Date: Tue, 1 Feb 2022 14:36:40 -0800
From: Nathan Bossart <nathandbossart@gmail.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: 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>,
	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>
Subject: Re: make MaxBackends available in _PG_init
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On Mon, Jan 31, 2022 at 09:32:21AM -0500, Robert Haas wrote:
> The main reason why this doesn't work for MaxBackends is that
> MaxBackends depends on the values of multiple GUCs. There is a further
> wrinkle too, which is that none of those GUCs can change, and
> therefore code does things with the resulting value on the assumption
> that they won't change, like size shared-memory data structures.
> Therefore, if you read the wrong value, you've got a big problem. So
> the real issues here IMHO are about the difficulty of making sure that
> (1) when a GUC changes, we update all of the things that depend on it
> including things which may also depend on other GUCs and (2) making
> sure that values which can't ever change are computed before they are
> used.
> 
> I don't know what the solution to problem #1 is, but the solution to
> problem #2 is simple: make people call a function to get the value
> rather than just reading a bare variable. GetConfigOption() is not a
> good solution for people aiming to write C code that does useful
> things, because it delivers the value as a string, and that is not
> what you want. But an accessor function like GetMaxBackends() for a
> quantity of this type is wonderful. Depending on the situation, you
> might choose to have the accessor function [a] fail an assertion if
> the value is not available yet or [b] compute the value if they value
> has not yet been computed or [c] do the latter if possible, otherwise
> the former. But the fact that you are making code call a function
> rather than just read a variable gives you a very strong tool to make
> sure that someone can't blindly read a 0 or whatever instead of the
> real value.

+1

I can work on a new patch if this is the direction we want to go.  There
were a couple of functions that called GetMaxBackends() repetitively that I
should probably fix before the patch should be seriously considered.

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com