Make vacuum a bit more verbose to debug BF failure.

This is temporary. While possibly some more error checking / debugging
in this path would be a good thing, it'll not look exactly like this.

Discussion: https://postgr.es/m/20200816181604.l54m6kss5ntd6xow@alap3.anarazel.de

src/backend/access/heap/heapam.c

@@ -6048,7 +6048,16 @@ FreezeMultiXactId(MultiXactId multi, uint16 t_infomask,
 				TransactionIdIsInProgress(members[i].xid))
 			{
 				/* running locker cannot possibly be older than the cutoff */
-				Assert(!TransactionIdPrecedes(members[i].xid, cutoff_xid));
+				if (TransactionIdPrecedes(members[i].xid, cutoff_xid))
+				{
+					/* temporary on-bf debugging */
+					elog(PANIC, "too old alive locker: multi: %u, member xid: %u, memb-current: %d, memb-progress: %d, cutoff: %u, cutoff-multi: %u, relfrozenxid: %u, relminmxid: %u",
+						 multi, members[i].xid,
+						 TransactionIdIsCurrentTransactionId(members[i].xid),
+						 TransactionIdIsInProgress(members[i].xid),
+						 cutoff_xid, cutoff_multi,
+						 relfrozenxid, relminmxid);
+				}
 				newmembers[nnewmembers++] = members[i];
 				has_lockers = true;
 			}

src/backend/access/heap/vacuumlazy.c

@@ -1350,7 +1350,14 @@ lazy_scan_heap(Relation onerel, VacuumParams *params, LVRelStats *vacrelstats,
 					if (HeapTupleIsHotUpdated(&tuple) ||
 						HeapTupleIsHeapOnly(&tuple) ||
 						params->index_cleanup == VACOPT_TERNARY_DISABLED)
+					{
+						/* temporary on-bf debugging */
+						elog(LOG, "treating dead HOT tuple (updated %d, heap only: %d, index cleanup: %d) as alive",
+							 HeapTupleIsHotUpdated(&tuple), HeapTupleIsHeapOnly(&tuple),
+							 params->index_cleanup == VACOPT_TERNARY_DISABLED);
+
 						nkeep += 1;
+					}
 					else
 						tupgone = true; /* we can delete the tuple */
 					all_visible = false;