Re: pg15b2: large objects lost on upgrade

Hi,

On 2022-08-03 09:59:40 -0400, Robert Haas wrote:
> On Tue, Aug 2, 2022 at 3:51 PM Tom Lane  wrote:
> > > The test does look helpful and it would catch regressions. Loosely
> > > quoting Robert on a different point upthread, we don't want to turn off
> > > the alarm just because it's spuriously going off.
> > > I think the weakened check is OK (and possibly mimics the real-world
> > > where autovacuum runs), unless you see a major drawback to it?
> >
> > I also think that ">=" is a sufficient requirement.  It'd be a
> > bit painful to test if we had to cope with potential XID wraparound,
> > but we know that these installations haven't been around nearly
> > long enough for that, so a plain ">=" test ought to be good enough.
> > (Replacing the simple "eq" code with something that can handle that
> > doesn't look like much fun, though.)
> 
> I don't really like this approach. Imagine that the code got broken in
> such a way that relfrozenxid and relminmxid were set to a value chosen
> at random - say, the contents of 4 bytes of unallocated memory that
> contained random garbage. Well, right now, the chances that this would
> cause a test failure are nearly 100%. With this change, they'd be
> nearly 0%.

Can't that pretty easily be addressed by subsequently querying txid_current(),
and checking that the value isn't newer than that?

Greetings,

Andres Freund