Re: [15] Custom WAL resource managers, single user mode, and recovery

Hi,

On 2022-07-14 20:48:57 -0400, Tom Lane wrote:
> Jeff Davis <pgsql(at)j-davis(dot)com> writes:
> > If a user has custom table access methods, those are likely to be
> > required during single user mode anyway (to VACUUM, for instance), so
> > we need to be able to load modules during single user mode. I can't
> > think of a reason specifically why we can't process
> > shared_preload_libraries. Thoughts?
>
> Hmm. There may have been some idea that a library might fail in
> single-user mode, and/or have been the cause of you needing to
> use single-user in the first place. But if so, you can always
> edit it out of shared_preload_libraries. I agree that not
> having the option to load it isn't great.

+1

> I think that the patch might be missing some stuff. In postmaster.c,
> there are several steps after process_shared_preload_libraries()
> that look to be there to support loadable libraries, such as
> process_shmem_requests(). Wouldn't it be reasonable to expect
> that a loadable library would malfunction without those?

+1

> (I wonder if we shouldn't refactor this so that the postmaster
> and standalone mode share more of the initialization logic.
> Keeping these bits in sync seems unlikely to happen otherwise.)

Yes, that might be worthwhile. OTOH, I wonder if we should spend that time to
remove single user mode instead - the architectural complexity really doesn't
seem worth it anymore, and IMO my prototype from a few months back showed that
it's feasible.

Greetings,

Andres Freund