Bug #32436 KILL QUERY completely deadlocks mysqld

Sending several "KILL QUERY" statements to target a connection running
"SELECT SLEEP" could freeze the server.

The locking order in Item_func_sleep was wrong and this could lead to a
dead lock.

This patch solves the issue by resolving the locking order properly.


sql/item_func.cc:
  - Moved LOCK_user_locks critical region so that it doesn't share space with
    mysys_var->mutex region; this can lead to deadlock.

sql/item_func.cc

@@ -3732,13 +3732,12 @@ longlong Item_func_sleep::val_int()
       break;
     error= 0;
   }
-
+  pthread_mutex_unlock(&LOCK_user_locks);
   pthread_mutex_lock(&thd->mysys_var->mutex);
   thd->mysys_var->current_mutex= 0;
   thd->mysys_var->current_cond=  0;
   pthread_mutex_unlock(&thd->mysys_var->mutex);
 
-  pthread_mutex_unlock(&LOCK_user_locks);
   pthread_cond_destroy(&cond);
 
   return test(!error); 		// Return 1 killed