From: Matheus Alcantara <matheusssilv97@gmail.com>
To: Joel Jacobson <joel@compiler.org>
To: Álvaro Herrera <alvherre@kurilemu.de>
To: Arseniy Mukhin <arseniy.mukhin.dev@gmail.com>
Cc: Matheus Alcantara <matheusssilv97@gmail.com>
Cc: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: Rishu Bagga <rishu.postgres@gmail.com>
Cc: Yura Sokolov <y.sokolov@postgrespro.ru>
Cc: Daniil Davydov <3danissimo@gmail.com>
Cc: Alexandra Wang <alexandra.wang.oss@gmail.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Subject: Re: LISTEN/NOTIFY bug: VACUUM sets frozenxid past a xid in async queue
Date: Mon, 20 Oct 2025 09:41:43 -0300
Message-ID: <DDN5OD2RQFF4.CW8X8CMVQAJC@gmail.com> (raw)
In-Reply-To: <25651193-da4e-4185-a564-f2efa6b0c8a4@app.fastmail.com>
References: <202510171347.twmkbitfsgfh@alvherre.pgsql>
<25651193-da4e-4185-a564-f2efa6b0c8a4@app.fastmail.com>
On Sun Oct 19, 2025 at 2:14 PM -03, Joel Jacobson wrote:
> On Fri, Oct 17, 2025, at 15:51, Álvaro Herrera wrote:
>> I have the impression that this thread has lost focus on the idea of
>> producing a backpatchable bugfix. The last proposed patch has a lot of
>> new mechanism that doesn't seem suitable for backpatch. I could be
>> wrong of course.
>
> I've tried to create a minimal isolated fix, hopefully suitable for
> backpatching, with no new mechanisms, other than the added
> GetOldestQueuedNotifyXid used by vac_update_datfrozenxid.
>
> It's based on the approach discussed earlier in this thread, that just
> goes through the notification queue from QUEUE_TAIL to QUEUE_HEAD, to
> find the oldestXid in the current database.
>
> Implementation:
>
> * Break out SLRU read page code from asyncQueueReadAllNotifications into
> new helper-function asyncQueueReadPageToBuffer.
>
> * Add GetOldestQueuedNotifyXid which uses the new helper-function
> asyncQueueReadPageToBuffer.
>
> It passes the 001_xid_freeze.pl test, not included in this patch.
>
This is similar to what was already proposed at [1]. This approach was
abandoned because a notification on the queue may block datfrozenxid
advance and clog truncation which can cause other issues for the users [2].
[1] https://www.postgresql.org/message-id/CAFY6G8cJm73_MM9SuynZUqtqcaTuepUDgDuvS661oLW7U0dgsg%40mail.gma...
[2] https://www.postgresql.org/message-id/d186fba0-dc65-4274-aa96-3906bbb2e530%40postgrespro.ru
--
Matheus Alcantara
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Subject: Re: LISTEN/NOTIFY bug: VACUUM sets frozenxid past a xid in async queue
In-Reply-To: <DDN5OD2RQFF4.CW8X8CMVQAJC@gmail.com>
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