Avoid special XID snapshotConflictHorizon values.

Don't allow VACUUM to WAL-log the value FrozenTransactionId as the
snapshotConflictHorizon of freezing or visibility map related WAL
records.

The only special XID value that's an allowable snapshotConflictHorizon
is InvalidTransactionId, which is interpreted as "record definitely
doesn't require a recovery conflict".

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-WznuNGSzF8v6OsgjaC5aYsb3cZ6HW6MLm30X0d65cmSH6A@mail.gmail.com

src/backend/access/heap/vacuumlazy.c

@@ -1698,7 +1698,8 @@ retry:
 					}
 
 					/* Track newest xmin on page. */
-					if (TransactionIdFollows(xmin, prunestate->visibility_cutoff_xid))
+					if (TransactionIdFollows(xmin, prunestate->visibility_cutoff_xid) &&
+						TransactionIdIsNormal(xmin))
 						prunestate->visibility_cutoff_xid = xmin;
 				}
 				break;
@@ -1863,7 +1864,7 @@ retry:
 		 * because visibility_cutoff_xid will be logged by our caller in a
 		 * moment.
 		 */
-		Assert(cutoff == FrozenTransactionId ||
+		Assert(!TransactionIdIsValid(cutoff) ||
 			   cutoff == prunestate->visibility_cutoff_xid);
 	}
 #endif
@@ -3293,7 +3294,8 @@ heap_page_is_all_visible(LVRelState *vacrel, Buffer buf,
 					}
 
 					/* Track newest xmin on page. */
-					if (TransactionIdFollows(xmin, *visibility_cutoff_xid))
+					if (TransactionIdFollows(xmin, *visibility_cutoff_xid) &&
+						TransactionIdIsNormal(xmin))
 						*visibility_cutoff_xid = xmin;
 
 					/* Check whether this tuple is already frozen or not */

src/backend/storage/ipc/standby.c

@@ -493,6 +493,7 @@ ResolveRecoveryConflictWithSnapshot(TransactionId snapshotConflictHorizon,
 	if (!TransactionIdIsValid(snapshotConflictHorizon))
 		return;
 
+	Assert(TransactionIdIsNormal(snapshotConflictHorizon));
 	backends = GetConflictingVirtualXIDs(snapshotConflictHorizon,
 										 locator.dbOid);
 	ResolveRecoveryConflictWithVirtualXIDs(backends,