WaitForOlderSnapshots in DETACH PARTITION causes deadlocks

Hi,

While recently looking into partition maintenance, I found a case in which
DETACH PARTITION FINALIZE could case deadlocks. This occurs when a
ALTER TABLE DETACH CONCURRENTLY, followed by a cancel, the followed by
an ALTER TABLE DETACH FINALIZE, and this sequence of steps occur in the
middle of other REPEATABLE READ/SERIALIZABLE transactions. These RR/SERIALIZABLE
should not accessed the partition until the FINALIZE step starts. See the attached
repro.

This seems to occur as the FINALIZE is calling WaitForOlderSnapshot [1]
to make sure that all older snapshots are completed before the finalize
is completed and the detached partition is removed from the parent table
association.

WaitForOlderSnapshots is used here to ensure that snapshots older than
the start of the ALTER TABLE DETACH CONCURRENTLY are completely removed
to guarantee consistency, however it does seem to cause deadlocks for at
least RR/SERIALIZABLE transactions.

There are cases [2] in which certain operations are accepted as not being
MVCC-safe, and now I am wondering if this is another case? Deadlocks are
not a good scenario, and WaitForOlderSnapshot does not appear to do
anything for READ COMMITTED transactions.

So do we actually need WaitForOlderSnapshot in the FINALIZE code? and
Could be acceptable that this operation is marked as not MVCC-safe like
the other aforementioned operations?

Perhaps I am missing some important point here, so any feedback will be
appreciated.

Regards,

Sami Imseih
Amazon Web Services (AWS)

[1] https://github.com/postgres/postgres/blob/master/src/backend/commands/tablecmds.c#L18757-L18764
[2] https://www.postgresql.org/docs/current/mvcc-caveats.html