Re: [COMMITTERS] pgsql: Fix freezing of a dead HOT-updated tuple

On Thu, Nov 2, 2017 at 9:44 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> The second commit (22576734b805fb0952f9be841ca8f643694ee868) is where
> I think things get a lot more dangerous. The problem (as Andres
> pointed out to me this afternoon) is that it seems possible to end up
> with a situation where there should be two HOT chains on a page, and
> because of the weakened xmin/xmax checking rules, we end up thinking
> that the second one is a continuation of the first one, which will be
> all kinds of bad news. That would be particularly likely to happen in
> releases from before we invented HEAP_XMIN_FROZEN, when there's no
> xmin/xmax matching at all, but could happen on later releases if we
> are extraordinarily unlucky (i.e. xmin of the first tuple in the
> second chain happens to be the same as the pre-freeze xmax in the old
> chain, probably because the same XID was used to update the page in
> two consecutive epochs). Fortunately, that commit is (I think) not
> released anywhere.

FWIW, if you look at the second commit
(22576734b805fb0952f9be841ca8f643694ee868) carefully, you'll realize
that it doesn't even treat those two cases differently. It was buggy
even on its own terms. The FrozenTransactionId test used an xmin from
HeapTupleHeaderGetXmin(), not HeapTupleHeaderGetRawXmin().

--
Peter Geoghegan