Re: pgsql: Fix remaining race condition with CLOG truncation and LISTEN/NOT

Hi Heikki,

On Wed, Nov 12, 2025 at 07:16:38PM +0000, Heikki Linnakangas wrote:
> Fix remaining race condition with CLOG truncation and LISTEN/NOTIFY
>
> Previous commit fixed a bug where VACUUM would truncate the CLOG
> that's still needed to check the commit status of XIDs in the async
> notify queue, but as mentioned in the commit message, it wasn't a full
> fix. If a backend is executing asyncQueueReadAllNotifications() and
> has just made a local copy of an async SLRU page which contains old
> XIDs, vacuum can concurrently truncate the CLOG covering those XIDs,
> and the backend still gets an error when it calls
> TransactionIdDidCommit() on those XIDs in the local copy. This commit
> fixes that race condition.
>
> To fix, hold the SLRU bank lock across the TransactionIdDidCommit()
> calls in NOTIFY processing.

While reading through this set of changes, the addition of this commit
across the v14~v16 range has made me smile:

+ * Now that we have let go of the SLRU bank lock, send the notifications
+ * to our backend

SLRU bank locks have been introduced in v17 by Alvaro in 53c2a97a9266.
Just noticed while going through.
--
Michael