Re: do only critical work during single-user vacuum?

On Sun, Feb 20, 2022 at 02:15:37PM -0800, Andres Freund wrote:
> On 2022-02-19 20:57:57 -0800, Noah Misch wrote:
> > 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(at)bowt(dot)ie> wrote:
> > > > On Tue, Feb 15, 2022 at 9:28 AM Peter Geoghegan <pg(at)bowt(dot)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.
>
> If we collected the information along the lines of I proposed in the second half of
> https://www.postgresql.org/message-id/20220204013539.qdegpqzvayq3d4y2%40alap3.anarazel.de
> we should be able to handle such cases more intelligently, I think?
>
> We could e.g. add an error if FreezeMultiXactId() needs to create a new
> multixact for a far-in-the-past xid. That's not great, of course, but if we
> include the precise cause (pid of backend / prepared xact name / slot name /
> ...) necessitating creating a new multi, it'd still be a significant
> improvement over the status quo.

Yes, exactly.