Feedback-ID: id4a34324:Fastmail
Date: Sun, 12 Apr 2026 10:05:34 -0400
From: Andres Freund <andres@anarazel.de>
To: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Cc: Antonin Houska <ah@cybertec.at>, Amit Kapila <amit.kapila16@gmail.com>,
	Alvaro Herrera <alvherre@alvh.no-ip.org>,
 Srinath Reddy Sadipiralla <srinath2133@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 2026-04-12 15:31:20 +0200, Mihail Nikalayeu wrote:
> On Thu, Apr 9, 2026 at 4:20 PM Andres Freund <andres@anarazel.de> wrote:
> > But with my proposal to properly teach the deadlock detector about
> assuming
> > there's a wait edge for the eventual lock upgrade by S1, the first example
> > would still work, because the lock upgrade would not be considered a hard
> > cycle, and the second example would have S2 error out.
> 
> Attached patch contains some (maybe naive) POC for similar approach.
> It adds a 'deadlock_protected' flag, which changes how the deadlock
> detector cancels backends.
> 
> Instead of cancelling the backend entered the deadlock detector - it
> cancel some another (nearest hard edge) until it is possible to get the
> lock (either by
> reordering or directly).

I don't think that's as good.  The problem is that that way you're only
detecting the deadlocks once they have materialized (i.e. once repack actually
does the lock upgrade), rather than cancelling when we know that the problem
starts.  Having sessions pointlessly blocked for many hours is bad.


> Also, I added test cases with the scenarios you mentioned into the repack
> spec - to ensure repack is still working while other backend are cancelled.

I think we should perhaps commit spec tests for these (I've not yet reviewed
them, but in principle), even before we fix the problem. It's good to document
the current behavior and have a comment that the wrongly cancelled case should
not trigger an error.

Greetings,

Andres Freund