Re: make MaxBackends available in _PG_init

Hi,

On 2022-03-25 14:35:42 +0800, Julien Rouhaud wrote:
> On Fri, Mar 25, 2022 at 02:08:09PM +0900, Michael Paquier wrote:
> > On Fri, Mar 25, 2022 at 11:11:46AM +0800, Julien Rouhaud wrote:
> > > As an example, here's a POC for a new shmem_request_hook hook after _PG_init().
> > > With it I could easily fix pg_wait_sampling shmem allocation (and checked that
> > > it's indeed requesting the correct size).
> >
> > Are you sure that the end of a release cycle is the good moment to
> > begin designing new hooks? Anything added is something we are going
> > to need supporting moving forward. My brain is telling me that we
> > ought to revisit the business with GetMaxBackends() properly instead,
> > and perhaps revert that.
>
> I agree, and as I mentioned in my original email I don't think that the
> committed patch is actually adding something on which we can really build on.
> So I'm also in favor of reverting, as it seems like be a better option in the
> long run to have a clean and broader solution.

I don't really understand. The issue that started this thread was bugs in
extensions due to accessing MaxBackends before it is initialized - which the
patch prevents. The stuff that you're complaining about / designing here
doesn't seem related to that. I like the idea of the hooks etc, but I fail to
see why we "ought to revisit the business with GetMaxBackends()"?

Greetings,

Andres Freund