Date: Thu, 14 Apr 2022 18:30:10 -0700
From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Robert Haas <robertmhaas@gmail.com>,
	Justin Pryzby <pryzby@telsasoft.com>,
	Kyotaro Horiguchi <horikyota.ntt@gmail.com>,
	Erik Rijkers <er@xs4all.nl>,
	Matthias van de Meent <boekewurm+postgres@gmail.com>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: TRAP: FailedAssertion("HaveRegisteredOrActiveSnapshot()", File:
 "toast_internals.c", Line: 670, PID: 19403)
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Hi,

On 2022-04-14 15:05:50 -0400, Tom Lane wrote:
> Andres' complaint is that that snapshot might get invalidated when you
> weren't expecting it, but I'm not really convinced that we have all
> that many bugs of that ilk.  Wouldn't CLOBBER_CACHE_ALWAYS testing
> find them?

Don't see why it would - we don't have any mechanism in place for
enforcing that we don't update / delete a tuple we've looked up with an
xmin that wasn't continually enforced.  A typical pattern is to use a
catalog cache (registered an all) for a syscache lookup, but then not
have a registered / active snapshot until an eventual update / delete
(after the syscache scan ends). Which isn't safe, because without a
MyProc->xmin set, the tuple we're updating / deleting could be updated,
removed and replaced with another tuple.

Greetings,

Andres Freund