Re: make MaxBackends available in _PG_init

Hi,

On Mon, Apr 18, 2022 at 08:17:04PM -0400, Tom Lane wrote:
> Nathan Bossart <nathandbossart(at)gmail(dot)com> writes:
> > I'm looking for a clean way to ERROR if someone attempts to call
> > RequestAddinShmemSpace() or RequestNamedLWLockTranche() outside of the
> > hook. Currently, we are using static variables in ipci.c and lwlock.c to
> > silently ignore invalid requests. I could add a new 'extern bool' called
> > 'process_shmem_requests_in_progress', but extensions could easily hack
> > around that to allow requests in _PG_init(). Maybe I am overthinking all
> > this and that is good enough.
>
> If they do that and it breaks something, that's their fault not ours.
> (It's not like there's not $BIGNUM ways for a C-language module to
> break the backend, anyway.)

Agreed. Similarly the process_shared_preload_libraries_in_progress flag could
be modified by extension, and that wouldn't be any better.

> BTW, I'd make such errors FATAL, as it's unlikely that we can recover
> cleanly from an error during initialization of a loadable module.
> The module's likely to be only partially initialized/hooked in.

While at it, should we make process_shmem_requests_in_progress true when the
new hook is called? The hook should only be called when that's the case, and
extension authors may feel like asserting it.