Horton

Re: make MaxBackends available in _PG_init

Nathan Bossart <nathandbossart@gmail.com>

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From: Nathan Bossart <nathandbossart@gmail.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Julien Rouhaud <rjuju123@gmail.com>, 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>, Tom Lane <tgl@sss.pgh.pa.us>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Date: 2022-03-30T16:30:51Z
Lists: pgsql-hackers 
On Tue, Mar 29, 2022 at 12:22:21PM -0400, Robert Haas wrote:
> It's not, though, because the original proposal was to change things
> around so that the value of MaxBackends would have been reliable in
> _PG_init(). If we'd done that, then extensions that are using it in
> _PG_init() would have gone from being buggy to being not-buggy. But
> since you advocated against that change, we instead committed
> something that caused them to go from being buggy to failing outright.
> That's pretty painful for people with such extensions. And IMHO, it's
> *much* more legitimate to want to size a data structure based on the
> value of MaxBackends than it is for extensions to override GUC values.
> If we can make the latter use case work in a sane way, OK, although I
> have my doubts about how sane it really is, but it can't be at the
> expense of telling extensions that have been (incorrectly) using
> MaxBackends in _PG_init() that we're throwing them under the bus.
> 
> IMHO, the proper thing to do if certain GUC values are required for an
> extension to work is to put that information in the documentation and
> error out at an appropriate point if the user does not follow the
> directions. Then this issue does not arise. But there's no reasonable
> workaround for being unable to size data structures based on
> MaxBackends.

FWIW I would be on board with reverting all the GetMaxBackends() stuff if
we made the value available in _PG_init() and stopped supporting GUC
overrides by extensions (e.g., ERROR-ing in SetConfigOption() when
process_shared_preload_libraries_in_progress is true).

-- 
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