Horton

Re: make MaxBackends available in _PG_init

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Nathan Bossart <nathandbossart@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: 2022-05-06T14:43:21Z
Lists: pgsql-hackers 
Robert Haas <robertmhaas@gmail.com> writes:
> On Fri, May 6, 2022 at 9:57 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> I agree that _PG_fini functions as they stand are worthless.
>> What I'm not getting is why we should care enough about that
>> to break just about everybody's extension.  Even if unloading
>> extensions were feasible, who would bother?

> Well, if we think that, then we ought to remove the NOT_USED code and
> all the random _PG_fini() stuff that's still floating around.

I think that's exactly what we should do, if it bugs you that stuff
is just sitting there.  I see no prospect that we'll ever make it
work, because the question of unhooking from hooks is just the tip
of the iceberg.  As an example, what should happen with any custom
GUCs the module has defined?  Dropping their values might not be
very nice, but if we leave them around then the next LOAD (if any)
will see a conflict.  Another fun question is whether it's ever
safe to unload a module that was preloaded by the postmaster.

In short, this seems like a can of very wriggly worms, with not
a lot of benefit that would ensue from opening it.

			regards, tom lane

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