pgsql: AccessExclusiveLock new relations just after assigning the OID.

AccessExclusiveLock new relations just after assigning the OID.

This has no user-visible, important consequences, since other sessions'
catalog scans can't find the relation until we commit. However, this
unblocks introducing a rule about locks required to heap_update() a
pg_class row. CREATE TABLE has been acquiring this lock eventually, but
it can heap_update() pg_class.relchecks earlier. create_toast_table()
has been acquiring only ShareLock. Back-patch to v12 (all supported
versions), the plan for the commit relying on the new rule.

Reviewed (in an earlier version) by Robert Haas.

Discussion: https://postgr.es/m/20240611024525.9f.nmisch@google.com

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/fc8c25806e84c38f3920fd3507a389eac34d62a5

Modified Files
--------------
src/backend/catalog/heap.c | 7 +++++++
1 file changed, 7 insertions(+)