Intermittent BRIN failures on hyrax and lousyjack

Buildfarm member hyrax has shown this failure twice recently:

--- /home/buildfarm/buildroot/HEAD/pgsql.build/src/test/regress/expected/brin.out    2020-01-23 11:10:05.730014075
-0500
+++ /home/buildfarm/buildroot/HEAD/pgsql.build/src/test/regress/results/brin.out    2020-06-30 03:50:23.651196117 -0400
@@ -490,6 +490,7 @@
 INSERT INTO brintest_2 VALUES (numrange(0, 2^1000::numeric));
 INSERT INTO brintest_2 VALUES ('(-1, 0)');
 SELECT brin_desummarize_range('brinidx', 0);
+WARNING:  leftover placeholder tuple detected in BRIN index "brinidx", deleting
  brin_desummarize_range
 ------------------------

This happened on HEAD:
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=hyrax&dt=2020-06-30%2001%3A41%3A50
and v13:
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=hyrax&dt=2020-06-17%2005%3A50%3A46
and lousyjack has also shown it once on HEAD:
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=lousyjack&dt=2020-03-02%2013%3A03%3A04

The warning is from this bit (commit c655899ba):

    /*
     * Because of ShareUpdateExclusive lock, this function shouldn't run
     * concurrently with summarization.  Placeholder tuples can only exist as
     * leftovers from crashed summarization, so if we detect any, we complain
     * but proceed.
     */
    if (BrinTupleIsPlaceholder(tup))
        ereport(WARNING,
                (errmsg("leftover placeholder tuple detected in BRIN index \"%s\", deleting",
                        RelationGetRelationName(idxrel))));

Now, there was no preceding crash in these tests, so the comment's claim
is evidently a lie.  But what's going wrong?  The postmaster log provides
a strong hint:

2020-06-30 03:50:14.603 EDT [3699:48] pg_regress/brin LOG:  statement: SELECT brin_desummarize_range('brinidx', 0);
2020-06-30 03:50:15.894 EDT [3699:49] pg_regress/brin LOG:  process 3699 still waiting for ShareUpdateExclusiveLock on
relation24795 of database 16384 after 1000.094 ms 
2020-06-30 03:50:15.894 EDT [3699:50] pg_regress/brin DETAIL:  Process holding the lock: 7237. Wait queue: 3699.
2020-06-30 03:50:15.894 EDT [3699:51] pg_regress/brin STATEMENT:  SELECT brin_desummarize_range('brinidx', 0);
2020-06-30 03:50:15.895 EDT [7237:1] ERROR:  canceling autovacuum task
2020-06-30 03:50:15.895 EDT [7237:2] CONTEXT:  while cleaning up index "brinidx" of relation "public.brintest"
    automatic vacuum of table "regression.public.brintest"
2020-06-30 03:50:15.899 EDT [3699:52] pg_regress/brin LOG:  process 3699 acquired ShareUpdateExclusiveLock on relation
24795of database 16384 after 1001.626 ms 
2020-06-30 03:50:15.899 EDT [3699:53] pg_regress/brin STATEMENT:  SELECT brin_desummarize_range('brinidx', 0);
2020-06-30 03:50:16.018 EDT [3699:54] pg_regress/brin WARNING:  leftover placeholder tuple detected in BRIN index
"brinidx",deleting 
2020-06-30 03:50:16.031 EDT [3699:55] pg_regress/brin LOG:  statement: SELECT brin_summarize_range('brinidx', 0);

I think the "crash" actually was the forced autovac cancellation we see
here.  Thus, the fact that both these animals use CLOBBER_CACHE_ALWAYS is
not a direct cause of the failure, though it might contribute to getting
the timing right for this to happen.

So (1) the comment needs to be adjusted to mention that vacuum
cancellation is enough to create the situation, and (2) probably the
message needs to be downgraded quite a lot, DEBUG2 or even less.
Or maybe we could just delete the quoted stanza altogether.

            regards, tom lane