Fix: action executor: ref count imbalance for session object

Observed issue
==============

The following scenario leads to a hang on `lttng destroy`.

 # Start lttng-sessiond under gdb
 $ gdb lttng-sessiond
     set pagination off
     set non-stop
     start
     break action_executor_snapshot_session_handler

 $ lttng add-trigger --name my_trigger --condition=event-rule-matches --type=user:tracepoint --name=sample_component:message --action=snapshot-session my_snapshot
 $ lttng create --snapshot my_snapshot
 $ lttng enable-event -u -a
 $ lttng start

 $ start an app producing a single sample_component:message

 # gdb should break on thread 6

 # inside gdb
	thread 6

 $ lttng destroy my_snapshot
 $ lttng create --snapshot my_snapshot
 $ lttng enable-event -u -a
 $ lttng start

 # inside gdb use `continue`

 $ lttng destroy my_snapshot

  The destroy command hang:

  Destroying session my_snapshot.... ....

Cause
=====

The scenario forces the usage of the following code path:

 if (session->id != LTTNG_OPTIONAL_GET(item->context.session_id)) {
  624├───────────────> DBG("Session id for session `%s` (id: %" PRIu64
  625│                     " is not the same that was sampled (id: %" PRIu64
  626│                     " at the moment the work item was enqueued for %s` action of trigger `%s`",
  627│                                 session_name, session->id,
  628│                                 LTTNG_OPTIONAL_GET(item->context.session_id),
  629│                                 get_action_name(action),
  630│                                 get_trigger_name(work_item->trigger));
  631│                 ret = 0;
  632│                 goto error_unlock_list;
  633│         }

At that point a reference on the session object was taken on line:

 610│         session = session_find_by_name(session_name);

But the reference is never put on `error_unlock_list` resulting in a ref
count problem.

Solution
========

Use `session_put` for the code path.

Note that most of the handler also have the same problem that was
introduced by commit 72365501d3148ca977a09bad8de0ec51b427bdd8 [1]

Known drawbacks
=========

None.

Refs
=========
[1] https://github.com/lttng/lttng-tools/commit/72365501d3148ca977a09bad8de0ec51b427bdd8

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I23c3c089866df74854bbfe64320310c4b28ee41d

diff --git a/src/bin/lttng-sessiond/action-executor.cpp b/src/bin/lttng-sessiond/action-executor.cpp
index cc7834f2745ca6e58d8299dd7c649fe68ecc0aaa..8c2374535195cc0cc59531a4d02133b3930c5e56 100644 (file)
--- a/src/bin/lttng-sessiond/action-executor.cpp
+++ b/src/bin/lttng-sessiond/action-executor.cpp
@@ -331,12 +331,12 @@ static int action_executor_start_session_handler(
                                get_action_name(action),
                                get_trigger_name(work_item->trigger));
                ret = 0;
-               goto error_unlock_list;
+               goto error_put_session;
        }
 
        session_lock(session);
        if (!is_trigger_allowed_for_session(work_item->trigger, session)) {
-               goto error_dispose_session;
+               goto error_unlock_session;
        }
 
        cmd_ret = (lttng_error_code) cmd_start_trace(session);
@@ -357,8 +357,9 @@ static int action_executor_start_session_handler(
                break;
        }
 
-error_dispose_session:
+error_unlock_session:
        session_unlock(session);
+error_put_session:
        session_put(session);
 error_unlock_list:
        session_unlock_list();
@@ -423,12 +424,12 @@ static int action_executor_stop_session_handler(
                                get_action_name(action),
                                get_trigger_name(work_item->trigger));
                ret = 0;
-               goto error_unlock_list;
+               goto error_put_session;
        }
 
        session_lock(session);
        if (!is_trigger_allowed_for_session(work_item->trigger, session)) {
-               goto error_dispose_session;
+               goto error_unlock_session;
        }
 
        cmd_ret = (lttng_error_code) cmd_stop_trace(session);
@@ -449,8 +450,9 @@ static int action_executor_stop_session_handler(
                break;
        }
 
-error_dispose_session:
+error_unlock_session:
        session_unlock(session);
+error_put_session:
        session_put(session);
 error_unlock_list:
        session_unlock_list();
@@ -515,12 +517,12 @@ static int action_executor_rotate_session_handler(
                                get_action_name(action),
                                get_trigger_name(work_item->trigger));
                ret = 0;
-               goto error_unlock_list;
+               goto error_put_session;
        }
 
        session_lock(session);
        if (!is_trigger_allowed_for_session(work_item->trigger, session)) {
-               goto error_dispose_session;
+               goto error_unlock_session;
        }
 
        cmd_ret = (lttng_error_code) cmd_rotate_session(session, NULL, false,
@@ -548,8 +550,9 @@ static int action_executor_rotate_session_handler(
                break;
        }
 
-error_dispose_session:
+error_unlock_session:
        session_unlock(session);
+error_put_session:
        session_put(session);
 error_unlock_list:
        session_unlock_list();
@@ -629,12 +632,12 @@ static int action_executor_snapshot_session_handler(
                                get_action_name(action),
                                get_trigger_name(work_item->trigger));
                ret = 0;
-               goto error_unlock_list;
+               goto error_put_session;
        }
 
        session_lock(session);
        if (!is_trigger_allowed_for_session(work_item->trigger, session)) {
-               goto error_dispose_session;
+               goto error_unlock_session;
        }
 
        cmd_ret = (lttng_error_code) cmd_snapshot_record(session, snapshot_output, 0);
@@ -651,8 +654,9 @@ static int action_executor_snapshot_session_handler(
                break;
        }
 
-error_dispose_session:
+error_unlock_session:
        session_unlock(session);
+error_put_session:
        session_put(session);
 error_unlock_list:
        session_unlock_list();