Re: Weird failure with latches in curculio on v15 - Mailing list pgsql-hackers

Hi,

On 2023-02-09 11:12:21 -0500, Robert Haas wrote:
> On Thu, Feb 9, 2023 at 10:51 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > I'm fairly concerned about the idea of making it common for people
> > to write their own main loop for the archiver.  That means that, if
> > we have a bug fix that requires the archiver to do X, we will not
> > just be patching our own code but trying to get an indeterminate
> > set of third parties to add the fix to their code.

I'm somewhat concerned about that too, but perhaps from a different
angle. First, I think we don't do our users a service by defaulting the
in-core implementation to something that doesn't scale to even a moderately
busy server. Second, I doubt we'll get the API for any of this right, without
an acutual user that does something more complicated than restoring one-by-one
in a blocking manner.


> I don't know what kind of bug we could really have in the main loop
> that would be common to every implementation. They're probably all
> going to check for interrupts, do some work, and then wait for I/O on
> some things by calling select() or some equivalent. But the work, and
> the wait for the I/O, would be different for every implementation. I
> would anticipate that the amount of common code would be nearly zero.

I don't think it's that hard to imagine problems. To be reasonably fast, a
decent restore implementation will have to 'restore ahead'. Which also
provides ample things to go wrong. E.g.

- WAL source is switched, restore module needs to react to that, but doesn't,
  we end up lots of wasted work, or worse, filename conflicts
- recovery follows a timeline, restore module doesn't catch on quickly enough
- end of recovery happens, restore just continues on


> > If we think we need primitives to let the archiver hooks get all
> > the pending files, or whatever, by all means add those.  But don't
> > cede fundamental control of the archiver.  The hooks need to be
> > decoration on a framework we provide, not the framework themselves.
>
> I don't quite see how you can make asynchronous and parallel archiving
> work if the archiver process only calls into the archive module at
> times that it chooses. That would mean that the module has to return
> control to the archiver when it's in the middle of archiving one or
> more files -- and then I don't see how it can get control back at the
> appropriate time. Do you have a thought about that?

I don't think archiver is the hard part, that already has a dedicated
process, and it also has something of a queuing system already. The startup
process imo is the complicated one...

If we had a 'restorer' process, startup fed some sort of a queue with things
to restore in the near future, it might be more realistic to do something you
describe?

Greetings,

Andres Freund