From: | Andres Freund <andres(at)anarazel(dot)de> |
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To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "Jonathan S(dot) Katz" <jkatz(at)postgresql(dot)org>, Noah Misch <noah(at)leadboat(dot)com>, Andrew Dunstan <andrew(at)dunslane(dot)net>, Bruce Momjian <bruce(at)momjian(dot)us>, Michael Paquier <michael(at)paquier(dot)xyz>, Justin Pryzby <pryzby(at)telsasoft(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, Shruthi Gowda <gowdashru(at)gmail(dot)com> |
Subject: | Re: pg15b2: large objects lost on upgrade |
Date: | 2022-08-03 20:20:14 |
Message-ID: | 20220803202014.77m6z5m7v525p2b4@awork3.anarazel.de |
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Lists: | pgsql-hackers |
Hi,
On 2022-08-03 09:59:40 -0400, Robert Haas wrote:
> On Tue, Aug 2, 2022 at 3:51 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> 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
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