Re: make MaxBackends available in _PG_init

From: Nathan Bossart <nathandbossart(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Julien Rouhaud <rjuju123(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: 2022-04-18 22:52:44
Message-ID: 20220418225244.GA2387491@nathanxps13
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On Thu, Apr 14, 2022 at 12:39:46PM -0400, Tom Lane wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Thu, Apr 14, 2022 at 12:22 PM Nathan Bossart
>> <nathandbossart(at)gmail(dot)com> wrote:
>>>> I'd be in favor of a hard break.
>>> Yeah, this is a good point. If we're okay with breaking existing
>>> extensions like this, I will work on a patch.
>> I tend to think it's a good idea.
> I've come around to that view as well.

Here is a new patch set that introduces the aforementioned "hard break."

I noticed that requests for more LWLocks follow a similar pattern as
regular shared memory requests, and I figured that we would want to do
something similar for those, but I wasn't sure exactly how to proceed. I
saw two options: 1) use shmem_request_hook for both regular requests and
LWLock requests or 2) introduce an lwlock_request_hook. My instinct was
that option 1 was preferable, but AFAICT this requires introducing a new
external variable for inspecting whether the request is made at a valid
time. This would be similar to
process_shared_preload_libraries_in_progress, which I believe means a
determined extension author could easily hack around the request
restrictions. I thought option 2 added too much machinery to work around
this problem. For now, I haven't made any changes for LWLock requests.
What are your thoughts?

Nathan Bossart
Amazon Web Services:

Attachment Content-Type Size
v4-0001-Fix-comments-about-bgworker-registration-before-M.patch text/x-diff 3.0 KB
v4-0002-Add-a-new-shmem_request_hook-hook.patch text/x-diff 10.4 KB

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