Date: Sat, 19 Feb 2022 20:57:57 -0800
From: Noah Misch <noah@leadboat.com>
To: John Naylor <john.naylor@enterprisedb.com>
Cc: Peter Geoghegan <pg@bowt.ie>, Andres Freund <andres@anarazel.de>,
	Robert Haas <robertmhaas@gmail.com>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: do only critical work during single-user vacuum?
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On Wed, Feb 16, 2022 at 03:43:12PM +0700, John Naylor wrote:
> On Wed, Feb 16, 2022 at 6:17 AM Peter Geoghegan <pg@bowt.ie> wrote:
> > On Tue, Feb 15, 2022 at 9:28 AM Peter Geoghegan <pg@bowt.ie> wrote:
> 
> > > I did notice from my own testing of the failsafe (by artificially
> > > inducing wraparound failure using an XID burning C function) that
> > > autovacuum seemed to totally correct the problem, even when the system
> > > had already crossed xidStopLimit - it came back on its own. I wasn't
> > > completely sure of how robust this effect was, though.
> 
> I'll put some effort in finding any way that it might not be robust.

A VACUUM may create a not-trivially-bounded number of multixacts via
FreezeMultiXactId().  In a cluster at multiStopLimit, completing VACUUM
without error needs preparation something like:

1. Kill each XID that might appear in a multixact.
2. Resolve each prepared transaction that might appear in a multixact.
3. Run VACUUM.  At this point, multiStopLimit is blocking new multixacts from
   other commands, and the lack of running multixact members removes the need
   for FreezeMultiXactId() to create multixacts.

Adding to the badness of single-user mode so well described upthread, one can
enter it without doing (2) and then wrap the nextMXact counter.