Feedback-ID: id4a34324:Fastmail
Date: Tue, 7 Apr 2026 16:24:04 -0400
From: Andres Freund <andres@anarazel.de>
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Mihail Nikalayeu <mihailnikalayeu@gmail.com>,
	Antonin Houska <ah@cybertec.at>,
 Srinath Reddy Sadipiralla <srinath2133@gmail.com>,
	Amit Kapila <amit.kapila16@gmail.com>,
 Matthias van de Meent <boekewurm+postgres@gmail.com>,
	Pg Hackers <pgsql-hackers@lists.postgresql.org>,
 Robert Treat <rob@xzilla.net>
Subject: Re: Adding REPACK [concurrently]
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Hi,

On 2026-04-07 00:22:32 +0200, Alvaro Herrera wrote:
> From 4303eea0a72408183f9f5afcf8d2801df20f8ffe Mon Sep 17 00:00:00 2001
> From: Antonin Houska <ah@cybertec.at>
> Date: Wed, 1 Apr 2026 17:35:47 +0200
> Subject: [PATCH v56 3/3] Error out any process that would block at REPACK
> 
> Any process waiting on REPACK to release its lock would actually cause
> it to deadlock when it tries to upgrade its lock to AEL, losing all work
> done to that point.  We avoid this by teaching the deadlock detector to
> raise an error when this condition is detected.

I'm rather doubtful that that is ok. Won't this make it basically unsafe to
use repack in way way too many situations? If sessions start erroring out,
rather than wait, because they briefly lock a relation or such it'll imo make
this rather unsafe in production.

ISTM that the proper fix here might be to allow repack to do a lock upgrade
that jumps to the front of the lock's wait queue. The lock upgrade is the
reason for the deadlock, right? And the reason it will often cause a deadlock
is that the repack will be at the tail, rather than the front of the wait
queue. So let's fix that.

Jumping the queue won't make the lock upgrade immediately, of course, but I
think it would fix the issue of the repack, rather than some other process,
getting cancelled?

Greetings,

Andres Freund