Feedback-ID: id4a34324:Fastmail
Date: Wed, 29 Nov 2023 10:55:55 -0800
From: Andres Freund <andres@anarazel.de>
To: Nathan Bossart <nathandbossart@gmail.com>
Cc: David Geier <geidav.pg@gmail.com>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: Fix assertion in autovacuum worker
Message-ID: <20231129185555.tnr3noi7qcjqqiw2@awork3.anarazel.de>
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 <20231129175201.GA902323@nathanxps13>
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Hi,

On 2023-11-29 11:52:01 -0600, Nathan Bossart wrote:
> On Tue, Nov 28, 2023 at 06:48:59PM -0800, Andres Freund wrote:
> > On 2023-11-28 20:42:47 -0600, Nathan Bossart wrote:
> >> Right.  Perhaps we could add a LWLockReleaseAll() to
> >> pgstat_shutdown_hook() instead of the autovacuum code, but I'm afraid that
> >> is still just a hack.
> >
> > Yea, we'd need that in just about all before_shmem_exit() callbacks.  I could
> > see an argument for doing it in proc_exit_prepare(). While that'd be a fairly
> > gross layering violation, we already do reset a number a bunch of stuff in
> > there:
>
> Gross layering violations aside, that at least seems more future-proof
> against other sigsetjmp() blocks that proc_exit() without doing any
> preliminary cleanup.

It's not just sigsetjmp() blocks not doing cleanup that are problematic -
consider what happens if there's either no sigsetjmp() block established (and
thus ERROR is promoted to FATAL), or if a FATAL error is raised. Then cleanup
in the sigsetjmp() site doesn't matter.  So we really need a better answer
here.

If we don't want to add LWLockReleaseAll() to proc_exit_prepare(), ISTM we
should all at least add some assertion infrastructure verifying it's being
called in relevant paths, triggering an assertion if there was no
LWLockReleaseAll() before reaching important before_shmem_exit() routines,
even if we don't actually hold an lwlock at the time of the error. Otherwise
problematic paths are way too hard to find.

Greetings,

Andres Freund