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

Matheus Alcantara <matheusssilv97@gmail.com>

From: Matheus Alcantara <matheusssilv97@gmail.com>

To: Masahiko Sawada <sawada.mshk@gmail.com>

Cc: Daniil Davydov <3danissimo@gmail.com>, Álvaro Herrera <alvherre@kurilemu.de>, Alexandra Wang <alexandra.wang.oss@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>

Date: 2025-08-20T21:22:25Z

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 Tue Aug 19, 2025 at 2:40 PM -03, Masahiko Sawada wrote:
> However, I have a more fundamental concern regarding the LISTEN/NOTIFY
> implementation. Since vacuum doesn't consider the XIDs of notification
> entries, there might be a critical issue with the truncation of clog
> entries that LISTEN/NOTIFY still requires. As I understand it,
> notification queue entries aren't ordered by XID, and it's possible
> for a notification with an older XID to be positioned at the queue's
> head. If vacuum freeze then truncates the corresponding clogs,
> listeners attempting to retrieve this notification would fail to
> obtain the transaction status. To address this, we likely need to
> either implement Álvaro's suggestion[1] to make vacuum aware of the
> oldest XID in the notification queue, or develop a mechanism to
> remove/freeze XIDs of the notification entries.
>
Thanks for the comments! Please see my reply at [1] that I mention that
I don't think that is too easy to have this specific scenario of a busy
backend loose dropped notifications.

[1] https://www.postgresql.org/message-id/DC7KGTXW3QSG.OZA24HONT78J%40gmail.com

--
Matheus Alcantara