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Date: Mon, 1 Jun 2026 19:12:53 +0200
From: =?utf-8?Q?=C3=81lvaro?= Herrera <alvherre@kurilemu.de>
To: n.kalinin@postgrespro.ru, pgsql-bugs@lists.postgresql.org,
	Antonin Houska <ah@cybertec.at>, b@ida.kurilemu.internal
Subject: Re: BUG #19500: pgrepack logical decoding plugin can crash assert
 builds via SQL decoding API
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On 2026-May-29, Álvaro Herrera wrote:

> On 2026-05-28, PG Bug reporting form wrote:
> 
> > It appears that the pgrepack output plugin is accessible through the
> > SQL logical decoding API, even though the plugin code explicitly
> > indicates that this interface is not supported. Reading changes from
> > such a slot can cause a backend process crash in builds with asserts
> > enabled.
> 
> Yeah, I would like to have a way to prevent this, if only for
> user-friendliness, but it's not terribly pressing since only a role
> with REPLICATION privs can create the replication slot, which as I
> recall are already pretty powerful.

How about something like this?  It makes your test case throw an error
instead of failing the assertion, which I suppose is an improvement.

The patch is a bit noisy because I moved more code than the minimum
necessary; but the gist of it is that we allocate RepackDecodingState in
repack_startup(), then have repack_setup_logical_decoding() fill in a
magic number, which we later check in repack_begin_txn().  This is a bit
wasteful, because we have to do that check once for each and every
transaction; however I see no other callback that would let us do this
kind of check after the slot is created but before we start to consume
from it.

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"Before you were born your parents weren't as boring as they are now. They
got that way paying your bills, cleaning up your room and listening to you
tell them how idealistic you are."  -- Charles J. Sykes' advice to teenagers

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