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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Nathan Bossart <nathandbossart@gmail.com>
Cc: Jelte Fennema-Nio <postgres@jeltef.nl>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Fujii Masao <masao.fujii@oss.nttdata.com>
Cc: pgsql-hackers@lists.postgresql.org
Subject: Re: Add a GUC check hook to ensure summarize_wal cannot be enabled when wal_level is minimal
Date: Wed, 10 Jul 2024 11:54:38 -0400
Message-ID: <1749638.1720626878@sss.pgh.pa.us> (raw)
In-Reply-To: <Zo6sTje4m0czRor9@nathan>
References: <6e30082e-041b-4e31-9633-95a66de76f5d@oss.nttdata.com>
	<ZoVgXuYWbzTLXyRY@nathan>
	<802ff3d5-fdc7-4b08-9046-2e9fc5fdae0e@oss.nttdata.com>
	<CA+TgmoYPNfgAT3OubjoYSiXTb3-urOjQxT3SCEUCrRBx5g6ihA@mail.gmail.com>
	<CA+TgmoY8DnzWNK_xu4=NTYdxGBGnFucrEpOALnS0awMp-s_pKQ@mail.gmail.com>
	<7c1b48d8-0f08-4733-a4e6-d55f04581db7@oss.nttdata.com>
	<CA+TgmoansG0B8ayQGw9Ozee6Jawz=HFUACpO_4gp-+SFJBxiYw@mail.gmail.com>
	<Zo6YPuvTmRW3b1s7@nathan>
	<CAGECzQQcLYiAdRj5xAm1G43MtpD_rm029ziqrbpqyDdxaCoEQA@mail.gmail.com>
	<1744272.1720624273@sss.pgh.pa.us>
	<Zo6sTje4m0czRor9@nathan>

Nathan Bossart <nathandbossart@gmail.com> writes:
> 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.

While it's been a little while since I actually traced the logic,
I believe the reason that case doesn't fail is this bit in
set_config_with_handle, about line 3477 as of HEAD:

        case PGC_POSTMASTER:
            if (context == PGC_SIGHUP)
            {
                /*
                 * We are re-reading a PGC_POSTMASTER variable from
                 * postgresql.conf.  We can't change the setting, so we should
                 * give a warning if the DBA tries to change it.  However,
                 * because of variant formats, canonicalization by check
                 * hooks, etc, we can't just compare the given string directly
                 * to what's stored.  Set a flag to check below after we have
                 * the final storable value.
                 */
                prohibitValueChange = true;
            }
            else if (context != PGC_POSTMASTER)
                // throw "cannot be changed now" error

			regards, tom lane





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