sessiond: add smart pointer utils for ltt_session

Add `find_session_by_id` and `find_locked_session_by_id` which return
smart pointers to ltt_session. In both cases, the smart pointers make
use of ltt_session's underlying reference counting mechanism.

In the case of `find_locked_session_by_id`, the session that is returned
is locked; it is automatically unlocked (and a reference is released)
when the pointer goes out of scope. This makes it easier to write
exception-safe code that uses the ltt_session API.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I125dc7592b8ef2de1da645029f311bf429a21767

diff --git a/src/bin/lttng-sessiond/session.cpp b/src/bin/lttng-sessiond/session.cpp
index d8accb87de12d9cab9eba739f553b6104657d048..1846024fcc60670d505a740db81125c7ec1e0aec 100644 (file)
--- a/src/bin/lttng-sessiond/session.cpp
+++ b/src/bin/lttng-sessiond/session.cpp
@@ -6,30 +6,32 @@
  */
 
 #define _LGPL_SOURCE
-#include <limits.h>
+#include <dirent.h>
 #include <inttypes.h>
+#include <limits.h>
+#include <pthread.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
-#include <urcu.h>
-#include <dirent.h>
 #include <sys/types.h>
-#include <pthread.h>
+#include <urcu.h>
 
 #include <common/common.hpp>
-#include <common/utils.hpp>
-#include <common/trace-chunk.hpp>
 #include <common/sessiond-comm/sessiond-comm.hpp>
-#include <lttng/location-internal.hpp>
+#include <common/trace-chunk.hpp>
+#include <common/urcu.hpp>
+#include <common/utils.hpp>
+
 #include "lttng-sessiond.hpp"
-#include "kernel.hpp"
+#include <lttng/location-internal.hpp>
 
+#include "cmd.hpp"
+#include "kernel.hpp"
 #include "session.hpp"
-#include "utils.hpp"
-#include "trace-ust.hpp"
 #include "timer.hpp"
-#include "cmd.hpp"
+#include "trace-ust.hpp"
+#include "utils.hpp"
 
 namespace {
 struct ltt_session_destroy_notifier_element {
@@ -42,6 +44,8 @@ struct ltt_session_clear_notifier_element {
        void *user_data;
 };
 
+namespace ls = lttng::sessiond;
+
 /*
  * NOTES:
  *
@@ -1461,3 +1465,38 @@ end:
        rcu_read_unlock();
        return found;
 }
+
+void ls::details::locked_session_release(ltt_session *session)
+{
+       session_unlock(session);
+       session_put(session);
+}
+
+ltt_session::locked_ptr ls::find_locked_session_by_id(ltt_session::id_t id)
+{
+       lttng::urcu::read_lock_guard rcu_lock;
+       auto session = session_find_by_id(id);
+
+       if (!session) {
+               return nullptr;
+       }
+
+       /*
+        * The pointer falling out of scope will unlock and release the reference to the
+        * session.
+        */
+       session_lock(session);
+       return ltt_session::locked_ptr(session);
+}
+
+ltt_session::sptr ls::find_session_by_id(ltt_session::id_t id)
+{
+       lttng::urcu::read_lock_guard rcu_lock;
+       auto session = session_find_by_id(id);
+
+       if (!session) {
+               return nullptr;
+       }
+
+       return {session, session_put};
+}

diff --git a/src/bin/lttng-sessiond/session.hpp b/src/bin/lttng-sessiond/session.hpp
index ad451336a5b5b430ad49a9d8e99c3bdb26a07429..17f3ca13970ae53f0e84001f7565fc92df8cbc89 100644 (file)
--- a/src/bin/lttng-sessiond/session.hpp
+++ b/src/bin/lttng-sessiond/session.hpp
@@ -14,6 +14,7 @@
 
 #include <common/hashtable/hashtable.hpp>
 #include <common/dynamic-array.hpp>
+#include <common/make-unique-wrapper.hpp>
 #include <lttng/rotation.h>
 #include <lttng/location.h>
 #include <lttng/lttng-error.h>
@@ -31,6 +32,14 @@ typedef void (*ltt_session_destroy_notifier)(const struct ltt_session *session,
 typedef void (*ltt_session_clear_notifier)(const struct ltt_session *session,
                void *user_data);
 
+namespace lttng {
+namespace sessiond {
+namespace details {
+void locked_session_release(ltt_session *session);
+} /* namespace details */
+} /* namespace sessiond */
+} /* namespace lttng */
+
 /*
  * Tracing session list
  *
@@ -67,6 +76,12 @@ struct ltt_session_list {
  * session for both LTTng and UST.
  */
 struct ltt_session {
+       using id_t = uint64_t;
+       using locked_ptr = std::unique_ptr<ltt_session,
+                       lttng::details::create_unique_class<ltt_session,
+                                       lttng::sessiond::details::locked_session_release>::deleter>;
+       using sptr = std::shared_ptr<ltt_session>;
+
        char name[NAME_MAX];
        bool has_auto_generated_name;
        bool name_contains_creation_time;
@@ -84,7 +99,8 @@ struct ltt_session {
         */
        pthread_mutex_t lock;
        struct cds_list_head list;
-       uint64_t id;            /* session unique identifier */
+       /* session unique identifier */
+       id_t id;
        /* Indicates if the session has been added to the session list and ht.*/
        bool published;
        /* Indicates if a destroy command has been applied to this session. */
@@ -240,7 +256,7 @@ struct lttng_trace_archive_location *session_get_trace_archive_location(
                const struct ltt_session *session);
 
 struct ltt_session *session_find_by_name(const char *name);
-struct ltt_session *session_find_by_id(uint64_t id);
+struct ltt_session *session_find_by_id(ltt_session::id_t id);
 
 struct ltt_session_list *session_get_list(void);
 void session_list_wait_empty(void);
@@ -294,4 +310,26 @@ bool session_output_supports_trace_chunks(const struct ltt_session *session);
  */
 bool sample_session_id_by_name(const char *name, uint64_t *id);
 
+namespace lttng {
+namespace sessiond {
+
+/*
+ * Session list lock must be acquired by the caller.
+ * The caller must not keep the ownership of the returned locked session
+ * for longer than strictly necessary. If your intention is to acquire
+ * a reference to an ltt_session, see `find_session_by_id()`.
+ */
+ltt_session::locked_ptr find_locked_session_by_id(ltt_session::id_t id);
+
+/*
+ * Session list lock must be acquired by the caller.
+ * The caller must not keep the ownership of the returned locked session
+ * for longer than strictly necessary. If your intention is to acquire
+ * a reference to an ltt_session, see `find_session_by_id()`.
+ */
+ltt_session::sptr find_session_by_id(ltt_session::id_t id);
+
+} /* namespace sessiond */
+} /* namespace lttng */
+
 #endif /* _LTT_SESSION_H */