Awhile back I added some code to StartupCLOG() to forcibly zero out

the remainder of the current clog page during system startup.  While
this was a good idea, it turns out the code fails if nextXid is
exactly at a page boundary, because we won't have created the "current"
clog page yet in that case.  Since the page will be correctly zeroed
when we execute the first transaction on it, the solution is just to
do nothing when exactly at a page boundary.  Per trouble report from
Dave Hartwig.

Author: tglsfdc

src/backend/access/transam/clog.c

@@ -24,7 +24,7 @@
  * Portions Copyright (c) 1996-2004, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/backend/access/transam/clog.c,v 1.26 2004/08/30 19:00:42 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/access/transam/clog.c,v 1.27 2004/12/22 18:45:49 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -212,10 +212,6 @@ StartupCLOG(void)
 {
 	TransactionId xid = ShmemVariableCache->nextXid;
 	int			pageno = TransactionIdToPage(xid);
-	int			byteno = TransactionIdToByte(xid);
-	int			bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
-	int			slotno;
-	char	   *byteptr;
 
 	LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
 
@@ -232,17 +228,27 @@ StartupCLOG(void)
 	 * marked by the previous database lifecycle (since subtransaction
 	 * commit writes clog but makes no WAL entry).  Let's just be safe.
 	 * (We need not worry about pages beyond the current one, since those
-	 * will be zeroed when first used.)
+	 * will be zeroed when first used.  For the same reason, there is no
+	 * need to do anything when nextXid is exactly at a page boundary; and
+	 * it's likely that the "current" page doesn't exist yet in that case.)
 	 */
-	slotno = SimpleLruReadPage(ClogCtl, pageno, xid);
-	byteptr = ClogCtl->shared->page_buffer[slotno] + byteno;
+	if (TransactionIdToPgIndex(xid) != 0)
+	{
+		int			byteno = TransactionIdToByte(xid);
+		int			bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
+		int			slotno;
+		char	   *byteptr;
 
-	/* Zero so-far-unused positions in the current byte */
-	*byteptr &= (1 << bshift) - 1;
-	/* Zero the rest of the page */
-	MemSet(byteptr + 1, 0, BLCKSZ - byteno - 1);
+		slotno = SimpleLruReadPage(ClogCtl, pageno, xid);
+		byteptr = ClogCtl->shared->page_buffer[slotno] + byteno;
 
-	ClogCtl->shared->page_status[slotno] = SLRU_PAGE_DIRTY;
+		/* Zero so-far-unused positions in the current byte */
+		*byteptr &= (1 << bshift) - 1;
+		/* Zero the rest of the page */
+		MemSet(byteptr + 1, 0, BLCKSZ - byteno - 1);
+
+		ClogCtl->shared->page_status[slotno] = SLRU_PAGE_DIRTY;
+	}
 
 	LWLockRelease(CLogControlLock);
 }