Commit 052026c9b9 made heap_vacuum_rel call RelationGetNumberOfBlocks before it calls vacuum_get_cutoffs -- it swapped the order. This is wrong, as explained by an intact comment above the call to vacuum_get_cutoffs.
In short, there is now a brief window during which the relation can be extended that'll allow heap pages with tuple headers < VACUUM's OldestXmin to be created, which are overlooked by that same VACUUM (they're beyond the same VACUUM's rel_pages). As a result of all this, VACUUM might advance pg_class.relfrozenxid to a later/younger value than a remaining/unfrozen XID value from one of these unscanned heap pages. -- Peter Geoghegan
