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Date: Mon, 20 Oct 2025 09:41:43 -0300
Message-Id: <DDN5OD2RQFF4.CW8X8CMVQAJC@gmail.com>
Cc: "Matheus Alcantara" <matheusssilv97@gmail.com>, "Masahiko Sawada"
 <sawada.mshk@gmail.com>, "Rishu Bagga" <rishu.postgres@gmail.com>, "Yura
 Sokolov" <y.sokolov@postgrespro.ru>, "Daniil Davydov"
 <3danissimo@gmail.com>, "Alexandra Wang" <alexandra.wang.oss@gmail.com>,
 "pgsql-hackers" <pgsql-hackers@postgresql.org>, "Tom Lane"
 <tgl@sss.pgh.pa.us>
Subject: Re: LISTEN/NOTIFY bug: VACUUM sets frozenxid past a xid in async
 queue
From: "Matheus Alcantara" <matheusssilv97@gmail.com>
To: "Joel Jacobson" <joel@compiler.org>, =?utf-8?q?=C3=81lvaro_Herrera?=
 <alvherre@kurilemu.de>, "Arseniy Mukhin" <arseniy.mukhin.dev@gmail.com>
References: <202510171347.twmkbitfsgfh@alvherre.pgsql>
 <25651193-da4e-4185-a564-f2efa6b0c8a4@app.fastmail.com>
In-Reply-To: <25651193-da4e-4185-a564-f2efa6b0c8a4@app.fastmail.com>
Archived-At: 
 <https://www.postgresql.org/message-id/DDN5OD2RQFF4.CW8X8CMVQAJC%40gmail.com>
Precedence: bulk

On Sun Oct 19, 2025 at 2:14 PM -03, Joel Jacobson wrote:
> On Fri, Oct 17, 2025, at 15:51, =C3=81lvaro 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_MM9SuynZUqtqcaTuepUD=
gDuvS661oLW7U0dgsg%40mail.gmail.com
[2] https://www.postgresql.org/message-id/d186fba0-dc65-4274-aa96-3906bbb2e=
530%40postgrespro.ru

--
Matheus Alcantara