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

diff --git a/src/backend/catalog/heap.c b/src/backend/catalog/heap.c
index a98a511583799b83ac36cbabc510e69489240cc2..8cf2584522234a1957c0e95d24b7a92b7672f7af 100644 (file)
--- a/src/backend/catalog/heap.c
+++ b/src/backend/catalog/heap.c
@@ -1226,6 +1226,13 @@ heap_create_with_catalog(const char *relname,
                                                                          relpersistence);
        }
 
+       /*
+        * Other sessions' catalog scans can't find this until we commit.  Hence,
+        * it doesn't hurt to hold AccessExclusiveLock.  Do it here so callers
+        * can't accidentally vary in their lock mode or acquisition timing.
+        */
+       LockRelationOid(relid, AccessExclusiveLock);
+
        /*
         * Determine the relation's initial permissions.
         */