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: Rishu Bagga <rishu.postgres@gmail.com>

Cc: Arseniy Mukhin <arseniy.mukhin.dev@gmail.com>, Yura Sokolov <y.sokolov@postgrespro.ru>, Masahiko Sawada <sawada.mshk@gmail.com>, Daniil Davydov <3danissimo@gmail.com>, Álvaro Herrera <alvherre@kurilemu.de>, Alexandra Wang <alexandra.wang.oss@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>, Tom Lane <tgl@sss.pgh.pa.us>, Joel Jacobson <joel@compiler.org>

Date: 2025-09-05T10:44:11Z

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 Wed Sep 3, 2025 at 8:51 PM -03, Rishu Bagga wrote:
> On Wed, Sep 3, 2025 at 2:14 PM Matheus Alcantara
>
> <matheusssilv97@gmail.com> wrote:
>
>
>> IIUC we don't store notifications of aborted transactions on the
>
>> queue. On PreCommit_Notify we add the notifications on the queue
>
>> and on Commit_Notify() we signal the backends.
>
>>
>
>> Or I'm missing something here?
>
>
> My understanding is that something could go wrong in between
>
> PreCommit_Notify and AtCommit_Notify, which could cause the
>
> transaction to abort, and the notification might be in the queue at
>
> this point, even though the transaction aborted, hence the dependency
>
> on the commit log.
>
Ok, I agree that that this may happen but I don't see this as a common
case to fix the issue based on this behaviour. I think that we check the
transaction status also to skip notifications that were added on the
queue by transactions that are not fully committed yet, and I see this
scenario as a more common case but I could be wrong.

--
Matheus Alcantara