Revert THD::THD(skip_global_sys_var_lock) argumentbb-10.3-svoj-MDEV-14984

Originally introduced by e972125f1 to avoid harmless wait for
LOCK_global_system_variables in a newly created thread, which creation was
initiated by system variable update.

At the same time it opens dangerous hole, when system variable update
thread already released LOCK_global_system_variables and ack_receiver
thread haven't yet completed new THD construction. In this case THD
constructor goes completely unprotected.

Since ack_receiver.stop() waits for the thread to go down, we have to
temporarily release LOCK_global_system_variables so that it doesn't
deadlock with ack_receiver.run(). Unfortunately it breaks atomicity
of rpl_semi_sync_master_enabled updates and makes them not serialized.

LOCK_rpl_semi_sync_master_enabled was introduced to workaround the above.
TODO: move ack_receiver start/stop into repl_semisync_master
enable_master/disable_master under LOCK_binlog protection?

Part of MDEV-14984 - regression in connect performance

1 files changed, 1 insertions, 0 deletions

diff --git a/sql/session_tracker.cc b/sql/session_tracker.cc
index 63a6770f7d1..f4dab11bb42 100644
--- a/sql/session_tracker.cc
+++ b/sql/session_tracker.cc
@@ -322,6 +322,7 @@ bool Session_sysvars_tracker::vars_list::construct_var_list(char *buf,
 
 void Session_sysvars_tracker::init(THD *thd)
 {
+  mysql_mutex_assert_owner(&LOCK_global_system_variables);
   DBUG_ASSERT(thd->variables.session_track_system_variables ==
               global_system_variables.session_track_system_variables);
   DBUG_ASSERT(global_system_variables.session_track_system_variables);