Horton

Re: make MaxBackends available in _PG_init

Andres Freund <andres@anarazel.de>

View thread View on postgr.es
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: 2022-03-26T17:23:16Z
Lists: pgsql-hackers 
Hi,

On 2022-03-26 15:22:03 +0800, Julien Rouhaud wrote:
> On Fri, Mar 25, 2022 at 03:23:17PM -0700, Andres Freund wrote:
> >
> > 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.
> 
> Well, the patch prevents accessing a 0-valued MaxBackends but doesn't do
> anything to solve the original complaint.  It's not like extensions won't need
> to access that information during _PG_init anymore.

It resolves the pretty common bug that an extension breaks once it's used via
s_p_l instead of loaded on-demand because MaxBackends isn't initialized in the
s_p_l case.


> And indeed, any third party code that previously needed to access what
> MaxBackends is supposed to store should already be using that formula, and
> the new GetMaxBackends() doesn't do anything about it.

It couldn't rely on MaxBackends before. It can't rely on GetMaxBackends()
now. You can see why I think that what you want is unrelated to the
introduction of GetMaxBackends().

If we introduce a separate hook that allows to influence things like
max_connections or whatnot we'd *even more* need a way to verify whether it's
legal to access MaxBackends in that moment.


> So all extensions will be as broken as before, except the few that were
> using MaxBackends without realizing it's 0. And if those exist (there's
> actually one) they're not that broken, probably because MaxBackend is only
> used to request additional shmem, with wanted value small enough so that
> it's compensated by the extra 100kB shmem postgres allocates.

I don't think it's rare at all.

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