Fix off-by-one error in txid_status().

The transaction ID returned by GetNextXidAndEpoch() is in the future,
so we can't attempt to access its status or we might try to read a
CLOG page that doesn't exist.  The > vs >= confusion probably stemmed
from the choice of a variable name containing the word "last" instead
of "next", so fix that too.

Back-patch to 10 where the function arrived.

Author: Thomas Munro
Discussion: https://postgr.es/m/CA%2BhUKG%2Buua_BV5cyfsioKVN2d61Lukg28ECsWTXKvh%3DBtN2DPA%40mail.gmail.com

src/backend/utils/adt/txid.c

@@ -113,9 +113,9 @@ TransactionIdInRecentPast(uint64 xid_with_epoch, TransactionId *extracted_xid)
 	uint32		xid_epoch = (uint32) (xid_with_epoch >> 32);
 	TransactionId xid = (TransactionId) xid_with_epoch;
 	uint32		now_epoch;
-	TransactionId now_epoch_last_xid;
+	TransactionId now_epoch_next_xid;
 
-	GetNextXidAndEpoch(&now_epoch_last_xid, &now_epoch);
+	GetNextXidAndEpoch(&now_epoch_next_xid, &now_epoch);
 
 	if (extracted_xid != NULL)
 		*extracted_xid = xid;
@@ -129,7 +129,7 @@ TransactionIdInRecentPast(uint64 xid_with_epoch, TransactionId *extracted_xid)
 
 	/* If the transaction ID is in the future, throw an error. */
 	if (xid_epoch > now_epoch
-		|| (xid_epoch == now_epoch && xid > now_epoch_last_xid))
+		|| (xid_epoch == now_epoch && xid >= now_epoch_next_xid))
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 errmsg("transaction ID %s is in the future",
@@ -151,7 +151,7 @@ TransactionIdInRecentPast(uint64 xid_with_epoch, TransactionId *extracted_xid)
 	 * CLOG entry is guaranteed to still exist.
 	 */
 	if (xid_epoch + 1 < now_epoch
-		|| (xid_epoch + 1 == now_epoch && xid < now_epoch_last_xid)
+		|| (xid_epoch + 1 == now_epoch && xid < now_epoch_next_xid)
 		|| TransactionIdPrecedes(xid, ShmemVariableCache->oldestClogXid))
 		return false;