Re: Wrong check in pg_visibility?

From: Konstantin Knizhnik <k(dot)knizhnik(at)postgrespro(dot)ru>
To: pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: Re: Wrong check in pg_visibility?
Date: 2020-12-08 09:59:25
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On 06.12.2020 23:50, Konstantin Knizhnik wrote:
> Hi hackers!
> Due to the error in PG-ProEE we have added the following test to
> pg_visibility:
> create table vacuum_test as select 42 i;
> vacuum vacuum_test;
> select count(*) > 0 from pg_check_visible('vacuum_test');
> drop table vacuum_test;
> Sometime (very rarely) this test failed: pg_visibility reports
> "corrupted" tuples.
> The same error can be reproduced not only with PG-Pro but also with
> vanilla REL_11_STABLE, REL_12_STABLE and REL_13_STABLE.
> It is not reproduced with master after Andres snapshot optimization -
> commit dc7420c2.
> It is not so easy to reproduce this error: it is necessary to repeat
> this tests many times.
> Probability increased with more aggressive autovacuum settings.
> But even with such settings and thousands of iterations I was not able
> to reproduce this error at my notebook - only at virtual machine.
> The error is reported here:
>             /*
>              * If we're checking whether the page is all-visible, we
> expect
>              * the tuple to be all-visible.
>              */
>             if (check_visible &&
>                 !tuple_all_visible(&tuple, OldestXmin, buffer))
>             {
>                 TransactionId RecomputedOldestXmin;
>                 /*
>                  * Time has passed since we computed OldestXmin, so it's
>                  * possible that this tuple is all-visible in reality
> even
>                  * though it doesn't appear so based on our
>                  * previously-computed value.  Let's compute a new
> value so we
>                  * can be certain whether there is a problem.
>                  *
>                  * From a concurrency point of view, it sort of sucks to
>                  * retake ProcArrayLock here while we're holding the
> buffer
>                  * exclusively locked, but it should be safe against
>                  * deadlocks, because surely GetOldestXmin() should
> never take
>                  * a buffer lock. And this shouldn't happen often, so
> it's
>                  * worth being careful so as to avoid false positives.
>                  */
>                 RecomputedOldestXmin = GetOldestXmin(NULL,
>                 if (!TransactionIdPrecedes(OldestXmin,
> RecomputedOldestXmin))
>                     record_corrupt_item(items, &tuple.t_self);
> I debugger I have checked that OldestXmin = RecomputedOldestXmin =
> tuple.t_data->xmin
> I wonder if this check in pg_visibility is really correct and it can
> not happen that OldestXmin=tuple.t_data->xmin?
> Please notice that tuple_all_visible returns false if
> !TransactionIdPrecedes(xmin, OldestXmin)

I did more investigations and have to say that this check in
pg_visibility.c is really not correct.
The process which is keeping oldest xmin is autovacuum.
It should be ignored by GetOldestXmin because of PROCARRAY_FLAGS_VACUUM
flags, but it is not actually skipped
because PROC_IN_VACUUM flag is not set yet. There is yet another flag -
which is always set in autovacuum, but it can not be passed to
GetOldestXmin? because is cleared by PROCARRAY_PROC_FLAGS_MASK.

If we just repeat RecomputedOldestXmin = GetOldestXmin(NULL,
several times, then finally we will get right xmin.

I wonder if such check should be excluded from pg_visibility or made in
more correct way?
Because nothing in documentation of pg_check_visible says that it may
return false positives:

|pg_check_visible(relation regclass, t_ctid OUT tid) returns setof tid|

    Returns the TIDs of non-all-visible tuples stored in pages
marked all-visible in the visibility map. If this function returns a
non-empty set of TIDs, the visibility map is corrupt.

And comment to this function is even morefrightening:

 * Return the TIDs of not-all-visible tuples in pages marked all-visible
 * in the visibility map.  We hope no one will ever find any, but there
 * be bugs, database corruption, etc.

Konstantin Knizhnik
Postgres Professional:
The Russian Postgres Company

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