bk commit into 6.0 tree (aelkin:1.2639) BUG#36929

[Andrei Elkin]

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ChangeSet@stripped, 2008-05-26 13:10:36+03:00, aelkin@stripped +2 -0
  Bug #36929  	crash in kill_zombie_dump_threads-> THD::awake() with replication tests
  
  There was a crash in THD::awake () at attempt to access concurrently resetable
  by the host thread THD::mysys_var.
  
  Fixed with forcing the host thread to reset only after acquiring LOCK_delete mutex.

  sql/scheduler.cc@stripped, 2008-05-26 13:10:34+03:00, aelkin@stripped +2 -0
    Resetting mysys_var under protection of LOCK_delete

  sql/sql_class.cc@stripped, 2008-05-26 13:10:34+03:00, aelkin@stripped +6 -1
    Moving resetting of mysys_var under LOCK_delete protection.

diff -Nrup a/sql/scheduler.cc b/sql/scheduler.cc
--- a/sql/scheduler.cc	2008-05-14 16:33:27 +03:00
+++ b/sql/scheduler.cc	2008-05-26 13:10:34 +03:00
@@ -222,7 +222,9 @@ void thd_scheduler::thread_detach()
   if (thread_attached)
   {
     THD* thd = (THD*)list.data;
+    pthread_mutex_lock(&thd->LOCK_delete);
     thd->mysys_var= NULL;
+    pthread_mutex_lock(&thd->LOCK_delete);
     thread_attached= FALSE;
 #ifndef DBUG_OFF
     /*
diff -Nrup a/sql/sql_class.cc b/sql/sql_class.cc
--- a/sql/sql_class.cc	2008-05-21 13:04:22 +03:00
+++ b/sql/sql_class.cc	2008-05-26 13:10:34 +03:00
@@ -902,6 +902,12 @@ THD::~THD()
   DBUG_ENTER("~THD()");
   /* Ensure that no one is using THD */
   pthread_mutex_lock(&LOCK_delete);
+  /*
+    resetting this pointer to the thread specific memory is guarded by
+    the same mutex as a killer thread acquires in order to get access
+    to the pointer
+  */
+  mysys_var= NULL;
   pthread_mutex_unlock(&LOCK_delete);
   add_to_status(&global_status_var, &status_var);
 
@@ -929,7 +935,6 @@ THD::~THD()
 #ifdef USING_TRANSACTIONS
   free_root(&transaction.mem_root,MYF(0));
 #endif
-  mysys_var=0;					// Safety (shouldn't be needed)
   pthread_mutex_destroy(&LOCK_delete);
 #ifndef DBUG_OFF
   dbug_sentry= THD_SENTRY_GONE;