Horton

Re: make MaxBackends available in _PG_init

Nathan Bossart <nathandbossart@gmail.com>

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From: Nathan Bossart <nathandbossart@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Robert Haas <robertmhaas@gmail.com>, Julien Rouhaud <rjuju123@gmail.com>, Andres Freund <andres@anarazel.de>, 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>
Date: 2022-04-18T22:52:44Z
Lists: pgsql-hackers

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On Thu, Apr 14, 2022 at 12:39:46PM -0400, Tom Lane wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> On Thu, Apr 14, 2022 at 12:22 PM Nathan Bossart
>> <nathandbossart@gmail.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: https://aws.amazon.com

Commits

  1. Add a new shmem_request_hook hook.

  2. Remove non-functional code for unloading loadable modules.

  3. Fix misleading comments about background worker registration.

  4. Revert the addition of GetMaxBackends() and related stuff.

  5. Fix typo in multixact.c

  6. Reduce more the number of calls to GetMaxBackends()

  7. Remove MaxBackends variable in favor of GetMaxBackends() function.

  8. Fix comments about bgworker registration before MaxBackends initialization

  9. Fix bogus assertion in BootstrapModeMain().

  10. Make sure MaxBackends is always set