Feedback-ID: i0fe9450f:Fastmail
Date: Thu, 18 Dec 2025 08:46:42 +0900
From: Michael Paquier <michael@paquier.xyz>
To: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Cc: pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: Fix remaining race condition with CLOG truncation and
 LISTEN/NOT
Message-ID: <aUNA4l9w1Flr9Y2z@paquier.xyz>
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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
>=20
> 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.
>=20
> 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

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