Horton

Re: make MaxBackends available in _PG_init

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Julien Rouhaud <rjuju123@gmail.com>
Cc: Michael Paquier <michael@paquier.xyz>, Robert Haas <robertmhaas@gmail.com>, Nathan Bossart <nathandbossart@gmail.com>, "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>, Tom Lane <tgl@sss.pgh.pa.us>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Date: 2022-03-25T22:23:17Z
Lists: pgsql-hackers 
Hi,

On 2022-03-25 14:35:42 +0800, Julien Rouhaud wrote:
> On Fri, Mar 25, 2022 at 02:08:09PM +0900, Michael Paquier wrote:
> > On Fri, Mar 25, 2022 at 11:11:46AM +0800, Julien Rouhaud wrote:
> > > As an example, here's a POC for a new shmem_request_hook hook after _PG_init().
> > > With it I could easily fix pg_wait_sampling shmem allocation (and checked that
> > > it's indeed requesting the correct size).
> > 
> > Are you sure that the end of a release cycle is the good moment to
> > begin designing new hooks?  Anything added is something we are going
> > to need supporting moving forward.  My brain is telling me that we
> > ought to revisit the business with GetMaxBackends() properly instead,
> > and perhaps revert that.
> 
> I agree, and as I mentioned in my original email I don't think that the
> committed patch is actually adding something on which we can really build on.
> So I'm also in favor of reverting, as it seems like be a better option in the
> long run to have a clean and broader solution.

I don't really understand. The issue that started this thread was bugs in
extensions due to accessing MaxBackends before it is initialized - which the
patch prevents. The stuff that you're complaining about / designing here
doesn't seem related to that. I like the idea of the hooks etc, but I fail to
see why we "ought to revisit the business with GetMaxBackends()"?

Greetings,

Andres Freund

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