Re: make MaxBackends available in _PG_init

From: Nathan Bossart <nathandbossart(at)gmail(dot)com>
To: Julien Rouhaud <rjuju123(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Michael Paquier <michael(at)paquier(dot)xyz>, "Bossart, Nathan" <bossartn(at)amazon(dot)com>, Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com>, "wangsh(dot)fnst(at)fujitsu(dot)com" <wangsh(dot)fnst(at)fujitsu(dot)com>, Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>, Greg Sabino Mullane <htamfids(at)gmail(dot)com>, "pgsql-hackers(at)lists(dot)postgresql(dot)org" <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: make MaxBackends available in _PG_init
Date: 2022-04-14 16:22:15
Message-ID: 20220414162215.GB2163833@nathanxps13
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On Thu, Apr 14, 2022 at 01:50:24PM +0800, Julien Rouhaud wrote:
> On Wed, Apr 13, 2022 at 11:30:40AM -0700, Nathan Bossart wrote:
>> If we do move forward with the shmem request hook, do we want to disallow
>> shmem requests anywhere else, or should we just leave it up to extension
>> authors to do the right thing? Many shmem requests in _PG_init() are
>> probably okay unless they depend on MaxBackends or another GUC that someone
>> might change. Given that, I think I currently prefer the latter (option B
>> from upthread).
>
> I'd be in favor of a hard break. There are already multiple extensions that
> relies on non final value of GUCs to size their shmem request. And as an
> extension author it can be hard to realize that, as those extensions work just
> fine until someone wants to try it with some other extension that changes some
> GUC. Forcing shmem request in a new hook will make sure that it's *always*
> correct, and that only requires very minimal work on the extension side.

Yeah, this is a good point. If we're okay with breaking existing
extensions like this, I will work on a patch.

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

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