Date: Wed, 10 Jul 2024 10:44:14 -0500
From: Nathan Bossart <nathandbossart@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Jelte Fennema-Nio <postgres@jeltef.nl>,
	Robert Haas <robertmhaas@gmail.com>,
	Fujii Masao <masao.fujii@oss.nttdata.com>,
	pgsql-hackers@lists.postgresql.org
Subject: Re: Add a GUC check hook to ensure summarize_wal cannot be enabled
 when wal_level is minimal
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On Wed, Jul 10, 2024 at 11:11:13AM -0400, Tom Lane wrote:
> Please, no.  We went through a ton of permutations of that kind of
> idea years ago, when it first became totally clear that cross-checks
> between GUCs do not work nicely if implemented in check_hooks.
> (You can find all the things we tried in the commit log, although
> I don't recall exactly when.)

Understood.

> A counter-example for what you just
> said is when a configuration file like the above is loaded after
> postmaster start.

I haven't tested it, but from skimming around the code, it looks like
ProcessConfigFileInternal() would deduplicate any previous entries in the
file prior to applying the values and running the check hooks.  Else,
reloading a configuration file with multiple startup-only GUC entries could
fail, even without bogus GUC check hooks.

-- 
nathan