Horton

Re: make MaxBackends available in _PG_init

Nathan Bossart <nathandbossart@gmail.com>

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From: Nathan Bossart <nathandbossart@gmail.com>
To: Michael Paquier <michael@paquier.xyz>
Cc: "Bossart, Nathan" <bossartn@amazon.com>, Robert Haas <robertmhaas@gmail.com>, Fujii Masao <masao.fujii@oss.nttdata.com>, "wangsh.fnst@fujitsu.com" <wangsh.fnst@fujitsu.com>, Andres Freund <andres@anarazel.de>, 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: 2022-01-29T04:33:42Z
Lists: pgsql-hackers 
On Sat, Jan 29, 2022 at 11:19:12AM +0900, Michael Paquier wrote:
> On Thu, Jan 27, 2022 at 10:18:15AM -0800, Nathan Bossart wrote:
>> Alright.  I think the comment adjustments still apply, so I split those out
>> to a new patch.
> 
> Looks fine after a second look, so applied.

Thanks!

> As of the issues of this thread, we really have two things to think
> about:
> 1) How do we want to control the access of some parameters in a
> context or another?  One idea would be more control through GUCs, say
> with a set of context-related flags that prevent the read of some
> variables until they are set.  We could encourage the use of
> GetConfigOption() for that.  For MaxBackends, we could add a read-only
> GUC for this purpose.  That's what Andres hinted at upthread, I
> guess.
> 2) How do we deal with unwanted access of shared parameters?  This one
> is not really controllable, is it?  And we are talking about much more
> than MaxBackends.  This could perhaps be addressed with more
> documentation in the headers for the concerned variables, as a first
> step.

Hm.  Perhaps we should understand the full scope of the problem first.
What else besides MaxBackends and the shared memory GUCs won't be properly
initialized when the shared_preload_libraries' _PG_init() functions are
called?  MaxBackends seems to be the only one that folks have experienced
problems with, which is why I initially zeroed in on it.

-- 
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