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

On Thu, Nov 2, 2017 at 10:26 PM, Peter Geoghegan <pg@bowt.ie> wrote:
> On Thu, Nov 2, 2017 at 9:44 AM, Robert Haas <robertmhaas@gmail.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().

Oh, wow.  You seem to be correct.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company


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