On Sat, Aug 27, 2022 at 1:06 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Sat, Aug 27, 2022 at 3:56 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Fri, Jul 29, 2022 at 12:15 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > >
> > > >
> > > > Yeah, your description makes sense to me. I've also considered how to
> > > > hit this path but I guess it is never hit. Thinking of it in another
> > > > way, first of all, at least 2 catalog modifying transactions have to
> > > > be running while writing a xl_running_xacts. The serialized snapshot
> > > > that is written when we decode the first xl_running_xact has two
> > > > transactions. Then, one of them is committed before the second
> > > > xl_running_xacts. The second serialized snapshot has only one
> > > > transaction. Then, the transaction is also committed after that. Now,
> > > > in order to execute the path, we need to start decoding from the first
> > > > serialized snapshot. However, if we start from there, we cannot decode
> > > > the full contents of the transaction that was committed later.
> > > >
> > >
> > > I think then we should change this code in the master branch patch
> > > with an additional comment on the lines of: "Either all the xacts got
> > > purged or none. It is only possible to partially remove the xids from
> > > this array if one or more of the xids are still running but not all.
> > > That can happen if we start decoding from a point (LSN where the
> > > snapshot state became consistent) where all the xacts in this were
> > > running and then at least one of those got committed and a few are
> > > still running. We will never start from such a point because we won't
> > > move the slot's restart_lsn past the point where the oldest running
> > > transaction's restart_decoding_lsn is."
> > >
> >
> > Unfortunately, this theory doesn't turn out to be true. While
> > investigating the latest buildfarm failure [1], I see that it is
> > possible that only part of the xacts in the restored catalog modifying
> > xacts list needs to be purged. See the attached where I have
> > demonstrated it via a reproducible test. It seems the point we were
> > missing was that to start from a point where two or more catalog
> > modifying were serialized, it requires another open transaction before
> > both get committed, and then we need the checkpoint or other way to
> > force running_xacts record in-between the commit of initial two
> > catalog modifying xacts. There could possibly be other ways as well
> > but the theory above wasn't correct.
> >
>
> Thank you for the analysis and the patch. I have the same conclusion.
> Since we took this approach only on the master the back branches are
> not affected.
>
> The new test scenario makes sense to me and looks better than the one
> I have. Regarding the fix, I think we should use
> TransactionIdFollowsOrEquals() instead of
> NormalTransactionIdPrecedes():
>
> + for (off = 0; off < builder->catchange.xcnt; off++)
> + {
> + if (NormalTransactionIdPrecedes(builder->catchange.xip[off],
> + builder->xmin))
> + break;
> + }
>
Right, fixed.
--
With Regards,
Amit Kapila.
