MDEV-22524 SIGABRT in safe_mutex_unlock with

session_track_system_variables and max_relay_log_size.

lock LOCK_global_system_variables around the get_one_variable() call
in the Session_sysvars_tracker::store_variable().

mysql-test/suite/sys_vars/r/session_track_system_variables_basic.result

@@ -159,6 +159,8 @@ SELECT @@session.session_track_system_variables;
 @@session.session_track_system_variables
 
 
+# MDEV-22524 SIGABRT in safe_mutex_unlock with session_track_system_variables and max_relay_log_size.
+SET SESSION session_track_system_variables="sql_slave_skip_counter", sql_slave_skip_counter= 0;
 # Restoring the original values.
 SET @@global.session_track_system_variables = @global_saved_tmp;
 # End of tests.

mysql-test/suite/sys_vars/t/session_track_system_variables_basic.test

@@ -122,6 +122,8 @@ SELECT @@global.session_track_system_variables;
 SELECT @@session.session_track_system_variables;
 --echo
 
+--echo # MDEV-22524 SIGABRT in safe_mutex_unlock with session_track_system_variables and max_relay_log_size.
+SET SESSION session_track_system_variables="sql_slave_skip_counter", sql_slave_skip_counter= 0;
 
 --echo # Restoring the original values.
 SET @@global.session_track_system_variables = @global_saved_tmp;

sql/session_tracker.cc

@@ -772,8 +772,11 @@ my_bool Session_sysvars_tracker::store_variable(void *ptr, void *data_ptr)
     show.name= svar->name.str;
     show.value= (char *) svar;
 
+    mysql_mutex_lock(&LOCK_global_system_variables);
     const char *value= get_one_variable(thd, &show, OPT_SESSION, SHOW_SYS, NULL,
                                         &charset, val_buf, &val_length);
+    mysql_mutex_unlock(&LOCK_global_system_variables);
+
     if (is_plugin)
       mysql_mutex_unlock(&LOCK_plugin);