Avoid using tuple from syscache for update of pg_database.datfrozenxid

pg_database.datfrozenxid gets updated using an in-place update at the
end of vacuum or autovacuum.  Since 96cdeae0, as pg_database has a toast
relation, it is possible for a pg_database tuple to have toast values
if there is a large set of ACLs in place.  In such a case, the in-place
update would fail because of the flattening of the toast values done for
the catcache entry fetched.  Instead of using a copy from the catcache,
this changes the logic to fetch the copy of the tuple by directly
scanning pg_database.

Per the lack of complaints on the matter, no backpatch is done.  Note
that before 96cdeae0, attempting to insert such a tuple to pg_database
would cause a "row is too big" error, so the end-of-vacuum problem was
not reachable.

Author: Ashwin Agrawal, Junfeng Yang
Discussion: https://postgr.es/m/DM5PR0501MB38800D9E4605BCA72DD35557CCE10@DM5PR0501MB3880.namprd05.prod.outlook.com

src/backend/access/heap/heapam.c

@@ -5710,6 +5710,10 @@ heap_abort_speculative(Relation relation, ItemPointer tid)
  *
  * tuple is an in-memory tuple structure containing the data to be written
  * over the target tuple.  Also, tuple->t_self identifies the target tuple.
+ *
+ * Note that the tuple updated here had better not come directly from the
+ * syscache if the relation has a toast relation as this tuple could
+ * include toast values that have been expanded, causing a failure here.
  */
 void
 heap_inplace_update(Relation relation, HeapTuple tuple)

src/backend/commands/vacuum.c

@@ -1361,6 +1361,7 @@ vac_update_datfrozenxid(void)
 	MultiXactId lastSaneMinMulti;
 	bool		bogus = false;
 	bool		dirty = false;
+	ScanKeyData key[1];
 
 	/*
 	 * Restrict this task to one backend per database.  This avoids race
@@ -1479,10 +1480,25 @@ vac_update_datfrozenxid(void)
 	/* Now fetch the pg_database tuple we need to update. */
 	relation = table_open(DatabaseRelationId, RowExclusiveLock);
 
-	/* Fetch a copy of the tuple to scribble on */
-	tuple = SearchSysCacheCopy1(DATABASEOID, ObjectIdGetDatum(MyDatabaseId));
+	/*
+	 * Get the pg_database tuple to scribble on.  Note that this does not
+	 * directly rely on the syscache to avoid issues with flattened toast
+	 * values for the in-place update.
+	 */
+	ScanKeyInit(&key[0],
+				Anum_pg_database_oid,
+				BTEqualStrategyNumber, F_OIDEQ,
+				ObjectIdGetDatum(MyDatabaseId));
+
+	scan = systable_beginscan(relation, DatabaseOidIndexId, true,
+							  NULL, 1, key);
+	tuple = systable_getnext(scan);
+	tuple = heap_copytuple(tuple);
+	systable_endscan(scan);
+
 	if (!HeapTupleIsValid(tuple))
 		elog(ERROR, "could not find tuple for database %u", MyDatabaseId);
+
 	dbform = (Form_pg_database) GETSTRUCT(tuple);
 
 	/*