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.

sql/events.cc

-/* 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
@@ -11,7 +11,7 @@
 
    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 */
 
 #include "mysql_priv.h"
 #include "events.h"
@@ -367,15 +367,14 @@ Events::create_event(THD *thd, Event_parse_data *parse_data,
       {
         sql_print_error("Event Error: An error occurred while creating query string, "
                         "before writing it into binary log.");
-        /* Restore the state of binlog format */
-        DBUG_ASSERT(!thd->is_current_stmt_binlog_format_row());
-        if (save_binlog_row_based)
-          thd->set_current_stmt_binlog_format_row();
-        DBUG_RETURN(TRUE);
+        ret= TRUE;
+      }
+      else
+      {
+        /* If the definer is not set or set to CURRENT_USER, the value of CURRENT_USER
+           will be written into the binary log as the definer for the SQL thread. */
+        ret= write_bin_log(thd, TRUE, log_query.c_ptr(), log_query.length());
       }
-      /* If the definer is not set or set to CURRENT_USER, the value of CURRENT_USER 
-         will be written into the binary log as the definer for the SQL thread. */
-      ret= write_bin_log(thd, TRUE, log_query.c_ptr(), log_query.length());
     }
   }
   mysql_mutex_unlock(&LOCK_event_metadata);
@@ -1017,7 +1016,11 @@ Events::dump_internal_status()
   puts("LLA = Last Locked At  LUA = Last Unlocked At");
   puts("WOC = Waiting On Condition  DL = Data Locked");
 
-  mysql_mutex_lock(&LOCK_event_metadata);
+  /*
+    opt_event_scheduler should only be accessed while
+    holding LOCK_global_system_variables.
+  */
+  mysql_mutex_lock(&LOCK_global_system_variables);
   if (opt_event_scheduler == EVENTS_DISABLED)
     puts("The Event Scheduler is disabled");
   else
@@ -1026,7 +1029,7 @@ Events::dump_internal_status()
     event_queue->dump_internal_status();
   }
 
-  mysql_mutex_unlock(&LOCK_event_metadata);
+  mysql_mutex_unlock(&LOCK_global_system_variables);
   DBUG_VOID_RETURN;
 }
 

sql/events.h

 #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();
@@ -106,9 +107,6 @@ public:
   static void
   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);
 

sql/sys_vars.cc

-/* Copyright (C) 2002-2006 MySQL AB, 2009-2010 Sun Microsystems, Inc.
+/* Copyright (c) 2002, 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
@@ -11,7 +11,7 @@
 
    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 */
 
 /*
   How to add new variables:
@@ -647,32 +647,40 @@ static bool event_scheduler_check(sys_var *self, THD *thd, set_var *var)
 }
 static bool event_scheduler_update(sys_var *self, THD *thd, enum_var_type type)
 {
+  uint opt_event_scheduler_value= Events::opt_event_scheduler;
   mysql_mutex_unlock(&LOCK_global_system_variables);
   /*
     Events::start() is heavyweight. In particular it creates a new THD,
     which takes LOCK_global_system_variables internally.
     Thus we have to release it here.
     We need to re-take it before returning, though.
-    And we need to take it *without* holding Events::LOCK_event_metadata.
+
+    Note that since we release LOCK_global_system_variables before calling
+    start/stop, there is a possibility that the server variable
+    can become out of sync with the real event scheduler state.
+
+    This can happen with two concurrent statments if the first gets
+    interrupted after start/stop but before retaking
+    LOCK_global_system_variables. However, this problem should be quite
+    rare and it's difficult to avoid it without opening up possibilities
+    for deadlocks. See bug#51160.
   */
-  bool ret= Events::opt_event_scheduler == Events::EVENTS_ON
+  bool ret= opt_event_scheduler_value == Events::EVENTS_ON
             ? Events::start()
             : Events::stop();
-  mysql_mutex_unlock(&Events::LOCK_event_metadata);
   mysql_mutex_lock(&LOCK_global_system_variables);
-  mysql_mutex_lock(&Events::LOCK_event_metadata);
   if (ret)
     my_error(ER_EVENT_SET_VAR_ERROR, MYF(0));
   return ret;
 }
-static PolyLock_mutex PLock_event_metadata(&Events::LOCK_event_metadata);
+
 static Sys_var_enum Sys_event_scheduler(
        "event_scheduler", "Enable the event scheduler. Possible values are "
        "ON, OFF, and DISABLED (keep the event scheduler completely "
        "deactivated, it cannot be activated run-time)",
        GLOBAL_VAR(Events::opt_event_scheduler), CMD_LINE(OPT_ARG),
        event_scheduler_names, DEFAULT(Events::EVENTS_OFF),
-       &PLock_event_metadata, NOT_IN_BINLOG,
+       NO_MUTEX_GUARD, NOT_IN_BINLOG,
        ON_CHECK(event_scheduler_check), ON_UPDATE(event_scheduler_update));
 #endif