Transaction IDs wrap around, per my proposal of 13-Aug-01. More

documentation to come, but the code is all here.  initdb forced.

src/backend/access/transam/clog.c

@@ -13,7 +13,7 @@
  * Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Header: /cvsroot/pgsql/src/backend/access/transam/clog.c,v 1.2 2001/08/25 23:24:39 tgl Exp $
+ * $Header: /cvsroot/pgsql/src/backend/access/transam/clog.c,v 1.3 2001/08/26 16:55:59 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -762,8 +762,12 @@ ExtendCLOG(TransactionId newestXact)
 {
 	int			pageno;
 
-	/* No work except at first XID of a page */
-	if (TransactionIdToPgIndex(newestXact) != 0)
+	/*
+	 * No work except at first XID of a page.  But beware: just after
+	 * wraparound, the first XID of page zero is FirstNormalTransactionId.
+	 */
+	if (TransactionIdToPgIndex(newestXact) != 0 &&
+		!TransactionIdEquals(newestXact, FirstNormalTransactionId))
 		return;
 
 	pageno = TransactionIdToPage(newestXact);
@@ -818,6 +822,18 @@ TruncateCLOG(TransactionId oldestXact)
 	S_LOCK(&(ClogCtl->control_lck));
 
 restart:;
+	/*
+	 * While we are holding the lock, make an important safety check:
+	 * the planned cutoff point must be <= the current CLOG endpoint page.
+	 * Otherwise we have already wrapped around, and proceeding with the
+	 * truncation would risk removing the current CLOG segment.
+	 */
+	if (CLOGPagePrecedes(ClogCtl->latest_page_number, cutoffPage))
+	{
+		S_UNLOCK(&(ClogCtl->control_lck));
+		elog(LOG, "unable to truncate commit log: apparent wraparound");
+		return;
+	}
 
 	for (slotno = 0; slotno < NUM_CLOG_BUFFERS; slotno++)
 	{

src/backend/access/transam/transam.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/access/transam/transam.c,v 1.47 2001/08/25 18:52:41 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/access/transam/transam.c,v 1.48 2001/08/26 16:55:59 tgl Exp $
  *
  * NOTES
  *	  This file contains the high level access-method interface to the
@@ -236,3 +236,68 @@ TransactionIdAbort(TransactionId transactionId)
 
 	TransactionLogUpdate(transactionId, TRANSACTION_STATUS_ABORTED);
 }
+
+
+/*
+ * TransactionIdPrecedes --- is id1 logically < id2?
+ */
+bool
+TransactionIdPrecedes(TransactionId id1, TransactionId id2)
+{
+	/*
+	 * If either ID is a permanent XID then we can just do unsigned
+	 * comparison.  If both are normal, do a modulo-2^31 comparison.
+	 */
+	int32		diff;
+
+	if (!TransactionIdIsNormal(id1) || !TransactionIdIsNormal(id2))
+		return (id1 < id2);
+
+	diff = (int32) (id1 - id2);
+	return (diff < 0);
+}
+
+/*
+ * TransactionIdPrecedesOrEquals --- is id1 logically <= id2?
+ */
+bool
+TransactionIdPrecedesOrEquals(TransactionId id1, TransactionId id2)
+{
+	int32		diff;
+
+	if (!TransactionIdIsNormal(id1) || !TransactionIdIsNormal(id2))
+		return (id1 <= id2);
+
+	diff = (int32) (id1 - id2);
+	return (diff <= 0);
+}
+
+/*
+ * TransactionIdFollows --- is id1 logically > id2?
+ */
+bool
+TransactionIdFollows(TransactionId id1, TransactionId id2)
+{
+	int32		diff;
+
+	if (!TransactionIdIsNormal(id1) || !TransactionIdIsNormal(id2))
+		return (id1 > id2);
+
+	diff = (int32) (id1 - id2);
+	return (diff > 0);
+}
+
+/*
+ * TransactionIdFollowsOrEquals --- is id1 logically >= id2?
+ */
+bool
+TransactionIdFollowsOrEquals(TransactionId id1, TransactionId id2)
+{
+	int32		diff;
+
+	if (!TransactionIdIsNormal(id1) || !TransactionIdIsNormal(id2))
+		return (id1 >= id2);
+
+	diff = (int32) (id1 - id2);
+	return (diff >= 0);
+}

src/backend/access/transam/xid.c

@@ -6,23 +6,18 @@
  * Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- *	$Id: xid.c,v 1.32 2001/08/23 23:06:37 tgl Exp $
- *
- * OLD COMMENTS
- * XXX WARNING
- *		Much of this file will change when we change our representation
- *		of transaction ids -cim 3/23/90
- *
- * It is time to make the switch from 5 byte to 4 byte transaction ids
- * This file was totally reworked. -mer 5/22/92
+ *	$Id: xid.c,v 1.33 2001/08/26 16:55:59 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
 
 #include "postgres.h"
 
+#include <limits.h>
+
 #include "access/xact.h"
 
+
 #define PG_GETARG_TRANSACTIONID(n)	DatumGetTransactionId(PG_GETARG_DATUM(n))
 #define PG_RETURN_TRANSACTIONID(x)	return TransactionIdGetDatum(x)
 
@@ -30,9 +25,9 @@
 Datum
 xidin(PG_FUNCTION_ARGS)
 {
-	char	   *representation = PG_GETARG_CSTRING(0);
+	char	   *str = PG_GETARG_CSTRING(0);
 
-	PG_RETURN_TRANSACTIONID((TransactionId) atol(representation));
+	PG_RETURN_TRANSACTIONID((TransactionId) strtoul(str, NULL, 0));
 }
 
 Datum
@@ -40,21 +35,15 @@ xidout(PG_FUNCTION_ARGS)
 {
 	TransactionId transactionId = PG_GETARG_TRANSACTIONID(0);
 	/* maximum 32 bit unsigned integer representation takes 10 chars */
-	char	   *representation = palloc(11);
+	char	   *str = palloc(11);
 
-	snprintf(representation, 11, "%lu", (unsigned long) transactionId);
+	snprintf(str, 11, "%lu", (unsigned long) transactionId);
 
-	PG_RETURN_CSTRING(representation);
+	PG_RETURN_CSTRING(str);
 }
 
-/* ----------------------------------------------------------------
- *		xideq
- * ----------------------------------------------------------------
- */
-
 /*
- *		xideq			- returns 1, iff xid1 == xid2
- *								  0  else;
+ *		xideq			- are two xids equal?
  */
 Datum
 xideq(PG_FUNCTION_ARGS)
@@ -64,3 +53,19 @@ xideq(PG_FUNCTION_ARGS)
 
 	PG_RETURN_BOOL(TransactionIdEquals(xid1, xid2));
 }
+
+/*
+ *		xid_age			- compute age of an XID (relative to current xact)
+ */
+Datum
+xid_age(PG_FUNCTION_ARGS)
+{
+	TransactionId xid = PG_GETARG_TRANSACTIONID(0);
+	TransactionId now = GetCurrentTransactionId();
+
+	/* Permanent XIDs are always infinitely old */
+	if (! TransactionIdIsNormal(xid))
+		PG_RETURN_INT32(INT_MAX);
+
+	PG_RETURN_INT32((int32) (now - xid));
+}