Bug #51160 Deadlock around SET GLOBAL EVENT_SCHEDULER = ON|OFF

This deadlock could occour betweeen one connection executing
SET GLOBAL EVENT_SCHEDULER= ON and another executing SET GLOBAL
EVENT_SCHEDULER= OFF. The bug was introduced by WL#4738.

The first connection would hold LOCK_event_metadata (protecting
the global variable) while trying to lock LOCK_global_system_variables
starting the event scheduler thread (in THD:init()).

The second connection would hold LOCK_global_system_variables
while trying to get LOCK_event_scheduler after stopping the event
scheduler inside event_scheduler_update().

This patch fixes the problem by not using LOCK_event_metadata to
protect the event_scheduler variable. It is still protected using
LOCK_global_system_variables. This fixes the deadlock as it removes 
one of the two mutexes used to produce it.

However, this patch opens up the possibility that the event_scheduler
variable and the real event_scheduler state can become out of sync
(e.g. variable = OFF, but scheduler running). But this can only
happen under very unlikely conditions - two concurrent SET GLOBAL
statments, with one thread interrupted at the exact wrong moment.
This is preferable to having the possibility of a deadlock.

This patch also fixes a bug where it was possible to exit create_event()
without releasing LOCK_event_metadata if running out of memory during
its exection.

No test case added since a repeatable test case would have required
excessive use of new sync points. Instead we rely on the fact that
this bug was easily reproduceable using RGQ tests.

1 files changed, 3 insertions, 5 deletions

diff --git a/sql/events.h b/sql/events.h
index 881ade37cbf..380b0d97f0a 100644
--- a/sql/events.h
+++ b/sql/events.h
@@ -1,6 +1,6 @@
 #ifndef _EVENT_H_
 #define _EVENT_H_
-/* Copyright (C) 2004-2006 MySQL AB, 2008-2009 Sun Microsystems, Inc
+/* Copyright (c) 2004, 2010, Oracle and/or its affiliates. All rights reserved.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -13,7 +13,7 @@
 
    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
-   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */
+   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA */
 
 /**
   @defgroup Event_Scheduler Event Scheduler
@@ -83,6 +83,7 @@ public:
     See sys_var.cc
   */
   enum enum_opt_event_scheduler { EVENTS_OFF, EVENTS_ON, EVENTS_DISABLED };
+  /* Protected using LOCK_global_system_variables only. */
   static uint opt_event_scheduler;
   static mysql_mutex_t LOCK_event_metadata;
   static bool check_if_system_tables_error();
@@ -107,9 +108,6 @@ public:
   destroy_mutexes();
 
   static bool
-  switch_event_scheduler_state(enum enum_opt_event_scheduler new_state);
-
-  static bool
   create_event(THD *thd, Event_parse_data *parse_data, bool if_exists);
 
   static bool