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

Álvaro Herrera <alvherre@kurilemu.de>

From: Álvaro Herrera <alvherre@kurilemu.de>

To: Heikki Linnakangas <hlinnaka@iki.fi>

Cc: Joel Jacobson <joel@compiler.org>, Tom Lane <tgl@sss.pgh.pa.us>, pgsql-hackers <pgsql-hackers@postgresql.org>

Date: 2025-11-04T16:27:48Z

Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

Clear 'xid' in dummy async notify entries written to fill up pages
- 84f1bf4afa5e 14.21 landed
- 21a9014cf00a 15.16 landed
- 0e8eaa2181d4 16.12 landed
- d80d5f099502 17.8 landed
- 82fa6b78dba1 18.2 landed
- 0bdc777e8007 19 (unreleased) landed
Fix remaining race condition with CLOG truncation and LISTEN/NOTIFY
- c2e58c0711fe 14.21 landed
- 0c862646cf2a 15.16 landed
- 44e8c60be66c 16.12 landed
- c2682810ab7d 17.8 landed
- 7b069a1876e4 18.2 landed
- 797e9ea6e54b 19 (unreleased) landed
Fix bug where we truncated CLOG that was still needed by LISTEN/NOTIFY
- eba917d360e7 14.21 landed
- 1a469d7b5b7d 15.16 landed
- 053e1868b7ee 16.12 landed
- d02c03ddc5e3 17.8 landed
- 321ec54625fd 18.2 landed
- 8eeb4a0f7c06 19 (unreleased) landed
Escalate ERRORs during async notify processing to FATAL
- 7cb05dd2d198 14.21 landed
- b1da37de21d4 15.16 landed
- c1a5bde003b8 16.12 landed
- b821c92920f0 17.8 landed
- aab4a84bb070 18.2 landed
- 1b4699090eaf 19 (unreleased) landed
Limit the size of TID lists during parallel GIN build
- c98dffcb7c70 19 (unreleased) cited

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)