Re: needless complexity in StartupXLOG

On Wed, Jul 28, 2021 at 3:28 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > Imagine if instead of
> > all the hairy logic we have now we just replaced this whole if
> > (IsInRecovery) stanza with this:
> >
> > if (InRecovery)
> > CreateEndOfRecoveryRecord();
> >
> > That would be WAY easier to reason about than the rat's nest we have
> > here today. Now, I am not sure what it would take to get there, but I
> > think that is the direction we ought to be heading.
>
> What are we going to do in the single user ([1]) case in this awesome future?
> I guess we could just not create a checkpoint until single user mode is shut
> down / creates a checkpoint for other reasons?

It probably depends on who writes this thus-far-hypothetical patch,
but my thought is that we'd unconditionally request a checkpoint after
writing the end-of-recovery record, same as we do now if (promoted).
If we happened to be in single-user mode, then that checkpoint request
would turn into performing a checkpoint on the spot in the one and
only process we've got, but StartupXLOG() wouldn't really need to care
what happens under the hood after it called RequestCheckpoint().

> [1] I really wish somebody had the energy to just remove single user and
> bootstrap modes. The degree to which they increase complexity in the rest of
> the system is entirely unreasonable. There's not actually any reason
> bootstrapping can't happen with checkpointer et al running, it's just
> autovacuum that'd need to be blocked.

I don't know whether this is the way forward or not. I think a lot of
the complexity of the current system is incidental rather than
intrinsic. If I were going to work on this, I'd probably working on
trying to tidy up the code and reduce the number of places that need
to care about IsUnderPostmaster and IsPostmasterEnvironment, rather
than trying to get rid of them. I suspect that's a necessary
prerequisite step anyway, and not a trivial effort either.

--
Robert Haas
EDB: http://www.enterprisedb.com