Re: relfrozenxid may disagree with row XIDs after 1ccc1e05ae

On Thu, 21 Mar 2024 at 17:15, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Sun, Mar 3, 2024 at 7:07 PM Noah Misch <noah(at)leadboat(dot)com> wrote:
> > I figure Matthias's upthread theory is more likely than not to hold. If it
> > does hold, commit 1ccc1e05ae created a new corruption route. Hence, I'm
> > adding a v17 open item for commit 1ccc1e05ae.
>
> I need some help understanding what's going on here. I became aware of
> this thread because I took a look at the open items list.
>
> This email seems to have branched off of the thread for bug #17257,
> reported 2021-10-29. The antecedent of "Matthias's upthread theory" is
> unclear to me. These emails seem like the most relevant ones:
>
> https://www.postgresql.org/message-id/CAEze2Wj7O5tnM_U151Baxr5ObTJafwH%3D71_JEmgJV%2B6eBgjL7g%40mail.gmail.com
> https://www.postgresql.org/message-id/CAEze2WhxhEQEx%2Bc%2BCXoDpQs1H1HgkYUK4BW-hFw5_eQxuVWqRw%40mail.gmail.com
> https://www.postgresql.org/message-id/20240106202413.e5%40rfd.leadboat.com
>
> But I'm having a hard time piecing it all together. The general
> picture seems to be that pruning and vacuum disagree about whether a
> particular tuple is prunable; before 1ccc1e05ae, that caused the retry
> loop in heap_page_prune() to retry forever. Now, it causes
> relfrozenxid to be set to too new a value, which is a data-corruption
> scenario. If that's right, I'm slightly miffed to find this being
> labeled as an open item, since that makes it seem like 1ccc1e05ae
> didn't create any new problem but only caused existing defects in the
> GlobalVisTest machinery to have different consequences. Perhaps it's
> all for the best, though. It's kind of embarrassing that we haven't
> fixed whatever the problem is here yet.
>
> But what exactly is the problem, and what's the fix? In the first of
> the emails linked above, Matthias argues that the problem is that
> GlobalVisState->maybe_needed can move backward. Peter Geoghegan seems
> to agree with that here:
>
> https://www.postgresql.org/message-id/CAH2-Wzk_L7Z7LREHTtg5vY08eeWdnHO70m98eWx4U1uwvW%3D0sA%40mail.gmail.com
>
> And Peter seems to have been trying to make sense of Andres's remarks
> here, which I think are saying the same thing:
>
> https://www.postgresql.org/message-id/20210616192202.6q63mu66h4uyn343%40alap3.anarazel.de
>
> So it seems like Matthias, Peter, and Andres all agree that
> GlobalVisState->maybe_needed going backward is bad and causes this
> problem. Unfortunately, I don't understand the mechanism.

There are 2 mechanisms I know of which allow this value to go backwards:

1. Replication slots that connect may set their backend's xmin to an
xmin < GlobalXmin.
This is known and has been documented, and was considered OK when this
was discussed on the list previously.

2. The commit abort path has a short window in which the backend's
xmin is unset and does not mirror the xmin of registered snapshots.
This is what I described in [0], and may be the worst (?) offender.

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

[0] https://www.postgresql.org/message-id/CAEze2Wj%2BV0kTx86xB_YbyaqTr5hnE_igdWAwuhSyjXBYscf5-Q%40mail.gmail.com