Fix for bug #21281 "Pending write lock is incorrectly removed when its

statement being KILLed".

When statement which was trying to obtain write lock on then table and
which was blocked by existing read lock was killed, concurrent statements
that were trying to obtain read locks on the same table and that were
blocked by the presence of this pending write lock were not woken up and
had to wait until this first read lock goes away.

This problem was caused by the fact that we forgot to wake up threads
which pending requests could have been satisfied after removing lock
request for the killed thread.

The patch solves the problem by waking up those threads in such situation.

Test for this bug will be added to 5.1 only as it has much better
facilities for its implementation. Particularly, by using I_S.PROCESSLIST
and wait_condition.inc script we can wait until thread will be blocked on
certain table lock without relying on unconditional sleep (which usage
increases time needed for test runs and might cause spurious test
failures on slower platforms).

mysys/thr_lock.c

@@ -383,6 +383,9 @@ static inline my_bool have_specific_lock(THR_LOCK_DATA *data,
 }
 
 
+static void wake_up_waiters(THR_LOCK *lock);
+
+
 static enum enum_thr_lock_result
 wait_for_lock(struct st_lock_list *wait, THR_LOCK_DATA *data,
               my_bool in_wait_list)
@@ -444,8 +447,13 @@ wait_for_lock(struct st_lock_list *wait, THR_LOCK_DATA *data,
       else
 	wait->last=data->prev;
       data->type= TL_UNLOCK;                    /* No lock */
+      check_locks(data->lock, "killed or timed out wait_for_lock", 1);
+      wake_up_waiters(data->lock);
+    }
+    else
+    {
+      check_locks(data->lock, "aborted wait_for_lock", 0);
     }
-    check_locks(data->lock,"failed wait_for_lock",0);
   }
   else
   {
@@ -771,6 +779,26 @@ void thr_unlock(THR_LOCK_DATA *data)
     lock->read_no_write_count--;
   data->type=TL_UNLOCK;				/* Mark unlocked */
   check_locks(lock,"after releasing lock",1);
+  wake_up_waiters(lock);
+  pthread_mutex_unlock(&lock->mutex);
+  DBUG_VOID_RETURN;
+}
+
+
+/**
+  @brief  Wake up all threads which pending requests for the lock
+          can be satisfied.
+
+  @param  lock  Lock for which threads should be woken up
+
+*/
+
+static void wake_up_waiters(THR_LOCK *lock)
+{
+  THR_LOCK_DATA *data;
+  enum thr_lock_type lock_type;
+
+  DBUG_ENTER("wake_up_waiters");
 
   if (!lock->write.data)			/* If no active write locks */
   {
@@ -820,11 +848,7 @@ void thr_unlock(THR_LOCK_DATA *data)
 	  data=lock->write_wait.data;		/* Free this too */
 	}
 	if (data->type >= TL_WRITE_LOW_PRIORITY)
-	{
-	  check_locks(lock,"giving write lock",0);
-	  pthread_mutex_unlock(&lock->mutex);
-	  DBUG_VOID_RETURN;
-	}
+          goto end;
 	/* Release possible read locks together with the write lock */
       }
       if (lock->read_wait.data)
@@ -879,8 +903,7 @@ void thr_unlock(THR_LOCK_DATA *data)
       free_all_read_locks(lock,0);
   }
 end:
-  check_locks(lock,"thr_unlock",0);
-  pthread_mutex_unlock(&lock->mutex);
+  check_locks(lock, "after waking up waiters", 0);
   DBUG_VOID_RETURN;
 }
 
@@ -1094,6 +1117,7 @@ my_bool thr_abort_locks_for_thread(THR_LOCK *lock, pthread_t thread)
 	lock->write_wait.last= data->prev;
     }
   }
+  wake_up_waiters(lock);
   pthread_mutex_unlock(&lock->mutex);
   DBUG_RETURN(found);
 }