Fix WaitOnLock() to ensure that the process's "waiting" flag is reset after

erroring out of a wait.  We can use a PG_TRY block for this, but add a comment
explaining why it'd be a bad idea to use it for any other state cleanup.

Back-patch to 8.2.  Prior releases had the same issue, but only with respect
to the process title, which is likely to get reset almost immediately anyway
after the transaction aborts, so it seems not worth changing them.  In 8.2
and HEAD, the pg_stat_activity "waiting" flag could remain set incorrectly
for a long time.

Per report from Gurjeet Singh.

Author: tglsfdc

src/backend/storage/lmgr/lock.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/lmgr/lock.c,v 1.180 2008/01/01 19:45:52 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/lmgr/lock.c,v 1.181 2008/02/02 22:26:17 tgl Exp $
  *
  * NOTES
  *	  A lock table is a shared memory hash table.  When
@@ -1102,16 +1102,17 @@ WaitOnLock(LOCALLOCK *locallock, ResourceOwner owner)
 {
 	LOCKMETHODID lockmethodid = LOCALLOCK_LOCKMETHOD(*locallock);
 	LockMethod	lockMethodTable = LockMethods[lockmethodid];
-	const char *old_status;
-	char	   *new_status = NULL;
-	int			len;
+	char	   * volatile new_status = NULL;
 
 	LOCK_PRINT("WaitOnLock: sleeping on lock",
 			   locallock->lock, locallock->tag.mode);
 
 	/* Report change to waiting status */
 	if (update_process_title)
 	{
+		const char *old_status;
+		int			len;
+
 		old_status = get_ps_display(&len);
 		new_status = (char *) palloc(len + 8 + 1);
 		memcpy(new_status, old_status, len);
@@ -1132,38 +1133,62 @@ WaitOnLock(LOCALLOCK *locallock, ResourceOwner owner)
 	 * that a cancel/die interrupt will interrupt ProcSleep after someone else
 	 * grants us the lock, but before we've noticed it. Hence, after granting,
 	 * the locktable state must fully reflect the fact that we own the lock;
-	 * we can't do additional work on return. Contrariwise, if we fail, any
-	 * cleanup must happen in xact abort processing, not here, to ensure it
-	 * will also happen in the cancel/die case.
+	 * we can't do additional work on return.
+	 *
+	 * We can and do use a PG_TRY block to try to clean up after failure,
+	 * but this still has a major limitation: elog(FATAL) can occur while
+	 * waiting (eg, a "die" interrupt), and then control won't come back here.
+	 * So all cleanup of essential state should happen in LockWaitCancel,
+	 * not here.  We can use PG_TRY to clear the "waiting" status flags,
+	 * since doing that is unimportant if the process exits.
 	 */
+	PG_TRY();
+	{
+		if (ProcSleep(locallock, lockMethodTable) != STATUS_OK)
+		{
+			/*
+			 * We failed as a result of a deadlock, see CheckDeadLock().
+			 * Quit now.
+			 */
+			awaitedLock = NULL;
+			LOCK_PRINT("WaitOnLock: aborting on lock",
+					   locallock->lock, locallock->tag.mode);
+			LWLockRelease(LockHashPartitionLock(locallock->hashcode));
 
-	if (ProcSleep(locallock, lockMethodTable) != STATUS_OK)
+			/*
+			 * Now that we aren't holding the partition lock, we can give an
+			 * error report including details about the detected deadlock.
+			 */
+			DeadLockReport();
+			/* not reached */
+		}
+	}
+	PG_CATCH();
 	{
-		/*
-		 * We failed as a result of a deadlock, see CheckDeadLock(). Quit now.
-		 */
-		awaitedLock = NULL;
-		LOCK_PRINT("WaitOnLock: aborting on lock",
-				   locallock->lock, locallock->tag.mode);
-		LWLockRelease(LockHashPartitionLock(locallock->hashcode));
+		/* In this path, awaitedLock remains set until LockWaitCancel */
 
-		/*
-		 * Now that we aren't holding the partition lock, we can give an error
-		 * report including details about the detected deadlock.
-		 */
-		DeadLockReport();
-		/* not reached */
+		/* Report change to non-waiting status */
+		pgstat_report_waiting(false);
+		if (update_process_title)
+		{
+			set_ps_display(new_status, false);
+			pfree(new_status);
+		}
+
+		/* and propagate the error */
+		PG_RE_THROW();
 	}
+	PG_END_TRY();
 
 	awaitedLock = NULL;
 
 	/* Report change to non-waiting status */
+	pgstat_report_waiting(false);
 	if (update_process_title)
 	{
 		set_ps_display(new_status, false);
 		pfree(new_status);
 	}
-	pgstat_report_waiting(false);
 
 	LOCK_PRINT("WaitOnLock: wakeup on lock",
 			   locallock->lock, locallock->tag.mode);