Re: LISTEN/NOTIFY bug: VACUUM sets frozenxid past a xid in async queue

On Fri Sep 19, 2025 at 12:13 PM -03, Arseniy Mukhin wrote:
> I think it's impossible: before pushing anything to the queue we
> acquire global lock in PreCommit_Notify():
>
> LockSharedObject(DatabaseRelationId, InvalidOid, 0, AccessExclusiveLock);
>
> While we are holding the lock, no writers can add anything to the
> queue. Then we save head position and add pending notifications to the
> queue. The moment we get in AtAbort_Notify(), we still hold the global
> lock (maybe it is worth adding Assert about it if we start relying on
> it), so we can be sure there are no notifications in the queue after
> the saved head position except ours. So it seems safe but maybe I
> missed something.
>
Thanks for the explanation! I'm just not sure if I understand why do we
need the LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE) on
PreCommit_Notify() if we already have the
LockSharedObject(DatabaseRelationId, InvalidOid, 0, AccessExclusiveLock);

See the attached patch that is based on your the previous comment of
resetting the QUEUE_HEAD at AtAbort_Notify()

--
Matheus Alcantara