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

On 2025-Nov-04, Heikki Linnakangas wrote:

> From 7c342e6efffc8d59c2e7658f6f2f3b138d02e0bb Mon Sep 17 00:00:00 2001
> From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
> Date: Tue, 4 Nov 2025 13:22:08 +0200
> Subject: [PATCH v2 1/2] Fix bug where we truncated CLOG that was still needed
>  by LISTEN/NOTIFY

> The async notification queue contains the XID of the sender, and when
> processing notifications we call TransactionIdDidCommit() on the
> XID. But we had no safeguards to prevent the CLOG segments containing
> those XIDs from being truncated away. As a result, if a backend didn't
> for some reason process its notifications for a long time, or when a
> new backend issued LISTEN, you could get an error like:
> 
> test=# listen c21;
> ERROR:  58P01: could not access status of transaction 14279685
> DETAIL:  Could not open file "pg_xact/000D": No such file or directory.
> LOCATION:  SlruReportIOError, slru.c:1087
> 
> Note: This commit is not a full fix. [...]

The commit message doesn't say just _what_ does the patch do to fix the
bug though.  Looking through the code, I think you're setting XIDs in
the async queue to frozenXid, but I think the commit message should
explain that.

Thanks for spending time on this.

-- 
Álvaro Herrera        Breisgau, Deutschland  —  https://www.EnterpriseDB.com/
"La fuerza no está en los medios físicos
sino que reside en una voluntad indomable" (Gandhi)