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From: Tom Lane <[email protected]>
To: Nathan Bossart <[email protected]>
Cc: Robert Haas <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: Julien Rouhaud <[email protected]>
Cc: Michael Paquier <[email protected]>
Cc: Bossart, Nathan <[email protected]>
Cc: Fujii Masao <[email protected]>
Cc: [email protected] <[email protected]>
Cc: Bharath Rupireddy <[email protected]>
Cc: Greg Sabino Mullane <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: make MaxBackends available in _PG_init
Date: Tue, 12 Apr 2022 16:58:42 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <20220412195908.GA2064996@nathanxps13>
References: <CA+TgmoZRnwF=9tMX0TiqZGC4qAK_27NweKd5r-S=OSaE_nL70A@mail.gmail.com>
	<20220411164408.GA1915258@nathanxps13>
	<CA+TgmoZVQznhWpUr-bjC9U=Won8ZnfozSkiXrW2HU80r6QyASQ@mail.gmail.com>
	<20220411204442.GA2004824@nathanxps13>
	<20220411211435.GA2057981@nathanxps13>
	<[email protected]>
	<20220412180306.GA2063516@nathanxps13>
	<CA+TgmoY4og1VADa78VySwsTjtTQbtXbnRPSq9x=e6ogS_wWfaw@mail.gmail.com>
	<[email protected]>
	<CA+TgmoZM7=VoAey4rf+sZEKFpGDWhqEqk_MHoBPXkYbAONGcyQ@mail.gmail.com>
	<20220412195908.GA2064996@nathanxps13>

Nathan Bossart <[email protected]> writes:
> I think it'd be reasonable to allow changing custom GUCs in _PG_init().
> Those aren't going to impact things like MaxBackends.

It's reasonable to allow changing *most* GUCs in _PG_init(); let us
please not break cases that work fine today.

It seems after a bit of reflection that what we ought to do is identify
the subset of PGC_POSTMASTER GUCs that feed into shared memory sizing,
mark those with a new GUC flag, and not allow them to be changed after
shared memory is set up.  (An alternative to a new flag bit is to
subdivide the PGC_POSTMASTER context into two values, but that seems
like it'd be far more invasive.  A flag bit needn't be user-visible.)
Then, what we'd need to do is arrange for shared-preload modules to
receive their _PG_init call before shared memory creation (when they
can still twiddle such GUCs) and then another callback later.

			regards, tom lane

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