Date: Sat, 18 May 2024 15:23:11 -0700
From: Noah Misch <noah@leadboat.com>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Robert Haas <robertmhaas@gmail.com>,
	Matthias van de Meent <boekewurm+postgres@gmail.com>,
	Peter Geoghegan <pg@bowt.ie>,
	Alexander Lakhin <exclusion@gmail.com>,
	PostgreSQL mailing lists <pgsql-bugs@lists.postgresql.org>
Subject: Re: relfrozenxid may disagree with row XIDs after 1ccc1e05ae
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On Thu, May 16, 2024 at 07:57:27PM -0400, Melanie Plageman wrote:
> On Thu, May 9, 2024 at 5:56 PM Melanie Plageman <melanieplageman@gmail.com> wrote:
> > I can repro the hang on 14 and 15 with the following:

> I'll probably add more robust comments to the test next week in
> preparation for writing a detailed commit message for the fix
> explaining the scenario.

Are there obstacles to fixing the hang by back-patching 1ccc1e05ae instead of
this?  We'll need to get confident about 1ccc1e05ae before v17, and that
sounds potentially easier than getting confident about both 1ccc1e05ae and
this other approach.  Regarding getting confident about 1ccc1e05ae, I think I
follow the upthread arguments that it does operate correctly.  As a cross
check, I looked at each mention of oldestxmin in vacuumlazy.c and vacuum.c.
Does the following heap_vacuum_rel() comment need an update?

	/*
	 * Get cutoffs that determine which deleted tuples are considered DEAD,
	 * not just RECENTLY_DEAD, and which XIDs/MXIDs to freeze.  Then determine
	 * the extent of the blocks that we'll scan in lazy_scan_heap.  It has to
	 * happen in this order to ensure that the OldestXmin cutoff field works
	 * as an upper bound on the XIDs stored in the pages we'll actually scan
	 * (NewRelfrozenXid tracking must never be allowed to miss unfrozen XIDs).
	 *
	 * Next acquire vistest, a related cutoff that's used in pruning.  We
	 * expect vistest will always make heap_page_prune_and_freeze() remove any
	 * deleted tuple whose xmax is < OldestXmin.