Message-ID: <08b4f1f7dcf321d38373f06acc65a8d62cc2f017.camel@j-davis.com>
Subject: Re: Is this a problem in GenericXLogFinish()?
From: Jeff Davis <pgsql@j-davis.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>, pgsql-hackers@postgresql.org
Date: Wed, 27 Sep 2023 08:03:45 -0700
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On Wed, 2023-09-27 at 10:36 -0400, Robert Haas wrote:
> On Tue, Sep 26, 2023 at 9:36=E2=80=AFPM Jeff Davis <pgsql@j-davis.com> wr=
ote:
> > That site is pretty trivial to fix, but there are also a couple
> > places
> > in hash.c and hashovfl.c that are registering a clean page and it's
> > not
> > clear to me exactly what's going on.
>=20
> Huh, I wonder if that's just a bug. Do you know where exactly?

hashovfl.c:665 and hash.c:831

In both cases it's registering the bucket_buf, and has a comment like:

 /*
  * bucket buffer needs to be registered to ensure that we can
  * acquire a cleanup lock on it during replay.
  */

And various redo routines have comments like:

 /*
  * Ensure we have a cleanup lock on primary bucket page before we
start
  * with the actual replay operation.  This is to ensure that neither a
  * scan can start nor a scan can be already-in-progress during the
replay
  * of this operation.  If we allow scans during this operation, then
they
  * can miss some records or show the same record multiple times.
  */

So it looks like it's intentionally registering a clean buffer so that
it can take a cleanup lock for reasons other than cleaning (or even
modiying) the page. I would think that there's a better way of
accomplishing that goal, so perhaps we can fix that?

Regards,
	Jeff Davis