Date: Tue, 12 Apr 2022 12:59:08 -0700
From: Nathan Bossart <nathandbossart@gmail.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Andres Freund <andres@anarazel.de>,
	Julien Rouhaud <rjuju123@gmail.com>,
	Michael Paquier <michael@paquier.xyz>,
	"Bossart, Nathan" <bossartn@amazon.com>,
	Fujii Masao <masao.fujii@oss.nttdata.com>,
	"wangsh.fnst@fujitsu.com" <wangsh.fnst@fujitsu.com>,
	Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>,
	Greg Sabino Mullane <htamfids@gmail.com>,
	"pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Subject: Re: make MaxBackends available in _PG_init
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On Tue, Apr 12, 2022 at 03:30:23PM -0400, Robert Haas wrote:
> On Tue, Apr 12, 2022 at 3:22 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Yeah.  It's a very long way from "changing shared memory sizes here
>> doesn't work" to "no extension is allowed to change any GUC at load
>> time".  A counterexample is that it's not clear why an extension
>> shouldn't be allowed to define a GUC using DefineCustomXXXVariable
>> and then immediately set it to some other value.  I think trying to
>> forbid that across-the-board is going to mostly result in breaking
>> a lot of cases that are perfectly safe.
> 
> Hmm, that's an interesting case which I hadn't considered. It seems
> like sort of a lame thing to do, because why wouldn't you just create
> the GUC with the right initial value instead of modifying it after the
> fact? But maybe there's some use case in which it makes sense to do it
> that way. A read-only GUC that advertises some calculated value,
> perhaps?

I think it'd be reasonable to allow changing custom GUCs in _PG_init().
Those aren't going to impact things like MaxBackends.

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com