node == nullptr) {
                DBG2("UST app no found with pid %d", pid);
                goto error;
        }

        DBG2("Found UST app by pid %d", pid);

        app = lttng::utils::container_of(node, &ust_app::pid_n);

error:
        return app;
}

/*
 * Allocate and init an UST app object using the registration information and
 * the command socket. This is called when the command socket connects to the
 * session daemon.
 *
 * The object is returned on success or else NULL.
 */
struct ust_app *ust_app_create(struct ust_register_msg *msg, int sock)
{
        int ret;
        struct ust_app *lta = nullptr;
        struct lttng_pipe *event_notifier_event_source_pipe = nullptr;

        LTTNG_ASSERT(msg);
        LTTNG_ASSERT(sock >= 0);

        DBG3("UST app creating application for socket %d", sock);

        if ((msg->bits_per_long == 64 && (uatomic_read(&the_ust_consumerd64_fd) == -EINVAL)) ||
            (msg->bits_per_long == 32 && (uatomic_read(&the_ust_consumerd32_fd) == -EINVAL))) {
                ERR("Registration failed: application \"%s\" (pid: %d) has "
                    "%d-bit long, but no consumerd for this size is available.\n",
                    msg->name,
                    msg->pid,
                    msg->bits_per_long);
                goto error;
        }

        /*
         * Reserve the two file descriptors of the event source pipe. The write
         * end will be closed once it is passed to the application, at which
         * point a single 'put' will be performed.
         */
        ret = lttng_fd_get(LTTNG_FD_APPS, 2);
        if (ret) {
                ERR("Failed to reserve two file descriptors for the event source pipe while creating a new application instance: app = '%s', pid = %d",
                    msg->name,
                    (int) msg->pid);
                goto error;
        }

        event_notifier_event_source_pipe = lttng_pipe_open(FD_CLOEXEC);
        if (!event_notifier_event_source_pipe) {
                PERROR("Failed to open application event source pipe: '%s' (pid = %d)",
                       msg->name,
                       msg->pid);
                goto error;
        }

        lta = zmalloc<ust_app>();
        if (lta == nullptr) {
                PERROR("malloc");
                goto error_free_pipe;
        }

        urcu_ref_init(&lta->ref);

        lta->event_notifier_group.event_pipe = event_notifier_event_source_pipe;

        lta->ppid = msg->ppid;
        lta->uid = msg->uid;
        lta->gid = msg->gid;

        lta->abi = {
                .bits_per_long = msg->bits_per_long,
                .long_alignment = msg->long_alignment,
                .uint8_t_alignment = msg->uint8_t_alignment,
                .uint16_t_alignment = msg->uint16_t_alignment,
                .uint32_t_alignment = msg->uint32_t_alignment,
                .uint64_t_alignment = msg->uint64_t_alignment,
                .byte_order = msg->byte_order == LITTLE_ENDIAN ?
                        lttng::sessiond::trace::byte_order::LITTLE_ENDIAN_ :
                        lttng::sessiond::trace::byte_order::BIG_ENDIAN_,
        };

        lta->v_major = msg->major;
        lta->v_minor = msg->minor;
        lta->sessions = lttng_ht_new(0, LTTNG_HT_TYPE_U64);
        lta->ust_objd = lttng_ht_new(0, LTTNG_HT_TYPE_ULONG);
        lta->ust_sessions_objd = lttng_ht_new(0, LTTNG_HT_TYPE_ULONG);
        lta->notify_sock = -1;
        lta->token_to_event_notifier_rule_ht = lttng_ht_new(0, LTTNG_HT_TYPE_U64);

        /* Copy name and make sure it's NULL terminated. */
        strncpy(lta->name, msg->name, sizeof(lta->name));
        lta->name[UST_APP_PROCNAME_LEN] = '\0';

        /*
         * Before this can be called, when receiving the registration information,
         * the application compatibility is checked. So, at this point, the
         * application can work with this session daemon.
         */
        lta->compatible = 1;

        lta->pid = msg->pid;
        lttng_ht_node_init_ulong(&lta->pid_n, (unsigned long) lta->pid);
        lta->sock = sock;
        pthread_mutex_init(&lta->sock_lock, nullptr);
        lttng_ht_node_init_ulong(&lta->sock_n, (unsigned long) lta->sock);

        CDS_INIT_LIST_HEAD(&lta->teardown_head);
        return lta;

error_free_pipe:
        lttng_pipe_destroy(event_notifier_event_source_pipe);
        lttng_fd_put(LTTNG_FD_APPS, 2);
error:
        return nullptr;
}

/*
 * For a given application object, add it to every hash table.
 */
void ust_app_add(struct ust_app *app)
{
        LTTNG_ASSERT(app);
        LTTNG_ASSERT(app->notify_sock >= 0);

        app->registration_time = time(nullptr);

        lttng::urcu::read_lock_guard read_lock;

        /*
         * On a re-registration, we want to kick out the previous registration of
         * that pid
         */
        lttng_ht_add_replace_ulong(ust_app_ht, &app->pid_n);

        /*
         * The socket _should_ be unique until _we_ call close. So, a add_unique
         * for the ust_app_ht_by_sock is used which asserts fail if the entry was
         * already in the table.
         */
        lttng_ht_add_unique_ulong(ust_app_ht_by_sock, &app->sock_n);

        /* Add application to the notify socket hash table. */
        lttng_ht_node_init_ulong(&app->notify_sock_n, app->notify_sock);
        lttng_ht_add_unique_ulong(ust_app_ht_by_notify_sock, &app->notify_sock_n);

        DBG("App registered with pid:%d ppid:%d uid:%d gid:%d sock =%d name:%s "
            "notify_sock =%d (version %d.%d)",
            app->pid,
            app->ppid,
            app->uid,
            app->gid,
            app->sock,
            app->name,
            app->notify_sock,
            app->v_major,
            app->v_minor);
}

/*
 * Set the application version into the object.
 *
 * Return 0 on success else a negative value either an errno code or a
 * LTTng-UST error code.
 */
int ust_app_version(struct ust_app *app)
{
        int ret;

        LTTNG_ASSERT(app);

        pthread_mutex_lock(&app->sock_lock);
        ret = lttng_ust_ctl_tracer_version(app->sock, &app->version);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret == -LTTNG_UST_ERR_EXITING || ret == -EPIPE) {
                        DBG3("UST app version failed. Application is dead: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else if (ret == -EAGAIN) {
                        WARN("UST app version failed. Communication time out: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else {
                        ERR("UST app version failed with ret %d: pid = %d, sock = %d",
                            ret,
                            app->pid,
                            app->sock);
                }
        }

        return ret;
}

bool ust_app_supports_notifiers(const struct ust_app *app)
{
        return app->v_major >= 9;
}

bool ust_app_supports_counters(const struct ust_app *app)
{
        return app->v_major >= 9;
}

/*
 * Setup the base event notifier group.
 *
 * Return 0 on success else a negative value either an errno code or a
 * LTTng-UST error code.
 */
int ust_app_setup_event_notifier_group(struct ust_app *app)
{
        int ret;
        int event_pipe_write_fd;
        struct lttng_ust_abi_object_data *event_notifier_group = nullptr;
        enum lttng_error_code lttng_ret;
        enum event_notifier_error_accounting_status event_notifier_error_accounting_status;

        LTTNG_ASSERT(app);

        if (!ust_app_supports_notifiers(app)) {
                ret = -ENOSYS;
                goto error;
        }

        /* Get the write side of the pipe. */
        event_pipe_write_fd = lttng_pipe_get_writefd(app->event_notifier_group.event_pipe);

        pthread_mutex_lock(&app->sock_lock);
        ret = lttng_ust_ctl_create_event_notifier_group(
                app->sock, event_pipe_write_fd, &event_notifier_group);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        ret = 0;
                        DBG3("UST app create event notifier group failed. Application is dead: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else if (ret == -EAGAIN) {
                        ret = 0;
                        WARN("UST app create event notifier group failed. Communication time out: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else {
                        ERR("UST app create event notifier group failed with ret %d: pid = %d, sock = %d, event_pipe_write_fd: %d",
                            ret,
                            app->pid,
                            app->sock,
                            event_pipe_write_fd);
                }
                goto error;
        }

        ret = lttng_pipe_write_close(app->event_notifier_group.event_pipe);
        if (ret) {
                ERR("Failed to close write end of the application's event source pipe: app = '%s' (pid = %d)",
                    app->name,
                    app->pid);
                goto error;
        }

        /*
         * Release the file descriptor that was reserved for the write-end of
         * the pipe.
         */
        lttng_fd_put(LTTNG_FD_APPS, 1);

        lttng_ret = notification_thread_command_add_tracer_event_source(
                the_notification_thread_handle,
                lttng_pipe_get_readfd(app->event_notifier_group.event_pipe),
                LTTNG_DOMAIN_UST);
        if (lttng_ret != LTTNG_OK) {
                ERR("Failed to add tracer event source to notification thread");
                ret = -1;
                goto error;
        }

        /* Assign handle only when the complete setup is valid. */
        app->event_notifier_group.object = event_notifier_group;

        event_notifier_error_accounting_status = event_notifier_error_accounting_register_app(app);
        switch (event_notifier_error_accounting_status) {
        case EVENT_NOTIFIER_ERROR_ACCOUNTING_STATUS_OK:
                break;
        case EVENT_NOTIFIER_ERROR_ACCOUNTING_STATUS_UNSUPPORTED:
                DBG3("Failed to setup event notifier error accounting (application does not support notifier error accounting): app socket fd = %d, app name = '%s', app pid = %d",
                     app->sock,
                     app->name,
                     (int) app->pid);
                ret = 0;
                goto error_accounting;
        case EVENT_NOTIFIER_ERROR_ACCOUNTING_STATUS_APP_DEAD:
                DBG3("Failed to setup event notifier error accounting (application is dead): app socket fd = %d, app name = '%s', app pid = %d",
                     app->sock,
                     app->name,
                     (int) app->pid);
                ret = 0;
                goto error_accounting;
        default:
                ERR("Failed to setup event notifier error accounting for app");
                ret = -1;
                goto error_accounting;
        }

        return ret;

error_accounting:
        lttng_ret = notification_thread_command_remove_tracer_event_source(
                the_notification_thread_handle,
                lttng_pipe_get_readfd(app->event_notifier_group.event_pipe));
        if (lttng_ret != LTTNG_OK) {
                ERR("Failed to remove application tracer event source from notification thread");
        }

error:
        lttng_ust_ctl_release_object(app->sock, app->event_notifier_group.object);
        free(app->event_notifier_group.object);
        app->event_notifier_group.object = nullptr;
        return ret;
}

static void ust_app_unregister(ust_app& app)
{
        struct lttng_ht_iter iter;
        struct ust_app_session *ua_sess;

        lttng::urcu::read_lock_guard read_lock;

        /*
         * For per-PID buffers, perform "push metadata" and flush all
         * application streams before removing app from hash tables,
         * ensuring proper behavior of data_pending check.
         * Remove sessions so they are not visible during deletion.
         */
        cds_lfht_for_each_entry (app.sessions->ht, &iter.iter, ua_sess, node.node) {
                const auto del_ret = lttng_ht_del(app.sessions, &iter);
                if (del_ret) {
                        /* The session was already removed so scheduled for teardown. */
                        continue;
                }

                if (ua_sess->buffer_type == LTTNG_BUFFER_PER_PID) {
                        (void) ust_app_flush_app_session(app, *ua_sess);
                }

                /*
                 * Add session to list for teardown. This is safe since at this point we
                 * are the only one using this list.
                 */
                lttng::pthread::lock_guard ust_app_session_lock(ua_sess->lock);

                if (ua_sess->deleted) {
                        continue;
                }

                /*
                 * Normally, this is done in the delete session process which is
                 * executed in the call rcu below. However, upon registration we can't
                 * afford to wait for the grace period before pushing data or else the
                 * data pending feature can race between the unregistration and stop
                 * command where the data pending command is sent *before* the grace
                 * period ended.
                 *
                 * The close metadata below nullifies the metadata pointer in the
                 * session so the delete session will NOT push/close a second time.
                 */
                auto locked_registry = get_locked_session_registry(ua_sess);
                if (locked_registry) {
                        /* Push metadata for application before freeing the application. */
                        (void) push_metadata(locked_registry, ua_sess->consumer);

                        /*
                         * Don't ask to close metadata for global per UID buffers. Close
                         * metadata only on destroy trace session in this case. Also, the
                         * previous push metadata could have flag the metadata registry to
                         * close so don't send a close command if closed.
                         */
                        if (ua_sess->buffer_type != LTTNG_BUFFER_PER_UID) {
                                const auto metadata_key = locked_registry->_metadata_key;
                                const auto consumer_bitness = locked_registry->abi.bits_per_long;

                                if (!locked_registry->_metadata_closed && metadata_key != 0) {
                                        locked_registry->_metadata_closed = true;
                                }

                                /* Release lock before communication, see comments in
                                 * close_metadata(). */
                                locked_registry.reset();
                                (void) close_metadata(
                                        metadata_key, consumer_bitness, ua_sess->consumer);
                        } else {
                                locked_registry.reset();
                        }
                }

                cds_list_add(&ua_sess->teardown_node, &app.teardown_head);
        }

        /*
         * Remove application from notify hash table. The thread handling the
         * notify socket could have deleted the node so ignore on error because
         * either way it's valid. The close of that socket is handled by the
         * apps_notify_thread.
         */
        iter.iter.node = &app.notify_sock_n.node;
        (void) lttng_ht_del(ust_app_ht_by_notify_sock, &iter);

        /*
         * Ignore return value since the node might have been removed before by an
         * add replace during app registration because the PID can be reassigned by
         * the OS.
         */
        iter.iter.node = &app.pid_n.node;
        if (lttng_ht_del(ust_app_ht, &iter)) {
                DBG3("Unregister app by PID %d failed. This can happen on pid reuse", app.pid);
        }
}

/*
 * Unregister app by removing it from the global traceable app list and freeing
 * the data struct.
 *
 * The socket is already closed at this point, so there is no need to close it.
 */
void ust_app_unregister_by_socket(int sock_fd)
{
        struct ust_app *app;
        struct lttng_ht_node_ulong *node;
        struct lttng_ht_iter ust_app_sock_iter;
        int ret;

        lttng::urcu::read_lock_guard read_lock;

        /* Get the node reference for a call_rcu */
        lttng_ht_lookup(ust_app_ht_by_sock, (void *) ((unsigned long) sock_fd), &ust_app_sock_iter);
        node = lttng_ht_iter_get_node_ulong(&ust_app_sock_iter);
        assert(node);

        app = caa_container_of(node, struct ust_app, sock_n);

        DBG_FMT("Application unregistering after socket activity: pid={}, socket_fd={}",
                app->pid,
                sock_fd);

        /* Remove application from socket hash table */
        ret = lttng_ht_del(ust_app_ht_by_sock, &ust_app_sock_iter);
        assert(!ret);

        /*
         * The socket is closed: release its reference to the application
         * to trigger its eventual teardown.
         */
        ust_app_put(app);
}

/*
 * Fill events array with all events name of all registered apps.
 */
int ust_app_list_events(struct lttng_event **events)
{
        int ret, handle;
        size_t nbmem, count = 0;
        struct lttng_ht_iter iter;
        struct ust_app *app;
        struct lttng_event *tmp_event;

        nbmem = UST_APP_EVENT_LIST_SIZE;
        tmp_event = calloc<lttng_event>(nbmem);
        if (tmp_event == nullptr) {
                PERROR("zmalloc ust app events");
                ret = -ENOMEM;
                goto error;
        }

        {
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        struct lttng_ust_abi_tracepoint_iter uiter;

                        health_code_update();

                        if (!app->compatible) {
                                /*
                                 * TODO: In time, we should notice the caller of this error by
                                 * telling him that this is a version error.
                                 */
                                continue;
                        }

                        pthread_mutex_lock(&app->sock_lock);
                        handle = lttng_ust_ctl_tracepoint_list(app->sock);
                        if (handle < 0) {
                                if (handle != -EPIPE && handle != -LTTNG_UST_ERR_EXITING) {
                                        ERR("UST app list events getting handle failed for app pid %d",
                                            app->pid);
                                }
                                pthread_mutex_unlock(&app->sock_lock);
                                continue;
                        }

                        while ((ret = lttng_ust_ctl_tracepoint_list_get(
                                        app->sock, handle, &uiter)) != -LTTNG_UST_ERR_NOENT) {
                                /* Handle ustctl error. */
                                if (ret < 0) {
                                        int release_ret;

                                        if (ret != -LTTNG_UST_ERR_EXITING && ret != -EPIPE) {
                                                ERR("UST app tp list get failed for app %d with ret %d",
                                                    app->sock,
                                                    ret);
                                        } else {
                                                DBG3("UST app tp list get failed. Application is dead");
                                                break;
                                        }

                                        free(tmp_event);
                                        release_ret =
                                                lttng_ust_ctl_release_handle(app->sock, handle);
                                        if (release_ret < 0 &&
                                            release_ret != -LTTNG_UST_ERR_EXITING &&
                                            release_ret != -EPIPE) {
                                                ERR("Error releasing app handle for app %d with ret %d",
                                                    app->sock,
                                                    release_ret);
                                        }

                                        pthread_mutex_unlock(&app->sock_lock);
                                        goto rcu_error;
                                }

                                health_code_update();
                                if (count >= nbmem) {
                                        /* In case the realloc fails, we free the memory */
                                        struct lttng_event *new_tmp_event;
                                        size_t new_nbmem;

                                        new_nbmem = nbmem << 1;
                                        DBG2("Reallocating event list from %zu to %zu entries",
                                             nbmem,
                                             new_nbmem);
                                        new_tmp_event = (lttng_event *) realloc(
                                                tmp_event, new_nbmem * sizeof(struct lttng_event));
                                        if (new_tmp_event == nullptr) {
                                                int release_ret;

                                                PERROR("realloc ust app events");
                                                free(tmp_event);
                                                ret = -ENOMEM;
                                                release_ret = lttng_ust_ctl_release_handle(
                                                        app->sock, handle);
                                                if (release_ret < 0 &&
                                                    release_ret != -LTTNG_UST_ERR_EXITING &&
                                                    release_ret != -EPIPE) {
                                                        ERR("Error releasing app handle for app %d with ret %d",
                                                            app->sock,
                                                            release_ret);
                                                }

                                                pthread_mutex_unlock(&app->sock_lock);
                                                goto rcu_error;
                                        }
                                        /* Zero the new memory */
                                        memset(new_tmp_event + nbmem,
                                               0,
                                               (new_nbmem - nbmem) * sizeof(struct lttng_event));
                                        nbmem = new_nbmem;
                                        tmp_event = new_tmp_event;
                                }

                                memcpy(tmp_event[count].name,
                                       uiter.name,
                                       LTTNG_UST_ABI_SYM_NAME_LEN);
                                tmp_event[count].loglevel = uiter.loglevel;
                                tmp_event[count].type =
                                        (enum lttng_event_type) LTTNG_UST_ABI_TRACEPOINT;
                                tmp_event[count].pid = app->pid;
                                tmp_event[count].enabled = -1;
                                count++;
                        }

                        ret = lttng_ust_ctl_release_handle(app->sock, handle);
                        pthread_mutex_unlock(&app->sock_lock);
                        if (ret < 0) {
                                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                                        DBG3("Error releasing app handle. Application died: pid = %d, sock = %d",
                                             app->pid,
                                             app->sock);
                                } else if (ret == -EAGAIN) {
                                        WARN("Error releasing app handle. Communication time out: pid = %d, sock = %d",
                                             app->pid,
                                             app->sock);
                                } else {
                                        ERR("Error releasing app handle with ret %d: pid = %d, sock = %d",
                                            ret,
                                            app->pid,
                                            app->sock);
                                }
                        }
                }
        }

        ret = count;
        *events = tmp_event;

        DBG2("UST app list events done (%zu events)", count);

rcu_error:
error:
        health_code_update();
        return ret;
}

/*
 * Fill events array with all events name of all registered apps.
 */
int ust_app_list_event_fields(struct lttng_event_field **fields)
{
        int ret, handle;
        size_t nbmem, count = 0;
        struct lttng_ht_iter iter;
        struct ust_app *app;
        struct lttng_event_field *tmp_event;

        nbmem = UST_APP_EVENT_LIST_SIZE;
        tmp_event = calloc<lttng_event_field>(nbmem);
        if (tmp_event == nullptr) {
                PERROR("zmalloc ust app event fields");
                ret = -ENOMEM;
                goto error;
        }

        {
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        struct lttng_ust_abi_field_iter uiter;

                        health_code_update();

                        if (!app->compatible) {
                                /*
                                 * TODO: In time, we should notice the caller of this error by
                                 * telling him that this is a version error.
                                 */
                                continue;
                        }

                        pthread_mutex_lock(&app->sock_lock);
                        handle = lttng_ust_ctl_tracepoint_field_list(app->sock);
                        if (handle < 0) {
                                if (handle != -EPIPE && handle != -LTTNG_UST_ERR_EXITING) {
                                        ERR("UST app list field getting handle failed for app pid %d",
                                            app->pid);
                                }
                                pthread_mutex_unlock(&app->sock_lock);
                                continue;
                        }

                        while ((ret = lttng_ust_ctl_tracepoint_field_list_get(
                                        app->sock, handle, &uiter)) != -LTTNG_UST_ERR_NOENT) {
                                /* Handle ustctl error. */
                                if (ret < 0) {
                                        int release_ret;

                                        if (ret != -LTTNG_UST_ERR_EXITING && ret != -EPIPE) {
                                                ERR("UST app tp list field failed for app %d with ret %d",
                                                    app->sock,
                                                    ret);
                                        } else {
                                                DBG3("UST app tp list field failed. Application is dead");
                                                break;
                                        }

                                        free(tmp_event);
                                        release_ret =
                                                lttng_ust_ctl_release_handle(app->sock, handle);
                                        pthread_mutex_unlock(&app->sock_lock);
                                        if (release_ret < 0 &&
                                            release_ret != -LTTNG_UST_ERR_EXITING &&
                                            release_ret != -EPIPE) {
                                                ERR("Error releasing app handle for app %d with ret %d",
                                                    app->sock,
                                                    release_ret);
                                        }

                                        goto rcu_error;
                                }

                                health_code_update();
                                if (count >= nbmem) {
                                        /* In case the realloc fails, we free the memory */
                                        struct lttng_event_field *new_tmp_event;
                                        size_t new_nbmem;

                                        new_nbmem = nbmem << 1;
                                        DBG2("Reallocating event field list from %zu to %zu entries",
                                             nbmem,
                                             new_nbmem);
                                        new_tmp_event = (lttng_event_field *) realloc(
                                                tmp_event,
                                                new_nbmem * sizeof(struct lttng_event_field));
                                        if (new_tmp_event == nullptr) {
                                                int release_ret;

                                                PERROR("realloc ust app event fields");
                                                free(tmp_event);
                                                ret = -ENOMEM;
                                                release_ret = lttng_ust_ctl_release_handle(
                                                        app->sock, handle);
                                                pthread_mutex_unlock(&app->sock_lock);
                                                if (release_ret &&
                                                    release_ret != -LTTNG_UST_ERR_EXITING &&
                                                    release_ret != -EPIPE) {
                                                        ERR("Error releasing app handle for app %d with ret %d",
                                                            app->sock,
                                                            release_ret);
                                                }

                                                goto rcu_error;
                                        }

                                        /* Zero the new memory */
                                        memset(new_tmp_event + nbmem,
                                               0,
                                               (new_nbmem - nbmem) *
                                                       sizeof(struct lttng_event_field));
                                        nbmem = new_nbmem;
                                        tmp_event = new_tmp_event;
                                }

                                memcpy(tmp_event[count].field_name,
                                       uiter.field_name,
                                       LTTNG_UST_ABI_SYM_NAME_LEN);
                                /* Mapping between these enums matches 1 to 1. */
                                tmp_event[count].type = (enum lttng_event_field_type) uiter.type;
                                tmp_event[count].nowrite = uiter.nowrite;

                                memcpy(tmp_event[count].event.name,
                                       uiter.event_name,
                                       LTTNG_UST_ABI_SYM_NAME_LEN);
                                tmp_event[count].event.loglevel = uiter.loglevel;
                                tmp_event[count].event.type = LTTNG_EVENT_TRACEPOINT;
                                tmp_event[count].event.pid = app->pid;
                                tmp_event[count].event.enabled = -1;
                                count++;
                        }

                        ret = lttng_ust_ctl_release_handle(app->sock, handle);
                        pthread_mutex_unlock(&app->sock_lock);
                        if (ret < 0 && ret != -LTTNG_UST_ERR_EXITING && ret != -EPIPE) {
                                ERR("Error releasing app handle for app %d with ret %d",
                                    app->sock,
                                    ret);
                        }
                }
        }

        ret = count;
        *fields = tmp_event;

        DBG2("UST app list event fields done (%zu events)", count);

rcu_error:
error:
        health_code_update();
        return ret;
}

/*
 * Free and clean all traceable apps of the global list.
 */
void ust_app_clean_list()
{
        int ret;
        struct ust_app *app;
        struct lttng_ht_iter iter;

        DBG2("UST app cleaning registered apps hash table");

        /* Cleanup notify socket hash table */
        if (ust_app_ht_by_notify_sock) {
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (
                        ust_app_ht_by_notify_sock->ht, &iter.iter, app, notify_sock_n.node) {
                        /*
                         * Assert that all notifiers are gone as all triggers
                         * are unregistered prior to this clean-up.
                         */
                        LTTNG_ASSERT(lttng_ht_get_count(app->token_to_event_notifier_rule_ht) == 0);
                        ust_app_notify_sock_unregister(app->notify_sock);
                }
        }

        /* Cleanup socket hash table */
        if (ust_app_ht_by_sock) {
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (ust_app_ht_by_sock->ht, &iter.iter, app, sock_n.node) {
                        ret = lttng_ht_del(ust_app_ht_by_sock, &iter);
                        LTTNG_ASSERT(!ret);
                        ust_app_put(app);
                }
        }

        /* Destroy is done only when the ht is empty */
        if (ust_app_ht) {
                lttng_ht_destroy(ust_app_ht);
        }
        if (ust_app_ht_by_sock) {
                lttng_ht_destroy(ust_app_ht_by_sock);
        }
        if (ust_app_ht_by_notify_sock) {
                lttng_ht_destroy(ust_app_ht_by_notify_sock);
        }
}

/*
 * Init UST app hash table.
 */
int ust_app_ht_alloc()
{
        ust_app_ht = lttng_ht_new(0, LTTNG_HT_TYPE_ULONG);
        if (!ust_app_ht) {
                return -1;
        }
        ust_app_ht_by_sock = lttng_ht_new(0, LTTNG_HT_TYPE_ULONG);
        if (!ust_app_ht_by_sock) {
                return -1;
        }
        ust_app_ht_by_notify_sock = lttng_ht_new(0, LTTNG_HT_TYPE_ULONG);
        if (!ust_app_ht_by_notify_sock) {
                return -1;
        }
        return 0;
}

/*
 * For a specific UST session, disable the channel for all registered apps.
 */
int ust_app_disable_channel_glb(struct ltt_ust_session *usess, struct ltt_ust_channel *uchan)
{
        int ret = 0;
        struct lttng_ht_iter iter;
        struct lttng_ht_node_str *ua_chan_node;
        struct ust_app *app;
        struct ust_app_session *ua_sess;
        struct ust_app_channel *ua_chan;

        LTTNG_ASSERT(usess->active);
        DBG2("UST app disabling channel %s from global domain for session id %" PRIu64,
             uchan->name,
             usess->id);

        {
                lttng::urcu::read_lock_guard read_lock;

                /* For every registered applications */
                cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        struct lttng_ht_iter uiter;
                        if (!app->compatible) {
                                /*
                                 * TODO: In time, we should notice the caller of this error by
                                 * telling him that this is a version error.
                                 */
                                continue;
                        }
                        ua_sess = lookup_session_by_app(usess, app);
                        if (ua_sess == nullptr) {
                                continue;
                        }

                        /* Get channel */
                        lttng_ht_lookup(ua_sess->channels, (void *) uchan->name, &uiter);
                        ua_chan_node = lttng_ht_iter_get_node_str(&uiter);
                        /* If the session if found for the app, the channel must be there */
                        LTTNG_ASSERT(ua_chan_node);

                        ua_chan = lttng::utils::container_of(ua_chan_node, &ust_app_channel::node);
                        /* The channel must not be already disabled */
                        LTTNG_ASSERT(ua_chan->enabled);

                        /* Disable channel onto application */
                        ret = disable_ust_app_channel(ua_sess, ua_chan, app);
                        if (ret < 0) {
                                /* XXX: We might want to report this error at some point... */
                                continue;
                        }
                }
        }

        return ret;
}

/*
 * For a specific UST session, enable the channel for all registered apps.
 */
int ust_app_enable_channel_glb(struct ltt_ust_session *usess, struct ltt_ust_channel *uchan)
{
        int ret = 0;
        struct lttng_ht_iter iter;
        struct ust_app *app;
        struct ust_app_session *ua_sess;

        LTTNG_ASSERT(usess->active);
        DBG2("UST app enabling channel %s to global domain for session id %" PRIu64,
             uchan->name,
             usess->id);

        {
                lttng::urcu::read_lock_guard read_lock;

                /* For every registered applications */
                cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        if (!app->compatible) {
                                /*
                                 * TODO: In time, we should notice the caller of this error by
                                 * telling him that this is a version error.
                                 */
                                continue;
                        }
                        ua_sess = lookup_session_by_app(usess, app);
                        if (ua_sess == nullptr) {
                                continue;
                        }

                        /* Enable channel onto application */
                        ret = enable_ust_app_channel(ua_sess, uchan, app);
                        if (ret < 0) {
                                /* XXX: We might want to report this error at some point... */
                                continue;
                        }
                }
        }

        return ret;
}

/*
 * Disable an event in a channel and for a specific session.
 */
int ust_app_disable_event_glb(struct ltt_ust_session *usess,
                              struct ltt_ust_channel *uchan,
                              struct ltt_ust_event *uevent)
{
        int ret = 0;
        struct lttng_ht_iter iter, uiter;
        struct lttng_ht_node_str *ua_chan_node;
        struct ust_app *app;
        struct ust_app_session *ua_sess;
        struct ust_app_channel *ua_chan;
        struct ust_app_event *ua_event;

        LTTNG_ASSERT(usess->active);
        DBG("UST app disabling event %s for all apps in channel "
            "%s for session id %" PRIu64,
            uevent->attr.name,
            uchan->name,
            usess->id);

        {
                lttng::urcu::read_lock_guard read_lock;

                /* For all registered applications */
                cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        if (!app->compatible) {
                                /*
                                 * TODO: In time, we should notice the caller of this error by
                                 * telling him that this is a version error.
                                 */
                                continue;
                        }
                        ua_sess = lookup_session_by_app(usess, app);
                        if (ua_sess == nullptr) {
                                /* Next app */
                                continue;
                        }

                        /* Lookup channel in the ust app session */
                        lttng_ht_lookup(ua_sess->channels, (void *) uchan->name, &uiter);
                        ua_chan_node = lttng_ht_iter_get_node_str(&uiter);
                        if (ua_chan_node == nullptr) {
                                DBG2("Channel %s not found in session id %" PRIu64
                                     " for app pid %d."
                                     "Skipping",
                                     uchan->name,
                                     usess->id,
                                     app->pid);
                                continue;
                        }
                        ua_chan = lttng::utils::container_of(ua_chan_node, &ust_app_channel::node);

                        ua_event = find_ust_app_event(
                                ua_chan->events,
                                uevent->attr.name,
                                uevent->filter,
                                (enum lttng_ust_abi_loglevel_type) uevent->attr.loglevel_type,
                                uevent->attr.loglevel,
                                uevent->exclusion);
                        if (ua_event == nullptr) {
                                DBG2("Event %s not found in channel %s for app pid %d."
                                     "Skipping",
                                     uevent->attr.name,
                                     uchan->name,
                                     app->pid);
                                continue;
                        }

                        ret = disable_ust_app_event(ua_event, app);
                        if (ret < 0) {
                                /* XXX: Report error someday... */
                                continue;
                        }
                }
        }

        return ret;
}

/* The ua_sess lock must be held by the caller.  */
static int ust_app_channel_create(struct ltt_ust_session *usess,
                                  struct ust_app_session *ua_sess,
                                  struct ltt_ust_channel *uchan,
                                  struct ust_app *app,
                                  struct ust_app_channel **_ua_chan)
{
        int ret = 0;
        struct ust_app_channel *ua_chan = nullptr;

        LTTNG_ASSERT(ua_sess);
        ASSERT_LOCKED(ua_sess->lock);

        if (!strncmp(uchan->name, DEFAULT_METADATA_NAME, sizeof(uchan->name))) {
                copy_channel_attr_to_ustctl(&ua_sess->metadata_attr, &uchan->attr);
                ret = 0;
        } else {
                struct ltt_ust_context *uctx = nullptr;

                /*
                 * Create channel onto application and synchronize its
                 * configuration.
                 */
                ret = ust_app_channel_allocate(
                        ua_sess, uchan, LTTNG_UST_ABI_CHAN_PER_CPU, usess, &ua_chan);
                if (ret < 0) {
                        goto error;
                }

                ret = ust_app_channel_send(app, usess, ua_sess, ua_chan);
                if (ret) {
                        goto error;
                }

                /* Add contexts. */
                cds_list_for_each_entry (uctx, &uchan->ctx_list, list) {
                        ret = create_ust_app_channel_context(ua_chan, &uctx->ctx, app);
                        if (ret) {
                                goto error;
                        }
                }
        }

error:
        if (ret < 0) {
                switch (ret) {
                case -ENOTCONN:
                        /*
                         * The application's socket is not valid. Either a bad socket
                         * or a timeout on it. We can't inform the caller that for a
                         * specific app, the session failed so lets continue here.
                         */
                        ret = 0; /* Not an error. */
                        break;
                case -ENOMEM:
                default:
                        break;
                }
        }

        if (ret == 0 && _ua_chan) {
                /*
                 * Only return the application's channel on success. Note
                 * that the channel can still be part of the application's
                 * channel hashtable on error.
                 */
                *_ua_chan = ua_chan;
        }
        return ret;
}

/*
 * Enable event for a specific session and channel on the tracer.
 */
int ust_app_enable_event_glb(struct ltt_ust_session *usess,
                             struct ltt_ust_channel *uchan,
                             struct ltt_ust_event *uevent)
{
        int ret = 0;
        struct lttng_ht_iter iter, uiter;
        struct lttng_ht_node_str *ua_chan_node;
        struct ust_app *app;
        struct ust_app_session *ua_sess;
        struct ust_app_channel *ua_chan;
        struct ust_app_event *ua_event;

        LTTNG_ASSERT(usess->active);
        DBG("UST app enabling event %s for all apps for session id %" PRIu64,
            uevent->attr.name,
            usess->id);

        /*
         * NOTE: At this point, this function is called only if the session and
         * channel passed are already created for all apps. and enabled on the
         * tracer also.
         */

        {
                lttng::urcu::read_lock_guard read_lock;

                /* For all registered applications */
                cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        if (!app->compatible) {
                                /*
                                 * TODO: In time, we should notice the caller of this error by
                                 * telling him that this is a version error.
                                 */
                                continue;
                        }
                        ua_sess = lookup_session_by_app(usess, app);
                        if (!ua_sess) {
                                /* The application has problem or is probably dead. */
                                continue;
                        }

                        pthread_mutex_lock(&ua_sess->lock);

                        if (ua_sess->deleted) {
                                pthread_mutex_unlock(&ua_sess->lock);
                                continue;
                        }

                        /* Lookup channel in the ust app session */
                        lttng_ht_lookup(ua_sess->channels, (void *) uchan->name, &uiter);
                        ua_chan_node = lttng_ht_iter_get_node_str(&uiter);
                        /*
                         * It is possible that the channel cannot be found is
                         * the channel/event creation occurs concurrently with
                         * an application exit.
                         */
                        if (!ua_chan_node) {
                                pthread_mutex_unlock(&ua_sess->lock);
                                continue;
                        }

                        ua_chan = lttng::utils::container_of(ua_chan_node, &ust_app_channel::node);

                        /* Get event node */
                        ua_event = find_ust_app_event(
                                ua_chan->events,
                                uevent->attr.name,
                                uevent->filter,
                                (enum lttng_ust_abi_loglevel_type) uevent->attr.loglevel_type,
                                uevent->attr.loglevel,
                                uevent->exclusion);
                        if (ua_event == nullptr) {
                                DBG3("UST app enable event %s not found for app PID %d."
                                     "Skipping app",
                                     uevent->attr.name,
                                     app->pid);
                                goto next_app;
                        }

                        ret = enable_ust_app_event(ua_event, app);
                        if (ret < 0) {
                                pthread_mutex_unlock(&ua_sess->lock);
                                goto error;
                        }
                next_app:
                        pthread_mutex_unlock(&ua_sess->lock);
                }
        }
error:
        return ret;
}

/*
 * For a specific existing UST session and UST channel, creates the event for
 * all registered apps.
 */
int ust_app_create_event_glb(struct ltt_ust_session *usess,
                             struct ltt_ust_channel *uchan,
                             struct ltt_ust_event *uevent)
{
        int ret = 0;
        struct lttng_ht_iter iter, uiter;
        struct lttng_ht_node_str *ua_chan_node;
        struct ust_app *app;
        struct ust_app_session *ua_sess;
        struct ust_app_channel *ua_chan;

        LTTNG_ASSERT(usess->active);
        DBG("UST app creating event %s for all apps for session id %" PRIu64,
            uevent->attr.name,
            usess->id);

        {
                lttng::urcu::read_lock_guard read_lock;

                /* For all registered applications */
                cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        if (!app->compatible) {
                                /*
                                 * TODO: In time, we should notice the caller of this error by
                                 * telling him that this is a version error.
                                 */
                                continue;
                        }

                        ua_sess = lookup_session_by_app(usess, app);
                        if (!ua_sess) {
                                /* The application has problem or is probably dead. */
                                continue;
                        }

                        pthread_mutex_lock(&ua_sess->lock);

                        if (ua_sess->deleted) {
                                pthread_mutex_unlock(&ua_sess->lock);
                                continue;
                        }

                        /* Lookup channel in the ust app session */
                        lttng_ht_lookup(ua_sess->channels, (void *) uchan->name, &uiter);
                        ua_chan_node = lttng_ht_iter_get_node_str(&uiter);
                        /* If the channel is not found, there is a code flow error */
                        LTTNG_ASSERT(ua_chan_node);

                        ua_chan = lttng::utils::container_of(ua_chan_node, &ust_app_channel::node);

                        ret = create_ust_app_event(ua_chan, uevent, app);
                        pthread_mutex_unlock(&ua_sess->lock);
                        if (ret < 0) {
                                if (ret != -LTTNG_UST_ERR_EXIST) {
                                        /* Possible value at this point: -ENOMEM. If so, we stop! */
                                        break;
                                }

                                DBG2("UST app event %s already exist on app PID %d",
                                     uevent->attr.name,
                                     app->pid);
                                continue;
                        }
                }
        }

        return ret;
}

/*
 * Start tracing for a specific UST session and app.
 *
 * Called with UST app session lock held.
 *
 */
static int ust_app_start_trace(struct ltt_ust_session *usess, struct ust_app *app)
{
        int ret = 0;
        struct ust_app_session *ua_sess;

        DBG("Starting tracing for ust app pid %d", app->pid);

        lttng::urcu::read_lock_guard read_lock;

        if (!app->compatible) {
                goto end;
        }

        ua_sess = lookup_session_by_app(usess, app);
        if (ua_sess == nullptr) {
                /* The session is in teardown process. Ignore and continue. */
                goto end;
        }

        pthread_mutex_lock(&ua_sess->lock);

        if (ua_sess->deleted) {
                pthread_mutex_unlock(&ua_sess->lock);
                goto end;
        }

        if (ua_sess->enabled) {
                pthread_mutex_unlock(&ua_sess->lock);
                goto end;
        }

        /* Upon restart, we skip the setup, already done */
        if (ua_sess->started) {
                goto skip_setup;
        }

        health_code_update();

skip_setup:
        /* This starts the UST tracing */
        pthread_mutex_lock(&app->sock_lock);
        ret = lttng_ust_ctl_start_session(app->sock, ua_sess->handle);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        DBG3("UST app start session failed. Application is dead: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                        pthread_mutex_unlock(&ua_sess->lock);
                        goto end;
                } else if (ret == -EAGAIN) {
                        WARN("UST app start session failed. Communication time out: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                        pthread_mutex_unlock(&ua_sess->lock);
                        goto end;

                } else {
                        ERR("UST app start session failed with ret %d: pid = %d, sock = %d",
                            ret,
                            app->pid,
                            app->sock);
                }
                goto error_unlock;
        }

        /* Indicate that the session has been started once */
        ua_sess->started = true;
        ua_sess->enabled = true;

        pthread_mutex_unlock(&ua_sess->lock);

        health_code_update();

        /* Quiescent wait after starting trace */
        pthread_mutex_lock(&app->sock_lock);
        ret = lttng_ust_ctl_wait_quiescent(app->sock);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        DBG3("UST app wait quiescent failed. Application is dead: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else if (ret == -EAGAIN) {
                        WARN("UST app wait quiescent failed. Communication time out: pid =  %d, sock = %d",
                             app->pid,
                             app->sock);
                } else {
                        ERR("UST app wait quiescent failed with ret %d: pid %d, sock = %d",
                            ret,
                            app->pid,
                            app->sock);
                }
        }

end:
        health_code_update();
        return 0;

error_unlock:
        pthread_mutex_unlock(&ua_sess->lock);
        health_code_update();
        return -1;
}

/*
 * Stop tracing for a specific UST session and app.
 */
static int ust_app_stop_trace(struct ltt_ust_session *usess, struct ust_app *app)
{
        int ret = 0;
        struct ust_app_session *ua_sess;

        DBG("Stopping tracing for ust app pid %d", app->pid);

        lttng::urcu::read_lock_guard read_lock;

        if (!app->compatible) {
                goto end_no_session;
        }

        ua_sess = lookup_session_by_app(usess, app);
        if (ua_sess == nullptr) {
                goto end_no_session;
        }

        pthread_mutex_lock(&ua_sess->lock);

        if (ua_sess->deleted) {
                pthread_mutex_unlock(&ua_sess->lock);
                goto end_no_session;
        }

        /*
         * If started = 0, it means that stop trace has been called for a session
         * that was never started. It's possible since we can have a fail start
         * from either the application manager thread or the command thread. Simply
         * indicate that this is a stop error.
         */
        if (!ua_sess->started) {
                goto error_rcu_unlock;
        }

        health_code_update();

        /* This inhibits UST tracing */
        pthread_mutex_lock(&app->sock_lock);
        ret = lttng_ust_ctl_stop_session(app->sock, ua_sess->handle);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        DBG3("UST app stop session failed. Application is dead: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                        goto end_unlock;
                } else if (ret == -EAGAIN) {
                        WARN("UST app stop session failed. Communication time out: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                        goto end_unlock;

                } else {
                        ERR("UST app stop session failed with ret %d: pid = %d, sock = %d",
                            ret,
                            app->pid,
                            app->sock);
                }
                goto error_rcu_unlock;
        }

        health_code_update();
        ua_sess->enabled = false;

        /* Quiescent wait after stopping trace */
        pthread_mutex_lock(&app->sock_lock);
        ret = lttng_ust_ctl_wait_quiescent(app->sock);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        DBG3("UST app wait quiescent failed. Application is dead: pid= %d, sock = %d",
                             app->pid,
                             app->sock);
                } else if (ret == -EAGAIN) {
                        WARN("UST app wait quiescent failed. Communication time out: pid= %d, sock = %d",
                             app->pid,
                             app->sock);
                } else {
                        ERR("UST app wait quiescent failed with ret %d: pid= %d, sock = %d",
                            ret,
                            app->pid,
                            app->sock);
                }
        }

        health_code_update();

        {
                auto locked_registry = get_locked_session_registry(ua_sess);

                /* The UST app session is held registry shall not be null. */
                LTTNG_ASSERT(locked_registry);

                /* Push metadata for application before freeing the application. */
                (void) push_metadata(locked_registry, ua_sess->consumer);
        }

end_unlock:
        pthread_mutex_unlock(&ua_sess->lock);
end_no_session:
        health_code_update();
        return 0;

error_rcu_unlock:
        pthread_mutex_unlock(&ua_sess->lock);
        health_code_update();
        return -1;
}

static int ust_app_flush_app_session(ust_app& app, ust_app_session& ua_sess)
{
        int ret, retval = 0;
        struct lttng_ht_iter iter;
        struct ust_app_channel *ua_chan;
        struct consumer_socket *socket;

        DBG("Flushing app session buffers for ust app pid %d", app.pid);

        if (!app.compatible) {
                goto end_not_compatible;
        }

        pthread_mutex_lock(&ua_sess.lock);

        if (ua_sess.deleted) {
                goto end_deleted;
        }

        health_code_update();

        /* Flushing buffers */
        socket = consumer_find_socket_by_bitness(app.abi.bits_per_long, ua_sess.consumer);

        /* Flush buffers and push metadata. */
        switch (ua_sess.buffer_type) {
        case LTTNG_BUFFER_PER_PID:
        {
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (ua_sess.channels->ht, &iter.iter, ua_chan, node.node) {
                        health_code_update();
                        ret = consumer_flush_channel(socket, ua_chan->key);
                        if (ret) {
                                ERR("Error flushing consumer channel");
                                retval = -1;
                                continue;
                        }
                }

                break;
        }
        case LTTNG_BUFFER_PER_UID:
        default:
                abort();
                break;
        }

        health_code_update();

end_deleted:
        pthread_mutex_unlock(&ua_sess.lock);

end_not_compatible:
        health_code_update();
        return retval;
}

/*
 * Flush buffers for all applications for a specific UST session.
 * Called with UST session lock held.
 */
static int ust_app_flush_session(struct ltt_ust_session *usess)

{
        int ret = 0;

        DBG("Flushing session buffers for all ust apps");

        /* Flush buffers and push metadata. */
        switch (usess->buffer_type) {
        case LTTNG_BUFFER_PER_UID:
        {
                struct buffer_reg_uid *reg;
                struct lttng_ht_iter iter;

                /* Flush all per UID buffers associated to that session. */
                cds_list_for_each_entry (reg, &usess->buffer_reg_uid_list, lnode) {
                        lttng::urcu::read_lock_guard read_lock;
                        lsu::registry_session *ust_session_reg;
                        struct buffer_reg_channel *buf_reg_chan;
                        struct consumer_socket *socket;

                        /* Get consumer socket to use to push the metadata.*/
                        socket = consumer_find_socket_by_bitness(reg->bits_per_long,
                                                                 usess->consumer);
                        if (!socket) {
                                /* Ignore request if no consumer is found for the session. */
                                continue;
                        }

                        cds_lfht_for_each_entry (
                                reg->registry->channels->ht, &iter.iter, buf_reg_chan, node.node) {
                                /*
                                 * The following call will print error values so the return
                                 * code is of little importance because whatever happens, we
                                 * have to try them all.
                                 */
                                (void) consumer_flush_channel(socket, buf_reg_chan->consumer_key);
                        }

                        ust_session_reg = reg->registry->reg.ust;
                        /* Push metadata. */
                        auto locked_registry = ust_session_reg->lock();
                        (void) push_metadata(locked_registry, usess->consumer);
                }

                break;
        }
        case LTTNG_BUFFER_PER_PID:
        {
                struct ust_app_session *ua_sess;
                struct lttng_ht_iter iter;
                struct ust_app *app;
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        ua_sess = lookup_session_by_app(usess, app);
                        if (ua_sess == nullptr) {
                                continue;
                        }

                        (void) ust_app_flush_app_session(*app, *ua_sess);
                }

                break;
        }
        default:
                ret = -1;
                abort();
                break;
        }

        health_code_update();
        return ret;
}

static int ust_app_clear_quiescent_app_session(struct ust_app *app, struct ust_app_session *ua_sess)
{
        int ret = 0;
        struct lttng_ht_iter iter;
        struct ust_app_channel *ua_chan;
        struct consumer_socket *socket;

        DBG("Clearing stream quiescent state for ust app pid %d", app->pid);

        lttng::urcu::read_lock_guard read_lock;

        if (!app->compatible) {
                goto end_not_compatible;
        }

        pthread_mutex_lock(&ua_sess->lock);

        if (ua_sess->deleted) {
                goto end_unlock;
        }

        health_code_update();

        socket = consumer_find_socket_by_bitness(app->abi.bits_per_long, ua_sess->consumer);
        if (!socket) {
                ERR("Failed to find consumer (%" PRIu32 ") socket", app->abi.bits_per_long);
                ret = -1;
                goto end_unlock;
        }

        /* Clear quiescent state. */
        switch (ua_sess->buffer_type) {
        case LTTNG_BUFFER_PER_PID:
                cds_lfht_for_each_entry (ua_sess->channels->ht, &iter.iter, ua_chan, node.node) {
                        health_code_update();
                        ret = consumer_clear_quiescent_channel(socket, ua_chan->key);
                        if (ret) {
                                ERR("Error clearing quiescent state for consumer channel");
                                ret = -1;
                                continue;
                        }
                }
                break;
        case LTTNG_BUFFER_PER_UID:
        default:
                abort();
                ret = -1;
                break;
        }

        health_code_update();

end_unlock:
        pthread_mutex_unlock(&ua_sess->lock);

end_not_compatible:
        health_code_update();
        return ret;
}

/*
 * Clear quiescent state in each stream for all applications for a
 * specific UST session.
 * Called with UST session lock held.
 */
static int ust_app_clear_quiescent_session(struct ltt_ust_session *usess)

{
        int ret = 0;

        DBG("Clearing stream quiescent state for all ust apps");

        switch (usess->buffer_type) {
        case LTTNG_BUFFER_PER_UID:
        {
                struct lttng_ht_iter iter;
                struct buffer_reg_uid *reg;

                /*
                 * Clear quiescent for all per UID buffers associated to
                 * that session.
                 */
                cds_list_for_each_entry (reg, &usess->buffer_reg_uid_list, lnode) {
                        struct consumer_socket *socket;
                        struct buffer_reg_channel *buf_reg_chan;
                        lttng::urcu::read_lock_guard read_lock;

                        /* Get associated consumer socket.*/
                        socket = consumer_find_socket_by_bitness(reg->bits_per_long,
                                                                 usess->consumer);
                        if (!socket) {
                                /*
                                 * Ignore request if no consumer is found for
                                 * the session.
                                 */
                                continue;
                        }

                        cds_lfht_for_each_entry (
                                reg->registry->channels->ht, &iter.iter, buf_reg_chan, node.node) {
                                /*
                                 * The following call will print error values so
                                 * the return code is of little importance
                                 * because whatever happens, we have to try them
                                 * all.
                                 */
                                (void) consumer_clear_quiescent_channel(socket,
                                                                        buf_reg_chan->consumer_key);
                        }
                }

                break;
        }
        case LTTNG_BUFFER_PER_PID:
        {
                struct ust_app_session *ua_sess;
                struct lttng_ht_iter iter;
                struct ust_app *app;
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        ua_sess = lookup_session_by_app(usess, app);
                        if (ua_sess == nullptr) {
                                continue;
                        }
                        (void) ust_app_clear_quiescent_app_session(app, ua_sess);
                }

                break;
        }
        default:
                ret = -1;
                abort();
                break;
        }

        health_code_update();
        return ret;
}

/*
 * Destroy a specific UST session in apps.
 */
static int destroy_trace(struct ltt_ust_session *usess, struct ust_app *app)
{
        int ret;
        struct ust_app_session *ua_sess;
        struct lttng_ht_iter iter;
        struct lttng_ht_node_u64 *node;

        DBG("Destroy tracing for ust app pid %d", app->pid);

        lttng::urcu::read_lock_guard read_lock;

        if (!app->compatible) {
                goto end;
        }

        __lookup_session_by_app(usess, app, &iter);
        node = lttng_ht_iter_get_node_u64(&iter);
        if (node == nullptr) {
                /* Session is being or is deleted. */
                goto end;
        }
        ua_sess = lttng::utils::container_of(node, &ust_app_session::node);

        health_code_update();
        destroy_app_session(app, ua_sess);

        health_code_update();

        /* Quiescent wait after stopping trace */
        pthread_mutex_lock(&app->sock_lock);
        ret = lttng_ust_ctl_wait_quiescent(app->sock);
        pthread_mutex_unlock(&app->sock_lock);
        if (ret < 0) {
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        DBG3("UST app wait quiescent failed. Application is dead: pid= %d, sock = %d",
                             app->pid,
                             app->sock);
                } else if (ret == -EAGAIN) {
                        WARN("UST app wait quiescent failed. Communication time out: pid= %d, sock = %d",
                             app->pid,
                             app->sock);
                } else {
                        ERR("UST app wait quiescent failed with ret %d: pid= %d, sock = %d",
                            ret,
                            app->pid,
                            app->sock);
                }
        }
end:
        health_code_update();
        return 0;
}

/*
 * Start tracing for the UST session.
 */
int ust_app_start_trace_all(struct ltt_ust_session *usess)
{
        struct lttng_ht_iter iter;
        struct ust_app *app;

        DBG("Starting all UST traces");

        /*
         * Even though the start trace might fail, flag this session active so
         * other application coming in are started by default.
         */
        usess->active = true;

        /*
         * In a start-stop-start use-case, we need to clear the quiescent state
         * of each channel set by the prior stop command, thus ensuring that a
         * following stop or destroy is sure to grab a timestamp_end near those
         * operations, even if the packet is empty.
         */
        (void) ust_app_clear_quiescent_session(usess);

        {
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        ust_app_global_update(usess, app);
                }
        }

        return 0;
}

/*
 * Start tracing for the UST session.
 * Called with UST session lock held.
 */
int ust_app_stop_trace_all(struct ltt_ust_session *usess)
{
        int ret = 0;
        struct lttng_ht_iter iter;
        struct ust_app *app;

        DBG("Stopping all UST traces");

        /*
         * Even though the stop trace might fail, flag this session inactive so
         * other application coming in are not started by default.
         */
        usess->active = false;

        {
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        ret = ust_app_stop_trace(usess, app);
                        if (ret < 0) {
                                /* Continue to next apps even on error */
                                continue;
                        }
                }
        }

        (void) ust_app_flush_session(usess);

        return 0;
}

/*
 * Destroy app UST session.
 */
int ust_app_destroy_trace_all(struct ltt_ust_session *usess)
{
        int ret = 0;
        struct lttng_ht_iter iter;
        struct ust_app *app;

        DBG("Destroy all UST traces");

        {
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        ret = destroy_trace(usess, app);
                        if (ret < 0) {
                                /* Continue to next apps even on error */
                                continue;
                        }
                }
        }

        return 0;
}

/* The ua_sess lock must be held by the caller. */
static int find_or_create_ust_app_channel(struct ltt_ust_session *usess,
                                          struct ust_app_session *ua_sess,
                                          struct ust_app *app,
                                          struct ltt_ust_channel *uchan,
                                          struct ust_app_channel **ua_chan)
{
        int ret = 0;
        struct lttng_ht_iter iter;
        struct lttng_ht_node_str *ua_chan_node;

        lttng_ht_lookup(ua_sess->channels, (void *) uchan->name, &iter);
        ua_chan_node = lttng_ht_iter_get_node_str(&iter);
        if (ua_chan_node) {
                *ua_chan = caa_container_of(ua_chan_node, struct ust_app_channel, node);
                goto end;
        }

        ret = ust_app_channel_create(usess, ua_sess, uchan, app, ua_chan);
        if (ret) {
                goto end;
        }
end:
        return ret;
}

static int ust_app_channel_synchronize_event(struct ust_app_channel *ua_chan,
                                             struct ltt_ust_event *uevent,
                                             struct ust_app *app)
{
        int ret = 0;
        struct ust_app_event *ua_event = nullptr;

        ua_event = find_ust_app_event(ua_chan->events,
                                      uevent->attr.name,
                                      uevent->filter,
                                      (enum lttng_ust_abi_loglevel_type) uevent->attr.loglevel_type,
                                      uevent->attr.loglevel,
                                      uevent->exclusion);
        if (!ua_event) {
                ret = create_ust_app_event(ua_chan, uevent, app);
                if (ret < 0) {
                        goto end;
                }
        } else {
                if (ua_event->enabled != uevent->enabled) {
                        ret = uevent->enabled ? enable_ust_app_event(ua_event, app) :
                                                disable_ust_app_event(ua_event, app);
                }
        }

end:
        return ret;
}

/* Called with RCU read-side lock held. */
static void ust_app_synchronize_event_notifier_rules(struct ust_app *app)
{
        int ret = 0;
        enum lttng_error_code ret_code;
        enum lttng_trigger_status t_status;
        struct lttng_ht_iter app_trigger_iter;
        struct lttng_triggers *triggers = nullptr;
        struct ust_app_event_notifier_rule *event_notifier_rule;
        unsigned int count, i;

        ASSERT_RCU_READ_LOCKED();

        if (!ust_app_supports_notifiers(app)) {
                goto end;
        }

        /*
         * Currrently, registering or unregistering a trigger with an
         * event rule condition causes a full synchronization of the event
         * notifiers.
         *
         * The first step attempts to add an event notifier for all registered
         * triggers that apply to the user space tracers. Then, the
         * application's event notifiers rules are all checked against the list
         * of registered triggers. Any event notifier that doesn't have a
         * matching trigger can be assumed to have been disabled.
         *
         * All of this is inefficient, but is put in place to get the feature
         * rolling as it is simpler at this moment. It will be optimized Soon™
         * to allow the state of enabled
         * event notifiers to be synchronized in a piece-wise way.
         */

        /* Get all triggers using uid 0 (root) */
        ret_code = notification_thread_command_list_triggers(
                the_notification_thread_handle, 0, &triggers);
        if (ret_code != LTTNG_OK) {
                goto end;
        }

        LTTNG_ASSERT(triggers);

        t_status = lttng_triggers_get_count(triggers, &count);
        if (t_status != LTTNG_TRIGGER_STATUS_OK) {
                goto end;
        }

        for (i = 0; i < count; i++) {
                const struct lttng_condition *condition;
                const struct lttng_event_rule *event_rule;
                struct lttng_trigger *trigger;
                const struct ust_app_event_notifier_rule *looked_up_event_notifier_rule;
                enum lttng_condition_status condition_status;
                uint64_t token;

                trigger = lttng_triggers_borrow_mutable_at_index(triggers, i);
                LTTNG_ASSERT(trigger);

                token = lttng_trigger_get_tracer_token(trigger);
                condition = lttng_trigger_get_const_condition(trigger);

                if (lttng_condition_get_type(condition) !=
                    LTTNG_CONDITION_TYPE_EVENT_RULE_MATCHES) {
                        /* Does not apply */
                        continue;
                }

                condition_status =
                        lttng_condition_event_rule_matches_get_rule(condition, &event_rule);
                LTTNG_ASSERT(condition_status == LTTNG_CONDITION_STATUS_OK);

                if (lttng_event_rule_get_domain_type(event_rule) == LTTNG_DOMAIN_KERNEL) {
                        /* Skip kernel related triggers. */
                        continue;
                }

                /*
                 * Find or create the associated token event rule. The caller
                 * holds the RCU read lock, so this is safe to call without
                 * explicitly acquiring it here.
                 */
                looked_up_event_notifier_rule = find_ust_app_event_notifier_rule(
                        app->token_to_event_notifier_rule_ht, token);
                if (!looked_up_event_notifier_rule) {
                        ret = create_ust_app_event_notifier_rule(trigger, app);
                        if (ret < 0) {
                                goto end;
                        }
                }
        }

        {
                lttng::urcu::read_lock_guard read_lock;

                /* Remove all unknown event sources from the app. */
                cds_lfht_for_each_entry (app->token_to_event_notifier_rule_ht->ht,
                                         &app_trigger_iter.iter,
                                         event_notifier_rule,
                                         node.node) {
                        const uint64_t app_token = event_notifier_rule->token;
                        bool found = false;

                        /*
                         * Check if the app event trigger still exists on the
                         * notification side.
                         */
                        for (i = 0; i < count; i++) {
                                uint64_t notification_thread_token;
                                const struct lttng_trigger *trigger =
                                        lttng_triggers_get_at_index(triggers, i);

                                LTTNG_ASSERT(trigger);

                                notification_thread_token = lttng_trigger_get_tracer_token(trigger);

                                if (notification_thread_token == app_token) {
                                        found = true;
                                        break;
                                }
                        }

                        if (found) {
                                /* Still valid. */
                                continue;
                        }

                        /*
                         * This trigger was unregistered, disable it on the tracer's
                         * side.
                         */
                        ret = lttng_ht_del(app->token_to_event_notifier_rule_ht, &app_trigger_iter);
                        LTTNG_ASSERT(ret == 0);

                        /* Callee logs errors. */
                        (void) disable_ust_object(app, event_notifier_rule->obj);

                        delete_ust_app_event_notifier_rule(app->sock, event_notifier_rule, app);
                }
        }

end:
        lttng_triggers_destroy(triggers);
        return;
}

/*
 * RCU read lock must be held by the caller.
 */
static void ust_app_synchronize_all_channels(struct ltt_ust_session *usess,
                                             struct ust_app_session *ua_sess,
                                             struct ust_app *app)
{
        int ret = 0;
        struct cds_lfht_iter uchan_iter;
        struct ltt_ust_channel *uchan;

        LTTNG_ASSERT(usess);
        LTTNG_ASSERT(ua_sess);
        LTTNG_ASSERT(app);
        ASSERT_RCU_READ_LOCKED();

        cds_lfht_for_each_entry (usess->domain_global.channels->ht, &uchan_iter, uchan, node.node) {
                struct ust_app_channel *ua_chan;
                struct cds_lfht_iter uevent_iter;
                struct ltt_ust_event *uevent;

                /*
                 * Search for a matching ust_app_channel. If none is found,
                 * create it. Creating the channel will cause the ua_chan
                 * structure to be allocated, the channel buffers to be
                 * allocated (if necessary) and sent to the application, and
                 * all enabled contexts will be added to the channel.
                 */
                ret = find_or_create_ust_app_channel(usess, ua_sess, app, uchan, &ua_chan);
                if (ret) {
                        /* Tracer is probably gone or ENOMEM. */
                        goto end;
                }

                if (!ua_chan) {
                        /* ua_chan will be NULL for the metadata channel */
                        continue;
                }

                cds_lfht_for_each_entry (uchan->events->ht, &uevent_iter, uevent, node.node) {
                        ret = ust_app_channel_synchronize_event(ua_chan, uevent, app);
                        if (ret) {
                                goto end;
                        }
                }

                if (ua_chan->enabled != uchan->enabled) {
                        ret = uchan->enabled ? enable_ust_app_channel(ua_sess, uchan, app) :
                                               disable_ust_app_channel(ua_sess, ua_chan, app);
                        if (ret) {
                                goto end;
                        }
                }
        }
end:
        return;
}

/*
 * The caller must ensure that the application is compatible and is tracked
 * by the process attribute trackers.
 */
static void ust_app_synchronize(struct ltt_ust_session *usess, struct ust_app *app)
{
        int ret = 0;
        struct ust_app_session *ua_sess = nullptr;

        /*
         * The application's configuration should only be synchronized for
         * active sessions.
         */
        LTTNG_ASSERT(usess->active);

        ret = find_or_create_ust_app_session(usess, app, &ua_sess, nullptr);
        if (ret < 0) {
                /* Tracer is probably gone or ENOMEM. */
                goto end;
        }

        LTTNG_ASSERT(ua_sess);

        pthread_mutex_lock(&ua_sess->lock);
        if (ua_sess->deleted) {
                goto deleted_session;
        }

        {
                lttng::urcu::read_lock_guard read_lock;

                ust_app_synchronize_all_channels(usess, ua_sess, app);

                /*
                 * Create the metadata for the application. This returns gracefully if a
                 * metadata was already set for the session.
                 *
                 * The metadata channel must be created after the data channels as the
                 * consumer daemon assumes this ordering. When interacting with a relay
                 * daemon, the consumer will use this assumption to send the
                 * "STREAMS_SENT" message to the relay daemon.
                 */
                ret = create_ust_app_metadata(ua_sess, app, usess->consumer);
                if (ret < 0) {
                        ERR("Metadata creation failed for app sock %d for session id %" PRIu64,
                            app->sock,
                            usess->id);
                }
        }

deleted_session:
        pthread_mutex_unlock(&ua_sess->lock);
end:
        return;
}

static void ust_app_global_destroy(struct ltt_ust_session *usess, struct ust_app *app)
{
        struct ust_app_session *ua_sess;

        ua_sess = lookup_session_by_app(usess, app);
        if (ua_sess == nullptr) {
                return;
        }
        destroy_app_session(app, ua_sess);
}

/*
 * Add channels/events from UST global domain to registered apps at sock.
 *
 * Called with session lock held.
 * Called with RCU read-side lock held.
 */
void ust_app_global_update(struct ltt_ust_session *usess, struct ust_app *app)
{
        LTTNG_ASSERT(usess);
        LTTNG_ASSERT(usess->active);
        ASSERT_RCU_READ_LOCKED();

        DBG2("UST app global update for app sock %d for session id %" PRIu64, app->sock, usess->id);

        if (!app->compatible) {
                return;
        }
        if (trace_ust_id_tracker_lookup(LTTNG_PROCESS_ATTR_VIRTUAL_PROCESS_ID, usess, app->pid) &&
            trace_ust_id_tracker_lookup(LTTNG_PROCESS_ATTR_VIRTUAL_USER_ID, usess, app->uid) &&
            trace_ust_id_tracker_lookup(LTTNG_PROCESS_ATTR_VIRTUAL_GROUP_ID, usess, app->gid)) {
                /*
                 * Synchronize the application's internal tracing configuration
                 * and start tracing.
                 */
                ust_app_synchronize(usess, app);
                ust_app_start_trace(usess, app);
        } else {
                ust_app_global_destroy(usess, app);
        }
}

/*
 * Add all event notifiers to an application.
 *
 * Called with session lock held.
 * Called with RCU read-side lock held.
 */
void ust_app_global_update_event_notifier_rules(struct ust_app *app)
{
        ASSERT_RCU_READ_LOCKED();

        DBG2("UST application global event notifier rules update: app = '%s', pid = %d",
             app->name,
             app->pid);

        if (!app->compatible || !ust_app_supports_notifiers(app)) {
                return;
        }

        if (app->event_notifier_group.object == nullptr) {
                WARN("UST app global update of event notifiers for app skipped since communication handle is null: app = '%s', pid = %d",
                     app->name,
                     app->pid);
                return;
        }

        ust_app_synchronize_event_notifier_rules(app);
}

/*
 * Called with session lock held.
 */
void ust_app_global_update_all(struct ltt_ust_session *usess)
{
        struct lttng_ht_iter iter;
        struct ust_app *app;

        {
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        ust_app_global_update(usess, app);
                }
        }
}

void ust_app_global_update_all_event_notifier_rules()
{
        struct lttng_ht_iter iter;
        struct ust_app *app;

        lttng::urcu::read_lock_guard read_lock;
        cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                ust_app_global_update_event_notifier_rules(app);
        }
}

/*
 * Add context to a specific channel for global UST domain.
 */
int ust_app_add_ctx_channel_glb(struct ltt_ust_session *usess,
                                struct ltt_ust_channel *uchan,
                                struct ltt_ust_context *uctx)
{
        int ret = 0;
        struct lttng_ht_node_str *ua_chan_node;
        struct lttng_ht_iter iter, uiter;
        struct ust_app_channel *ua_chan = nullptr;
        struct ust_app_session *ua_sess;
        struct ust_app *app;

        LTTNG_ASSERT(usess->active);

        {
                lttng::urcu::read_lock_guard read_lock;
                cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        if (!app->compatible) {
                                /*
                                 * TODO: In time, we should notice the caller of this error by
                                 * telling him that this is a version error.
                                 */
                                continue;
                        }
                        ua_sess = lookup_session_by_app(usess, app);
                        if (ua_sess == nullptr) {
                                continue;
                        }

                        pthread_mutex_lock(&ua_sess->lock);

                        if (ua_sess->deleted) {
                                pthread_mutex_unlock(&ua_sess->lock);
                                continue;
                        }

                        /* Lookup channel in the ust app session */
                        lttng_ht_lookup(ua_sess->channels, (void *) uchan->name, &uiter);
                        ua_chan_node = lttng_ht_iter_get_node_str(&uiter);
                        if (ua_chan_node == nullptr) {
                                goto next_app;
                        }
                        ua_chan = caa_container_of(ua_chan_node, struct ust_app_channel, node);
                        ret = create_ust_app_channel_context(ua_chan, &uctx->ctx, app);
                        if (ret < 0) {
                                goto next_app;
                        }
                next_app:
                        pthread_mutex_unlock(&ua_sess->lock);
                }
        }

        return ret;
}

/*
 * Receive registration and populate the given msg structure.
 *
 * On success return 0 else a negative value returned by the ustctl call.
 */
int ust_app_recv_registration(int sock, struct ust_register_msg *msg)
{
        int ret;
        uint32_t pid, ppid, uid, gid;

        LTTNG_ASSERT(msg);

        ret = lttng_ust_ctl_recv_reg_msg(sock,
                                         &msg->type,
                                         &msg->major,
                                         &msg->minor,
                                         &pid,
                                         &ppid,
                                         &uid,
                                         &gid,
                                         &msg->bits_per_long,
                                         &msg->uint8_t_alignment,
                                         &msg->uint16_t_alignment,
                                         &msg->uint32_t_alignment,
                                         &msg->uint64_t_alignment,
                                         &msg->long_alignment,
                                         &msg->byte_order,
                                         msg->name);
        if (ret < 0) {
                switch (-ret) {
                case EPIPE:
                case ECONNRESET:
                case LTTNG_UST_ERR_EXITING:
                        DBG3("UST app recv reg message failed. Application died");
                        break;
                case LTTNG_UST_ERR_UNSUP_MAJOR:
                        ERR("UST app recv reg unsupported version %d.%d. Supporting %d.%d",
                            msg->major,
                            msg->minor,
                            LTTNG_UST_ABI_MAJOR_VERSION,
                            LTTNG_UST_ABI_MINOR_VERSION);
                        break;
                default:
                        ERR("UST app recv reg message failed with ret %d", ret);
                        break;
                }
                goto error;
        }
        msg->pid = (pid_t) pid;
        msg->ppid = (pid_t) ppid;
        msg->uid = (uid_t) uid;
        msg->gid = (gid_t) gid;

error:
        return ret;
}

/*
 * Return a ust app session object using the application object and the
 * session object descriptor has a key. If not found, NULL is returned.
 * A RCU read side lock MUST be acquired when calling this function.
 */
static struct ust_app_session *find_session_by_objd(struct ust_app *app, int objd)
{
        struct lttng_ht_node_ulong *node;
        struct lttng_ht_iter iter;
        struct ust_app_session *ua_sess = nullptr;

        LTTNG_ASSERT(app);
        ASSERT_RCU_READ_LOCKED();

        lttng_ht_lookup(app->ust_sessions_objd, (void *) ((unsigned long) objd), &iter);
        node = lttng_ht_iter_get_node_ulong(&iter);
        if (node == nullptr) {
                DBG2("UST app session find by objd %d not found", objd);
                goto error;
        }

        ua_sess = lttng::utils::container_of(node, &ust_app_session::ust_objd_node);

error:
        return ua_sess;
}

/*
 * Return a ust app channel object using the application object and the channel
 * object descriptor has a key. If not found, NULL is returned. A RCU read side
 * lock MUST be acquired before calling this function.
 */
static struct ust_app_channel *find_channel_by_objd(struct ust_app *app, int objd)
{
        struct lttng_ht_node_ulong *node;
        struct lttng_ht_iter iter;
        struct ust_app_channel *ua_chan = nullptr;

        LTTNG_ASSERT(app);
        ASSERT_RCU_READ_LOCKED();

        lttng_ht_lookup(app->ust_objd, (void *) ((unsigned long) objd), &iter);
        node = lttng_ht_iter_get_node_ulong(&iter);
        if (node == nullptr) {
                DBG2("UST app channel find by objd %d not found", objd);
                goto error;
        }

        ua_chan = lttng::utils::container_of(node, &ust_app_channel::ust_objd_node);

error:
        return ua_chan;
}

/*
 * Reply to a register channel notification from an application on the notify
 * socket. The channel metadata is also created.
 *
 * The session UST registry lock is acquired in this function.
 *
 * On success 0 is returned else a negative value.
 */
static int handle_app_register_channel_notification(int sock,
                                                    int cobjd,
                                                    struct lttng_ust_ctl_field *raw_context_fields,
                                                    size_t context_field_count)
{
        int ret, ret_code = 0;
        uint32_t chan_id;
        uint64_t chan_reg_key;
        struct ust_app *app;
        struct ust_app_channel *ua_chan;
        struct ust_app_session *ua_sess;
        auto ust_ctl_context_fields =
                lttng::make_unique_wrapper<lttng_ust_ctl_field, lttng::memory::free>(
                        raw_context_fields);

        lttng::urcu::read_lock_guard read_lock_guard;

        /* Lookup application. If not found, there is a code flow error. */
        app = find_app_by_notify_sock(sock);
        if (!app) {
                DBG("Application socket %d is being torn down. Abort event notify", sock);
                return -1;
        }

        /* Lookup channel by UST object descriptor. */
        ua_chan = find_channel_by_objd(app, cobjd);
        if (!ua_chan) {
                DBG("Application channel is being torn down. Abort event notify");
                return 0;
        }

        LTTNG_ASSERT(ua_chan->session);
        ua_sess = ua_chan->session;

        /* Get right session registry depending on the session buffer type. */
        auto locked_registry_session = get_locked_session_registry(ua_sess);
        if (!locked_registry_session) {
                DBG("Application session is being torn down. Abort event notify");
                return 0;
        };

        /* Depending on the buffer type, a different channel key is used. */
        if (ua_sess->buffer_type == LTTNG_BUFFER_PER_UID) {
                chan_reg_key = ua_chan->tracing_channel_id;
        } else {
                chan_reg_key = ua_chan->key;
        }

        auto& ust_reg_chan = locked_registry_session->channel(chan_reg_key);

        /* Channel id is set during the object creation. */
        chan_id = ust_reg_chan.id;

        /*
         * The application returns the typing information of the channel's
         * context fields. In per-PID buffering mode, this is the first and only
         * time we get this information. It is our chance to finalize the
         * initialiation of the channel and serialize it's layout's description
         * to the trace's metadata.
         *
         * However, in per-UID buffering mode, every application will provide
         * this information (redundantly). The first time will allow us to
         * complete the initialization. The following times, we simply validate
         * that all apps provide the same typing for the context fields as a
         * sanity check.
         */
        try {
                auto app_context_fields = lsu::create_trace_fields_from_ust_ctl_fields(
                        *locked_registry_session,
                        ust_ctl_context_fields.get(),
                        context_field_count,
                        lst::field_location::root::EVENT_RECORD_COMMON_CONTEXT,
                        lsu::ctl_field_quirks::UNDERSCORE_PREFIXED_VARIANT_TAG_MAPPINGS);

                if (!ust_reg_chan.is_registered()) {
                        lst::type::cuptr event_context = app_context_fields.size() ?
                                lttng::make_unique<lst::structure_type>(
                                        0, std::move(app_context_fields)) :
                                nullptr;

                        ust_reg_chan.event_context(std::move(event_context));
                } else {
                        /*
                         * Validate that the context fields match between
                         * registry and newcoming application.
                         */
                        bool context_fields_match;
                        const auto *previous_event_context = ust_reg_chan.event_context();

                        if (!previous_event_context) {
                                context_fields_match = app_context_fields.size() == 0;
                        } else {
                                const lst::structure_type app_event_context_struct(
                                        0, std::move(app_context_fields));

                                context_fields_match = *previous_event_context ==
                                        app_event_context_struct;
                        }

                        if (!context_fields_match) {
                                ERR("Registering application channel due to context field mismatch: pid = %d, sock = %d",
                                    app->pid,
                                    app->sock);
                                ret_code = -EINVAL;
                                goto reply;
                        }
                }
        } catch (const std::exception& ex) {
                ERR("Failed to handle application context: %s", ex.what());
                ret_code = -EINVAL;
                goto reply;
        }

reply:
        DBG3("UST app replying to register channel key %" PRIu64 " with id %u, ret = %d",
             chan_reg_key,
             chan_id,
             ret_code);

        ret = lttng_ust_ctl_reply_register_channel(
                sock,
                chan_id,
                ust_reg_chan.header_type_ == lst::stream_class::header_type::COMPACT ?
                        LTTNG_UST_CTL_CHANNEL_HEADER_COMPACT :
                        LTTNG_UST_CTL_CHANNEL_HEADER_LARGE,
                ret_code);
        if (ret < 0) {
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        DBG3("UST app reply channel failed. Application died: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else if (ret == -EAGAIN) {
                        WARN("UST app reply channel failed. Communication time out: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else {
                        ERR("UST app reply channel failed with ret %d: pid = %d, sock = %d",
                            ret,
                            app->pid,
                            app->sock);
                }

                return ret;
        }

        /* This channel registry's registration is completed. */
        ust_reg_chan.set_as_registered();

        return ret;
}

/*
 * Add event to the UST channel registry. When the event is added to the
 * registry, the metadata is also created. Once done, this replies to the
 * application with the appropriate error code.
 *
 * The session UST registry lock is acquired in the function.
 *
 * On success 0 is returned else a negative value.
 */
static int add_event_ust_registry(int sock,
                                  int sobjd,
                                  int cobjd,
                                  const char *name,
                                  char *raw_signature,
                                  size_t nr_fields,
                                  struct lttng_ust_ctl_field *raw_fields,
                                  int loglevel_value,
                                  char *raw_model_emf_uri)
{
        int ret, ret_code;
        uint32_t event_id = 0;
        uint64_t chan_reg_key;
        struct ust_app *app;
        struct ust_app_channel *ua_chan;
        struct ust_app_session *ua_sess;
        lttng::urcu::read_lock_guard rcu_lock;
        auto signature = lttng::make_unique_wrapper<char, lttng::memory::free>(raw_signature);
        auto fields =
                lttng::make_unique_wrapper<lttng_ust_ctl_field, lttng::memory::free>(raw_fields);
        auto model_emf_uri =
                lttng::make_unique_wrapper<char, lttng::memory::free>(raw_model_emf_uri);

        /* Lookup application. If not found, there is a code flow error. */
        app = find_app_by_notify_sock(sock);
        if (!app) {
                DBG("Application socket %d is being torn down. Abort event notify", sock);
                return -1;
        }

        /* Lookup channel by UST object descriptor. */
        ua_chan = find_channel_by_objd(app, cobjd);
        if (!ua_chan) {
                DBG("Application channel is being torn down. Abort event notify");
                return 0;
        }

        LTTNG_ASSERT(ua_chan->session);
        ua_sess = ua_chan->session;

        if (ua_sess->buffer_type == LTTNG_BUFFER_PER_UID) {
                chan_reg_key = ua_chan->tracing_channel_id;
        } else {
                chan_reg_key = ua_chan->key;
        }

        {
                auto locked_registry = get_locked_session_registry(ua_sess);
                if (locked_registry) {
                        /*
                         * From this point on, this call acquires the ownership of the signature,
                         * fields and model_emf_uri meaning any free are done inside it if needed.
                         * These three variables MUST NOT be read/write after this.
                         */
                        try {
                                auto& channel = locked_registry->channel(chan_reg_key);

                                /* event_id is set on success. */
                                channel.add_event(
                                        sobjd,
                                        cobjd,
                                        name,
                                        signature.get(),
                                        lsu::create_trace_fields_from_ust_ctl_fields(
                                                *locked_registry,
                                                fields.get(),
                                                nr_fields,
                                                lst::field_location::root::EVENT_RECORD_PAYLOAD,
                                                lsu::ctl_field_quirks::
                                                        UNDERSCORE_PREFIXED_VARIANT_TAG_MAPPINGS),
                                        loglevel_value,
                                        model_emf_uri.get() ?
                                                nonstd::optional<std::string>(model_emf_uri.get()) :
                                                nonstd::nullopt,
                                        ua_sess->buffer_type,
                                        *app,
                                        event_id);
                                ret_code = 0;
                        } catch (const std::exception& ex) {
                                ERR("Failed to add event `%s` to registry session: %s",
                                    name,
                                    ex.what());
                                /* Inform the application of the error; don't return directly. */
                                ret_code = -EINVAL;
                        }
                } else {
                        DBG("Application session is being torn down. Abort event notify");
                        return 0;
                }
        }

        /*
         * The return value is returned to ustctl so in case of an error, the
         * application can be notified. In case of an error, it's important not to
         * return a negative error or else the application will get closed.
         */
        ret = lttng_ust_ctl_reply_register_event(sock, event_id, ret_code);
        if (ret < 0) {
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        DBG3("UST app reply event failed. Application died: pid = %d, sock = %d.",
                             app->pid,
                             app->sock);
                } else if (ret == -EAGAIN) {
                        WARN("UST app reply event failed. Communication time out: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else {
                        ERR("UST app reply event failed with ret %d: pid = %d, sock = %d",
                            ret,
                            app->pid,
                            app->sock);
                }
                /*
                 * No need to wipe the create event since the application socket will
                 * get close on error hence cleaning up everything by itself.
                 */
                return ret;
        }

        DBG3("UST registry event %s with id %" PRId32 " added successfully", name, event_id);
        return ret;
}

/*
 * Add enum to the UST session registry. Once done, this replies to the
 * application with the appropriate error code.
 *
 * The session UST registry lock is acquired within this function.
 *
 * On success 0 is returned else a negative value.
 */
static int add_enum_ust_registry(int sock,
                                 int sobjd,
                                 const char *name,
                                 struct lttng_ust_ctl_enum_entry *raw_entries,
                                 size_t nr_entries)
{
        int ret = 0;
        struct ust_app *app;
        struct ust_app_session *ua_sess;
        uint64_t enum_id = -1ULL;
        lttng::urcu::read_lock_guard read_lock_guard;
        auto entries =
                lttng::make_unique_wrapper<struct lttng_ust_ctl_enum_entry, lttng::memory::free>(
                        raw_entries);

        /* Lookup application. If not found, there is a code flow error. */
        app = find_app_by_notify_sock(sock);
        if (!app) {
                /* Return an error since this is not an error */
                DBG("Application socket %d is being torn down. Aborting enum registration", sock);
                return -1;
        }

        /* Lookup session by UST object descriptor. */
        ua_sess = find_session_by_objd(app, sobjd);
        if (!ua_sess) {
                /* Return an error since this is not an error */
                DBG("Application session is being torn down (session not found). Aborting enum registration.");
                return 0;
        }

        auto locked_registry = get_locked_session_registry(ua_sess);
        if (!locked_registry) {
                DBG("Application session is being torn down (registry not found). Aborting enum registration.");
                return 0;
        }

        /*
         * From this point on, the callee acquires the ownership of
         * entries. The variable entries MUST NOT be read/written after
         * call.
         */
        int application_reply_code;
        try {
                locked_registry->create_or_find_enum(
                        sobjd, name, entries.release(), nr_entries, &enum_id);
                application_reply_code = 0;
        } catch (const std::exception& ex) {
                ERR("%s: %s",
                    lttng::format(
                            "Failed to create or find enumeration provided by application: app = {}, enumeration name = {}",
                            *app,
                            name)
                            .c_str(),
                    ex.what());
                application_reply_code = -1;
        }

        /*
         * The return value is returned to ustctl so in case of an error, the
         * application can be notified. In case of an error, it's important not to
         * return a negative error or else the application will get closed.
         */
        ret = lttng_ust_ctl_reply_register_enum(sock, enum_id, application_reply_code);
        if (ret < 0) {
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        DBG3("UST app reply enum failed. Application died: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else if (ret == -EAGAIN) {
                        WARN("UST app reply enum failed. Communication time out: pid = %d, sock = %d",
                             app->pid,
                             app->sock);
                } else {
                        ERR("UST app reply enum failed with ret %d: pid = %d, sock = %d",
                            ret,
                            app->pid,
                            app->sock);
                }
                /*
                 * No need to wipe the create enum since the application socket will
                 * get close on error hence cleaning up everything by itself.
                 */
                return ret;
        }

        DBG3("UST registry enum %s added successfully or already found", name);
        return 0;
}

/*
 * Handle application notification through the given notify socket.
 *
 * Return 0 on success or else a negative value.
 */
int ust_app_recv_notify(int sock)
{
        int ret;
        enum lttng_ust_ctl_notify_cmd cmd;

        DBG3("UST app receiving notify from sock %d", sock);

        ret = lttng_ust_ctl_recv_notify(sock, &cmd);
        if (ret < 0) {
                if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                        DBG3("UST app recv notify failed. Application died: sock = %d", sock);
                } else if (ret == -EAGAIN) {
                        WARN("UST app recv notify failed. Communication time out: sock = %d", sock);
                } else {
                        ERR("UST app recv notify failed with ret %d: sock = %d", ret, sock);
                }
                goto error;
        }

        switch (cmd) {
        case LTTNG_UST_CTL_NOTIFY_CMD_EVENT:
        {
                int sobjd, cobjd, loglevel_value;
                char name[LTTNG_UST_ABI_SYM_NAME_LEN], *sig, *model_emf_uri;
                size_t nr_fields;
                struct lttng_ust_ctl_field *fields;

                DBG2("UST app ustctl register event received");

                ret = lttng_ust_ctl_recv_register_event(sock,
                                                        &sobjd,
                                                        &cobjd,
                                                        name,
                                                        &loglevel_value,
                                                        &sig,
                                                        &nr_fields,
                                                        &fields,
                                                        &model_emf_uri);
                if (ret < 0) {
                        if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                                DBG3("UST app recv event failed. Application died: sock = %d",
                                     sock);
                        } else if (ret == -EAGAIN) {
                                WARN("UST app recv event failed. Communication time out: sock = %d",
                                     sock);
                        } else {
                                ERR("UST app recv event failed with ret %d: sock = %d", ret, sock);
                        }
                        goto error;
                }

                {
                        lttng::urcu::read_lock_guard rcu_lock;
                        const struct ust_app *app = find_app_by_notify_sock(sock);
                        if (!app) {
                                DBG("Application socket %d is being torn down. Abort event notify",
                                    sock);
                                ret = -1;
                                goto error;
                        }
                }

                if ((!fields && nr_fields > 0) || (fields && nr_fields == 0)) {
                        ERR("Invalid return value from lttng_ust_ctl_recv_register_event: fields = %p, nr_fields = %zu",
                            fields,
                            nr_fields);
                        ret = -1;
                        free(fields);
                        goto error;
                }

                /*
                 * Add event to the UST registry coming from the notify socket. This
                 * call will free if needed the sig, fields and model_emf_uri. This
                 * code path loses the ownsership of these variables and transfer them
                 * to the this function.
                 */
                ret = add_event_ust_registry(sock,
                                             sobjd,
                                             cobjd,
                                             name,
                                             sig,
                                             nr_fields,
                                             fields,
                                             loglevel_value,
                                             model_emf_uri);
                if (ret < 0) {
                        goto error;
                }

                break;
        }
        case LTTNG_UST_CTL_NOTIFY_CMD_CHANNEL:
        {
                int sobjd, cobjd;
                size_t field_count;
                struct lttng_ust_ctl_field *context_fields;

                DBG2("UST app ustctl register channel received");

                ret = lttng_ust_ctl_recv_register_channel(
                        sock, &sobjd, &cobjd, &field_count, &context_fields);
                if (ret < 0) {
                        if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                                DBG3("UST app recv channel failed. Application died: sock = %d",
                                     sock);
                        } else if (ret == -EAGAIN) {
                                WARN("UST app recv channel failed. Communication time out: sock = %d",
                                     sock);
                        } else {
                                ERR("UST app recv channel failed with ret %d: sock = %d",
                                    ret,
                                    sock);
                        }
                        goto error;
                }

                /*
                 * The fields ownership are transfered to this function call meaning
                 * that if needed it will be freed. After this, it's invalid to access
                 * fields or clean them up.
                 */
                ret = handle_app_register_channel_notification(
                        sock, cobjd, context_fields, field_count);
                if (ret < 0) {
                        goto error;
                }

                break;
        }
        case LTTNG_UST_CTL_NOTIFY_CMD_ENUM:
        {
                int sobjd;
                char name[LTTNG_UST_ABI_SYM_NAME_LEN];
                size_t nr_entries;
                struct lttng_ust_ctl_enum_entry *entries;

                DBG2("UST app ustctl register enum received");

                ret = lttng_ust_ctl_recv_register_enum(sock, &sobjd, name, &entries, &nr_entries);
                if (ret < 0) {
                        if (ret == -EPIPE || ret == -LTTNG_UST_ERR_EXITING) {
                                DBG3("UST app recv enum failed. Application died: sock = %d", sock);
                        } else if (ret == -EAGAIN) {
                                WARN("UST app recv enum failed. Communication time out: sock = %d",
                                     sock);
                        } else {
                                ERR("UST app recv enum failed with ret %d: sock = %d", ret, sock);
                        }
                        goto error;
                }

                /* Callee assumes ownership of entries. */
                ret = add_enum_ust_registry(sock, sobjd, name, entries, nr_entries);
                if (ret < 0) {
                        goto error;
                }

                break;
        }
        default:
                /* Should NEVER happen. */
                abort();
        }

error:
        return ret;
}

/*
 * Once the notify socket hangs up, this is called. First, it tries to find the
 * corresponding application. On failure, the call_rcu to close the socket is
 * executed. If an application is found, it tries to delete it from the notify
 * socket hash table. Whathever the result, it proceeds to the call_rcu.
 *
 * Note that an object needs to be allocated here so on ENOMEM failure, the
 * call RCU is not done but the rest of the cleanup is.
 */
void ust_app_notify_sock_unregister(int sock)
{
        int err_enomem = 0;
        struct lttng_ht_iter iter;
        struct ust_app *app;
        struct ust_app_notify_sock_obj *obj;

        LTTNG_ASSERT(sock >= 0);

        lttng::urcu::read_lock_guard read_lock;

        obj = zmalloc<ust_app_notify_sock_obj>();
        if (!obj) {
                /*
                 * An ENOMEM is kind of uncool. If this strikes we continue the
                 * procedure but the call_rcu will not be called. In this case, we
                 * accept the fd leak rather than possibly creating an unsynchronized
                 * state between threads.
                 *
                 * TODO: The notify object should be created once the notify socket is
                 * registered and stored independantely from the ust app object. The
                 * tricky part is to synchronize the teardown of the application and
                 * this notify object. Let's keep that in mind so we can avoid this
                 * kind of shenanigans with ENOMEM in the teardown path.
                 */
                err_enomem = 1;
        } else {
                obj->fd = sock;
        }

        DBG("UST app notify socket unregister %d", sock);

        /*
         * Lookup application by notify socket. If this fails, this means that the
         * hash table delete has already been done by the application
         * unregistration process so we can safely close the notify socket in a
         * call RCU.
         */
        app = find_app_by_notify_sock(sock);
        if (!app) {
                goto close_socket;
        }

        iter.iter.node = &app->notify_sock_n.node;

        /*
         * Whatever happens here either we fail or succeed, in both cases we have
         * to close the socket after a grace period to continue to the call RCU
         * here. If the deletion is successful, the application is not visible
         * anymore by other threads and is it fails it means that it was already
         * deleted from the hash table so either way we just have to close the
         * socket.
         */
        (void) lttng_ht_del(ust_app_ht_by_notify_sock, &iter);

close_socket:

        /*
         * Close socket after a grace period to avoid for the socket to be reused
         * before the application object is freed creating potential race between
         * threads trying to add unique in the global hash table.
         */
        if (!err_enomem) {
                call_rcu(&obj->head, close_notify_sock_rcu);
        }
}

/*
 * Destroy a ust app data structure and free its memory.
 */
static void ust_app_destroy(ust_app& app)
{
        call_rcu(&app.pid_n.head, delete_ust_app_rcu);
}

/*
 * Take a snapshot for a given UST session. The snapshot is sent to the given
 * output.
 *
 * Returns LTTNG_OK on success or a LTTNG_ERR error code.
 */
enum lttng_error_code ust_app_snapshot_record(const struct ltt_ust_session *usess,
                                              const struct consumer_output *output,
                                              uint64_t nb_packets_per_stream)
{
        int ret = 0;
        enum lttng_error_code status = LTTNG_OK;
        struct lttng_ht_iter iter;
        struct ust_app *app;
        char *trace_path = nullptr;

        LTTNG_ASSERT(usess);
        LTTNG_ASSERT(output);

        switch (usess->buffer_type) {
        case LTTNG_BUFFER_PER_UID:
        {
                struct buffer_reg_uid *reg;

                lttng::urcu::read_lock_guard read_lock;

                cds_list_for_each_entry (reg, &usess->buffer_reg_uid_list, lnode) {
                        struct buffer_reg_channel *buf_reg_chan;
                        struct consumer_socket *socket;
                        char pathname[PATH_MAX];
                        size_t consumer_path_offset = 0;

                        if (!reg->registry->reg.ust->_metadata_key) {
                                /* Skip since no metadata is present */
                                continue;
                        }

                        /* Get consumer socket to use to push the metadata.*/
                        socket = consumer_find_socket_by_bitness(reg->bits_per_long,
                                                                 usess->consumer);
                        if (!socket) {
                                status = LTTNG_ERR_INVALID;
                                goto error;
                        }

                        memset(pathname, 0, sizeof(pathname));
                        ret = snprintf(pathname,
                                       sizeof(pathname),
                                       DEFAULT_UST_TRACE_UID_PATH,
                                       reg->uid,
                                       reg->bits_per_long);
                        if (ret < 0) {
                                PERROR("snprintf snapshot path");
                                status = LTTNG_ERR_INVALID;
                                goto error;
                        }
                        /* Free path allowed on previous iteration. */
                        free(trace_path);
                        trace_path = setup_channel_trace_path(
                                usess->consumer, pathname, &consumer_path_offset);
                        if (!trace_path) {
                                status = LTTNG_ERR_INVALID;
                                goto error;
                        }
                        /* Add the UST default trace dir to path. */
                        cds_lfht_for_each_entry (
                                reg->registry->channels->ht, &iter.iter, buf_reg_chan, node.node) {
                                status =
                                        consumer_snapshot_channel(socket,
                                                                  buf_reg_chan->consumer_key,
                                                                  output,
                                                                  0,
                                                                  &trace_path[consumer_path_offset],
                                                                  nb_packets_per_stream);
                                if (status != LTTNG_OK) {
                                        goto error;
                                }
                        }
                        status = consumer_snapshot_channel(socket,
                                                           reg->registry->reg.ust->_metadata_key,
                                                           output,
                                                           1,
                                                           &trace_path[consumer_path_offset],
                                                           0);
                        if (status != LTTNG_OK) {
                                goto error;
                        }
                }

                break;
        }
        case LTTNG_BUFFER_PER_PID:
        {
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        struct consumer_socket *socket;
                        struct lttng_ht_iter chan_iter;
                        struct ust_app_channel *ua_chan;
                        struct ust_app_session *ua_sess;
                        lsu::registry_session *registry;
                        char pathname[PATH_MAX];
                        size_t consumer_path_offset = 0;

                        ua_sess = lookup_session_by_app(usess, app);
                        if (!ua_sess) {
                                /* Session not associated with this app. */
                                continue;
                        }

                        /* Get the right consumer socket for the application. */
                        socket = consumer_find_socket_by_bitness(app->abi.bits_per_long, output);
                        if (!socket) {
                                status = LTTNG_ERR_INVALID;
                                goto error;
                        }

                        /* Add the UST default trace dir to path. */
                        memset(pathname, 0, sizeof(pathname));
                        ret = snprintf(pathname, sizeof(pathname), "%s", ua_sess->path);
                        if (ret < 0) {
                                status = LTTNG_ERR_INVALID;
                                PERROR("snprintf snapshot path");
                                goto error;
                        }
                        /* Free path allowed on previous iteration. */
                        free(trace_path);
                        trace_path = setup_channel_trace_path(
                                usess->consumer, pathname, &consumer_path_offset);
                        if (!trace_path) {
                                status = LTTNG_ERR_INVALID;
                                goto error;
                        }
                        cds_lfht_for_each_entry (
                                ua_sess->channels->ht, &chan_iter.iter, ua_chan, node.node) {
                                status =
                                        consumer_snapshot_channel(socket,
                                                                  ua_chan->key,
                                                                  output,
                                                                  0,
                                                                  &trace_path[consumer_path_offset],
                                                                  nb_packets_per_stream);
                                switch (status) {
                                case LTTNG_OK:
                                        break;
                                case LTTNG_ERR_CHAN_NOT_FOUND:
                                        continue;
                                default:
                                        goto error;
                                }
                        }

                        registry = get_session_registry(ua_sess);
                        if (!registry) {
                                DBG("Application session is being torn down. Skip application.");
                                continue;
                        }
                        status = consumer_snapshot_channel(socket,
                                                           registry->_metadata_key,
                                                           output,
                                                           1,
                                                           &trace_path[consumer_path_offset],
                                                           0);
                        switch (status) {
                        case LTTNG_OK:
                                break;
                        case LTTNG_ERR_CHAN_NOT_FOUND:
                                continue;
                        default:
                                goto error;
                        }
                }
                break;
        }
        default:
                abort();
                break;
        }

error:
        free(trace_path);
        return status;
}

/*
 * Return the size taken by one more packet per stream.
 */
uint64_t ust_app_get_size_one_more_packet_per_stream(const struct ltt_ust_session *usess,
                                                     uint64_t cur_nr_packets)
{
        uint64_t tot_size = 0;
        struct ust_app *app;
        struct lttng_ht_iter iter;

        LTTNG_ASSERT(usess);

        switch (usess->buffer_type) {
        case LTTNG_BUFFER_PER_UID:
        {
                struct buffer_reg_uid *reg;

                cds_list_for_each_entry (reg, &usess->buffer_reg_uid_list, lnode) {
                        struct buffer_reg_channel *buf_reg_chan;

                        lttng::urcu::read_lock_guard read_lock;

                        cds_lfht_for_each_entry (
                                reg->registry->channels->ht, &iter.iter, buf_reg_chan, node.node) {
                                if (cur_nr_packets >= buf_reg_chan->num_subbuf) {
                                        /*
                                         * Don't take channel into account if we
                                         * already grab all its packets.
                                         */
                                        continue;
                                }
                                tot_size += buf_reg_chan->subbuf_size * buf_reg_chan->stream_count;
                        }
                }
                break;
        }
        case LTTNG_BUFFER_PER_PID:
        {
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        struct ust_app_channel *ua_chan;
                        struct ust_app_session *ua_sess;
                        struct lttng_ht_iter chan_iter;

                        ua_sess = lookup_session_by_app(usess, app);
                        if (!ua_sess) {
                                /* Session not associated with this app. */
                                continue;
                        }

                        cds_lfht_for_each_entry (
                                ua_sess->channels->ht, &chan_iter.iter, ua_chan, node.node) {
                                if (cur_nr_packets >= ua_chan->attr.num_subbuf) {
                                        /*
                                         * Don't take channel into account if we
                                         * already grab all its packets.
                                         */
                                        continue;
                                }
                                tot_size += ua_chan->attr.subbuf_size * ua_chan->streams.count;
                        }
                }
                break;
        }
        default:
                abort();
                break;
        }

        return tot_size;
}

int ust_app_uid_get_channel_runtime_stats(uint64_t ust_session_id,
                                          struct cds_list_head *buffer_reg_uid_list,
                                          struct consumer_output *consumer,
                                          uint64_t uchan_id,
                                          int overwrite,
                                          uint64_t *discarded,
                                          uint64_t *lost)
{
        int ret;
        uint64_t consumer_chan_key;

        *discarded = 0;
        *lost = 0;

        ret = buffer_reg_uid_consumer_channel_key(
                buffer_reg_uid_list, uchan_id, &consumer_chan_key);
        if (ret < 0) {
                /* Not found */
                ret = 0;
                goto end;
        }

        if (overwrite) {
                ret = consumer_get_lost_packets(ust_session_id, consumer_chan_key, consumer, lost);
        } else {
                ret = consumer_get_discarded_events(
                        ust_session_id, consumer_chan_key, consumer, discarded);
        }

end:
        return ret;
}

int ust_app_pid_get_channel_runtime_stats(struct ltt_ust_session *usess,
                                          struct ltt_ust_channel *uchan,
                                          struct consumer_output *consumer,
                                          int overwrite,
                                          uint64_t *discarded,
                                          uint64_t *lost)
{
        int ret = 0;
        struct lttng_ht_iter iter;
        struct lttng_ht_node_str *ua_chan_node;
        struct ust_app *app;
        struct ust_app_session *ua_sess;
        struct ust_app_channel *ua_chan;

        *discarded = 0;
        *lost = 0;

        /*
         * Iterate over every registered applications. Sum counters for
         * all applications containing requested session and channel.
         */
        lttng::urcu::read_lock_guard read_lock;

        cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                struct lttng_ht_iter uiter;

                ua_sess = lookup_session_by_app(usess, app);
                if (ua_sess == nullptr) {
                        continue;
                }

                /* Get channel */
                lttng_ht_lookup(ua_sess->channels, (void *) uchan->name, &uiter);
                ua_chan_node = lttng_ht_iter_get_node_str(&uiter);
                /* If the session is found for the app, the channel must be there */
                LTTNG_ASSERT(ua_chan_node);

                ua_chan = lttng::utils::container_of(ua_chan_node, &ust_app_channel::node);

                if (overwrite) {
                        uint64_t _lost;

                        ret = consumer_get_lost_packets(usess->id, ua_chan->key, consumer, &_lost);
                        if (ret < 0) {
                                break;
                        }
                        (*lost) += _lost;
                } else {
                        uint64_t _discarded;

                        ret = consumer_get_discarded_events(
                                usess->id, ua_chan->key, consumer, &_discarded);
                        if (ret < 0) {
                                break;
                        }
                        (*discarded) += _discarded;
                }
        }

        return ret;
}

static int ust_app_regenerate_statedump(struct ltt_ust_session *usess, struct ust_app *app)
{
        int ret = 0;
        struct ust_app_session *ua_sess;

        DBG("Regenerating the metadata for ust app pid %d", app->pid);

        lttng::urcu::read_lock_guard read_lock;

        ua_sess = lookup_session_by_app(usess, app);
        if (ua_sess == nullptr) {
                /* The session is in teardown process. Ignore and continue. */
                goto end;
        }

        pthread_mutex_lock(&ua_sess->lock);

        if (ua_sess->deleted) {
                goto end_unlock;
        }

        pthread_mutex_lock(&app->sock_lock);
        ret = lttng_ust_ctl_regenerate_statedump(app->sock, ua_sess->handle);
        pthread_mutex_unlock(&app->sock_lock);

end_unlock:
        pthread_mutex_unlock(&ua_sess->lock);

end:
        health_code_update();
        return ret;
}

/*
 * Regenerate the statedump for each app in the session.
 */
int ust_app_regenerate_statedump_all(struct ltt_ust_session *usess)
{
        int ret = 0;
        struct lttng_ht_iter iter;
        struct ust_app *app;

        DBG("Regenerating the metadata for all UST apps");

        lttng::urcu::read_lock_guard read_lock;

        cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                if (!app->compatible) {
                        continue;
                }

                ret = ust_app_regenerate_statedump(usess, app);
                if (ret < 0) {
                        /* Continue to the next app even on error */
                        continue;
                }
        }

        return 0;
}

/*
 * Rotate all the channels of a session.
 *
 * Return LTTNG_OK on success or else an LTTng error code.
 */
enum lttng_error_code ust_app_rotate_session(struct ltt_session *session)
{
        int ret;
        enum lttng_error_code cmd_ret = LTTNG_OK;
        struct lttng_ht_iter iter;
        struct ltt_ust_session *usess = session->ust_session;

        LTTNG_ASSERT(usess);

        switch (usess->buffer_type) {
        case LTTNG_BUFFER_PER_UID:
        {
                struct buffer_reg_uid *reg;

                cds_list_for_each_entry (reg, &usess->buffer_reg_uid_list, lnode) {
                        struct buffer_reg_channel *buf_reg_chan;
                        struct consumer_socket *socket;
                        lttng::urcu::read_lock_guard read_lock;

                        /* Get consumer socket to use to push the metadata.*/
                        socket = consumer_find_socket_by_bitness(reg->bits_per_long,
                                                                 usess->consumer);
                        if (!socket) {
                                cmd_ret = LTTNG_ERR_INVALID;
                                goto error;
                        }

                        /* Rotate the data channels. */
                        cds_lfht_for_each_entry (
                                reg->registry->channels->ht, &iter.iter, buf_reg_chan, node.node) {
                                ret = consumer_rotate_channel(socket,
                                                              buf_reg_chan->consumer_key,
                                                              usess->consumer,
                                                              /* is_metadata_channel */ false);
                                if (ret < 0) {
                                        cmd_ret = LTTNG_ERR_ROTATION_FAIL_CONSUMER;
                                        goto error;
                                }
                        }

                        /*
                         * The metadata channel might not be present.
                         *
                         * Consumer stream allocation can be done
                         * asynchronously and can fail on intermediary
                         * operations (i.e add context) and lead to data
                         * channels created with no metadata channel.
                         */
                        if (!reg->registry->reg.ust->_metadata_key) {
                                /* Skip since no metadata is present. */
                                continue;
                        }

                        {
                                auto locked_registry = reg->registry->reg.ust->lock();
                                (void) push_metadata(locked_registry, usess->consumer);
                        }

                        ret = consumer_rotate_channel(socket,
                                                      reg->registry->reg.ust->_metadata_key,
                                                      usess->consumer,
                                                      /* is_metadata_channel */ true);
                        if (ret < 0) {
                                cmd_ret = LTTNG_ERR_ROTATION_FAIL_CONSUMER;
                                goto error;
                        }
                }
                break;
        }
        case LTTNG_BUFFER_PER_PID:
        {
                lttng::urcu::read_lock_guard read_lock;
                ust_app *raw_app;

                cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, raw_app, pid_n.node) {
                        struct consumer_socket *socket;
                        struct lttng_ht_iter chan_iter;
                        struct ust_app_channel *ua_chan;
                        struct ust_app_session *ua_sess;
                        lsu::registry_session *registry;
                        bool app_reference_taken;

                        app_reference_taken = ust_app_get(*raw_app);
                        if (!app_reference_taken) {
                                /* Application unregistered concurrently, skip it. */
                                DBG("Could not get application reference as it is being torn down; skipping application");
                                continue;
                        }

                        ust_app_reference app(raw_app);
                        raw_app = nullptr;

                        ua_sess = lookup_session_by_app(usess, app.get());
                        if (!ua_sess) {
                                /* Session not associated with this app. */
                                continue;
                        }

                        /* Get the right consumer socket for the application. */
                        socket = consumer_find_socket_by_bitness(app->abi.bits_per_long,
                                                                 usess->consumer);
                        if (!socket) {
                                cmd_ret = LTTNG_ERR_INVALID;
                                goto error;
                        }

                        registry = get_session_registry(ua_sess);
                        LTTNG_ASSERT(registry);

                        /* Rotate the data channels. */
                        cds_lfht_for_each_entry (
                                ua_sess->channels->ht, &chan_iter.iter, ua_chan, node.node) {
                                ret = consumer_rotate_channel(socket,
                                                              ua_chan->key,
                                                              ua_sess->consumer,
                                                              /* is_metadata_channel */ false);
                                if (ret < 0) {
                                        cmd_ret = LTTNG_ERR_ROTATION_FAIL_CONSUMER;
                                        goto error;
                                }
                        }

                        /* Rotate the metadata channel. */
                        {
                                auto locked_registry = registry->lock();

                                (void) push_metadata(locked_registry, usess->consumer);
                        }

                        ret = consumer_rotate_channel(socket,
                                                      registry->_metadata_key,
                                                      ua_sess->consumer,
                                                      /* is_metadata_channel */ true);
                        if (ret < 0) {
                                cmd_ret = LTTNG_ERR_ROTATION_FAIL_CONSUMER;
                                goto error;
                        }
                }

                break;
        }
        default:
                abort();
                break;
        }

        cmd_ret = LTTNG_OK;

error:
        return cmd_ret;
}

enum lttng_error_code ust_app_create_channel_subdirectories(const struct ltt_ust_session *usess)
{
        enum lttng_error_code ret = LTTNG_OK;
        struct lttng_ht_iter iter;
        enum lttng_trace_chunk_status chunk_status;
        char *pathname_index;
        int fmt_ret;

        LTTNG_ASSERT(usess->current_trace_chunk);

        switch (usess->buffer_type) {
        case LTTNG_BUFFER_PER_UID:
        {
                struct buffer_reg_uid *reg;
                lttng::urcu::read_lock_guard read_lock;

                cds_list_for_each_entry (reg, &usess->buffer_reg_uid_list, lnode) {
                        fmt_ret = asprintf(&pathname_index,
                                           DEFAULT_UST_TRACE_DIR "/" DEFAULT_UST_TRACE_UID_PATH
                                                                 "/" DEFAULT_INDEX_DIR,
                                           reg->uid,
                                           reg->bits_per_long);
                        if (fmt_ret < 0) {
                                ERR("Failed to format channel index directory");
                                ret = LTTNG_ERR_CREATE_DIR_FAIL;
                                goto error;
                        }

                        /*
                         * Create the index subdirectory which will take care
                         * of implicitly creating the channel's path.
                         */
                        chunk_status = lttng_trace_chunk_create_subdirectory(
                                usess->current_trace_chunk, pathname_index);
                        free(pathname_index);
                        if (chunk_status != LTTNG_TRACE_CHUNK_STATUS_OK) {
                                ret = LTTNG_ERR_CREATE_DIR_FAIL;
                                goto error;
                        }
                }
                break;
        }
        case LTTNG_BUFFER_PER_PID:
        {
                struct ust_app *app;
                lttng::urcu::read_lock_guard read_lock;

                /*
                 * Create the toplevel ust/ directory in case no apps are running.
                 */
                chunk_status = lttng_trace_chunk_create_subdirectory(usess->current_trace_chunk,
                                                                     DEFAULT_UST_TRACE_DIR);
                if (chunk_status != LTTNG_TRACE_CHUNK_STATUS_OK) {
                        ret = LTTNG_ERR_CREATE_DIR_FAIL;
                        goto error;
                }

                cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        struct ust_app_session *ua_sess;
                        lsu::registry_session *registry;

                        ua_sess = lookup_session_by_app(usess, app);
                        if (!ua_sess) {
                                /* Session not associated with this app. */
                                continue;
                        }

                        registry = get_session_registry(ua_sess);
                        if (!registry) {
                                DBG("Application session is being torn down. Skip application.");
                                continue;
                        }

                        fmt_ret = asprintf(&pathname_index,
                                           DEFAULT_UST_TRACE_DIR "/%s/" DEFAULT_INDEX_DIR,
                                           ua_sess->path);
                        if (fmt_ret < 0) {
                                ERR("Failed to format channel index directory");
                                ret = LTTNG_ERR_CREATE_DIR_FAIL;
                                goto error;
                        }
                        /*
                         * Create the index subdirectory which will take care
                         * of implicitly creating the channel's path.
                         */
                        chunk_status = lttng_trace_chunk_create_subdirectory(
                                usess->current_trace_chunk, pathname_index);
                        free(pathname_index);
                        if (chunk_status != LTTNG_TRACE_CHUNK_STATUS_OK) {
                                ret = LTTNG_ERR_CREATE_DIR_FAIL;
                                goto error;
                        }
                }
                break;
        }
        default:
                abort();
        }

        ret = LTTNG_OK;
error:
        return ret;
}

/*
 * Clear all the channels of a session.
 *
 * Return LTTNG_OK on success or else an LTTng error code.
 */
enum lttng_error_code ust_app_clear_session(struct ltt_session *session)
{
        int ret;
        enum lttng_error_code cmd_ret = LTTNG_OK;
        struct lttng_ht_iter iter;
        struct ust_app *app;
        struct ltt_ust_session *usess = session->ust_session;

        LTTNG_ASSERT(usess);

        if (usess->active) {
                ERR("Expecting inactive session %s (%" PRIu64 ")", session->name, session->id);
                cmd_ret = LTTNG_ERR_FATAL;
                goto end;
        }

        switch (usess->buffer_type) {
        case LTTNG_BUFFER_PER_UID:
        {
                struct buffer_reg_uid *reg;
                lttng::urcu::read_lock_guard read_lock;

                cds_list_for_each_entry (reg, &usess->buffer_reg_uid_list, lnode) {
                        struct buffer_reg_channel *buf_reg_chan;
                        struct consumer_socket *socket;

                        /* Get consumer socket to use to push the metadata.*/
                        socket = consumer_find_socket_by_bitness(reg->bits_per_long,
                                                                 usess->consumer);
                        if (!socket) {
                                cmd_ret = LTTNG_ERR_INVALID;
                                goto error_socket;
                        }

                        /* Clear the data channels. */
                        cds_lfht_for_each_entry (
                                reg->registry->channels->ht, &iter.iter, buf_reg_chan, node.node) {
                                ret = consumer_clear_channel(socket, buf_reg_chan->consumer_key);
                                if (ret < 0) {
                                        goto error;
                                }
                        }

                        {
                                auto locked_registry = reg->registry->reg.ust->lock();
                                (void) push_metadata(locked_registry, usess->consumer);
                        }

                        /*
                         * Clear the metadata channel.
                         * Metadata channel is not cleared per se but we still need to
                         * perform a rotation operation on it behind the scene.
                         */
                        ret = consumer_clear_channel(socket, reg->registry->reg.ust->_metadata_key);
                        if (ret < 0) {
                                goto error;
                        }
                }
                break;
        }
        case LTTNG_BUFFER_PER_PID:
        {
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        struct consumer_socket *socket;
                        struct lttng_ht_iter chan_iter;
                        struct ust_app_channel *ua_chan;
                        struct ust_app_session *ua_sess;
                        lsu::registry_session *registry;

                        ua_sess = lookup_session_by_app(usess, app);
                        if (!ua_sess) {
                                /* Session not associated with this app. */
                                continue;
                        }

                        /* Get the right consumer socket for the application. */
                        socket = consumer_find_socket_by_bitness(app->abi.bits_per_long,
                                                                 usess->consumer);
                        if (!socket) {
                                cmd_ret = LTTNG_ERR_INVALID;
                                goto error_socket;
                        }

                        registry = get_session_registry(ua_sess);
                        if (!registry) {
                                DBG("Application session is being torn down. Skip application.");
                                continue;
                        }

                        /* Clear the data channels. */
                        cds_lfht_for_each_entry (
                                ua_sess->channels->ht, &chan_iter.iter, ua_chan, node.node) {
                                ret = consumer_clear_channel(socket, ua_chan->key);
                                if (ret < 0) {
                                        /* Per-PID buffer and application going away. */
                                        if (ret == -LTTNG_ERR_CHAN_NOT_FOUND) {
                                                continue;
                                        }
                                        goto error;
                                }
                        }

                        {
                                auto locked_registry = registry->lock();
                                (void) push_metadata(locked_registry, usess->consumer);
                        }

                        /*
                         * Clear the metadata channel.
                         * Metadata channel is not cleared per se but we still need to
                         * perform rotation operation on it behind the scene.
                         */
                        ret = consumer_clear_channel(socket, registry->_metadata_key);
                        if (ret < 0) {
                                /* Per-PID buffer and application going away. */
                                if (ret == -LTTNG_ERR_CHAN_NOT_FOUND) {
                                        continue;
                                }
                                goto error;
                        }
                }
                break;
        }
        default:
                abort();
                break;
        }

        cmd_ret = LTTNG_OK;
        goto end;

error:
        switch (-ret) {
        case LTTCOMM_CONSUMERD_RELAYD_CLEAR_DISALLOWED:
                cmd_ret = LTTNG_ERR_CLEAR_RELAY_DISALLOWED;
                break;
        default:
                cmd_ret = LTTNG_ERR_CLEAR_FAIL_CONSUMER;
        }

error_socket:
end:
        return cmd_ret;
}

/*
 * This function skips the metadata channel as the begin/end timestamps of a
 * metadata packet are useless.
 *
 * Moreover, opening a packet after a "clear" will cause problems for live
 * sessions as it will introduce padding that was not part of the first trace
 * chunk. The relay daemon expects the content of the metadata stream of
 * successive metadata trace chunks to be strict supersets of one another.
 *
 * For example, flushing a packet at the beginning of the metadata stream of
 * a trace chunk resulting from a "clear" session command will cause the
 * size of the metadata stream of the new trace chunk to not match the size of
 * the metadata stream of the original chunk. This will confuse the relay
 * daemon as the same "offset" in a metadata stream will no longer point
 * to the same content.
 */
enum lttng_error_code ust_app_open_packets(struct ltt_session *session)
{
        enum lttng_error_code ret = LTTNG_OK;
        struct lttng_ht_iter iter;
        struct ltt_ust_session *usess = session->ust_session;

        LTTNG_ASSERT(usess);

        switch (usess->buffer_type) {
        case LTTNG_BUFFER_PER_UID:
        {
                struct buffer_reg_uid *reg;

                cds_list_for_each_entry (reg, &usess->buffer_reg_uid_list, lnode) {
                        struct buffer_reg_channel *buf_reg_chan;
                        struct consumer_socket *socket;
                        lttng::urcu::read_lock_guard read_lock;

                        socket = consumer_find_socket_by_bitness(reg->bits_per_long,
                                                                 usess->consumer);
                        if (!socket) {
                                ret = LTTNG_ERR_FATAL;
                                goto error;
                        }

                        cds_lfht_for_each_entry (
                                reg->registry->channels->ht, &iter.iter, buf_reg_chan, node.node) {
                                const int open_ret = consumer_open_channel_packets(
                                        socket, buf_reg_chan->consumer_key);

                                if (open_ret < 0) {
                                        ret = LTTNG_ERR_UNK;
                                        goto error;
                                }
                        }
                }
                break;
        }
        case LTTNG_BUFFER_PER_PID:
        {
                struct ust_app *app;
                lttng::urcu::read_lock_guard read_lock;

                cds_lfht_for_each_entry (ust_app_ht->ht, &iter.iter, app, pid_n.node) {
                        struct consumer_socket *socket;
                        struct lttng_ht_iter chan_iter;
                        struct ust_app_channel *ua_chan;
                        struct ust_app_session *ua_sess;
                        lsu::registry_session *registry;

                        ua_sess = lookup_session_by_app(usess, app);
                        if (!ua_sess) {
                                /* Session not associated with this app. */
                                continue;
                        }

                        /* Get the right consumer socket for the application. */
                        socket = consumer_find_socket_by_bitness(app->abi.bits_per_long,
                                                                 usess->consumer);
                        if (!socket) {
                                ret = LTTNG_ERR_FATAL;
                                goto error;
                        }

                        registry = get_session_registry(ua_sess);
                        if (!registry) {
                                DBG("Application session is being torn down. Skip application.");
                                continue;
                        }

                        cds_lfht_for_each_entry (
                                ua_sess->channels->ht, &chan_iter.iter, ua_chan, node.node) {
                                const int open_ret =
                                        consumer_open_channel_packets(socket, ua_chan->key);

                                if (open_ret < 0) {
                                        /*
                                         * Per-PID buffer and application going
                                         * away.
                                         */
                                        if (open_ret == -LTTNG_ERR_CHAN_NOT_FOUND) {
                                                continue;
                                        }

                                        ret = LTTNG_ERR_UNK;
                                        goto error;
                                }
                        }
                }
                break;
        }
        default:
                abort();
                break;
        }

error:
        return ret;
}

lsu::ctl_field_quirks ust_app::ctl_field_quirks() const
{
        /*
         * Application contexts are expressed as variants. LTTng-UST announces
         * those by registering an enumeration named `..._tag`. It then registers a
         * variant as part of the event context that contains the various possible
         * types.
         *
         * Unfortunately, the names used in the enumeration and variant don't
         * match: the enumeration names are all prefixed with an underscore while
         * the variant type tag fields aren't.
         *
         * While the CTF 1.8.3 specification mentions that
         * underscores *should* (not *must*) be removed by CTF readers. Babeltrace
         * 1.x (and possibly others) expect a perfect match between the names used
         * by tags and variants.
         *
         * When the UNDERSCORE_PREFIXED_VARIANT_TAG_MAPPINGS quirk is enabled,
         * the variant's fields are modified to match the mappings of its tag.
         *
         * From ABI version >= 10.x, the variant fields and tag mapping names
         * correctly match, making this quirk unnecessary.
         */
        return v_major <= 9 ? lsu::ctl_field_quirks::UNDERSCORE_PREFIXED_VARIANT_TAG_MAPPINGS :
                              lsu::ctl_field_quirks::NONE;
}

static void ust_app_release(urcu_ref *ref)
{
        auto& app = *lttng::utils::container_of(ref, &ust_app::ref);

        ust_app_unregister(app);
        ust_app_destroy(app);
}

bool ust_app_get(ust_app& app)
{
        return urcu_ref_get_unless_zero(&app.ref);
}

void ust_app_put(struct ust_app *app)
{
        if (!app) {
                return;
        }

        urcu_ref_put(&app->ref, ust_app_release);
}