Re: make MaxBackends available in _PG_init

On Wed, Mar 23, 2022 at 09:03:18PM +0800, Julien Rouhaud wrote:
> On Wed, Mar 23, 2022 at 08:32:39AM -0400, Robert Haas wrote:
>> Well, the conclusion upthread was that extensions might change the
>> values of those GUCs from _PG_init(). If that's a real thing, then
>> what you're asking for here is impossible, because the final value is
>> indeterminate until all such extensions have finished twiddling those
>> the GUCs. On the other hand, it's definitely intended that extensions
>> should RequestAddinShmemSpace() from _PG_init(), and wanting to size
>> that memory based on MaxBackends is totally reasonable. Do we need to
>> add another function, alongside _PG_init(), that gets called after
>> MaxBackends is determined and before it's too late to
>> RequestAddinShmemSpace()?
> 
> Yes, I don't see how we can support such extensions without an additional hook,
> probably called right after InitializeMaxBackends() since at least
> InitializeShmemGUCs() will need to know about those extra memory needs.

Another possibility could be to add a hook that is called _before_
_PG_init() where libraries are permitted to adjust GUCs.  After the library
is loaded, we first call this _PG_change_GUCs() function, then we
initialize MaxBackends, and then we finally call _PG_init().  This way,
extensions would have access to MaxBackends within _PG_init(), and if an
extension really needed to alter GUCs, іt could define this new function.

> I'm not sure how to prevent third party code from messing with the gucs in it,
> but a clear indication in the hook comment should probably be enough.  It's not
> like it's hard for third-party code author to break something anyway.

ERROR-ing in SetConfigOption() might be another way to dissuade folks from
messing with GUCs.  This is obviously not a perfect solution.

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