Repair incorrect order of operations in GetNewTransactionId(). We must

complete ExtendCLOG() before advancing nextXid, so that if that routine
fails, the next incoming transaction will try it again.  Per trouble
report from Christopher Kings-Lynne.

src/backend/access/transam/varsup.c

@@ -6,7 +6,7 @@
  * Copyright (c) 2000-2003, PostgreSQL Global Development Group
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/transam/varsup.c,v 1.54 2004/01/07 18:56:24 neilc Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/transam/varsup.c,v 1.55 2004/01/26 19:15:59 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -45,10 +45,8 @@ GetNewTransactionId(void)
 
 	xid = ShmemVariableCache->nextXid;
 
-	TransactionIdAdvance(ShmemVariableCache->nextXid);
-
 	/*
-	 * If we have just allocated the first XID of a new page of the commit
+	 * If we are allocating the first XID of a new page of the commit
 	 * log, zero out that commit-log page before returning. We must do
 	 * this while holding XidGenLock, else another xact could acquire and
 	 * commit a later XID before we zero the page.	Fortunately, a page of
@@ -57,6 +55,14 @@ GetNewTransactionId(void)
 	 */
 	ExtendCLOG(xid);
 
+	/*
+	 * Now advance the nextXid counter.  This must not happen until after
+	 * we have successfully completed ExtendCLOG() --- if that routine fails,
+	 * we want the next incoming transaction to try it again.  We cannot
+	 * assign more XIDs until there is CLOG space for them.
+	 */
+	TransactionIdAdvance(ShmemVariableCache->nextXid);
+
 	/*
 	 * Must set MyProc->xid before releasing XidGenLock.  This ensures
 	 * that when GetSnapshotData calls ReadNewTransactionId, all active
@@ -74,7 +80,7 @@ GetNewTransactionId(void)
 	 *
 	 * A solution to the atomic-store problem would be to give each PGPROC
 	 * its own spinlock used only for fetching/storing that PGPROC's xid.
-	 * (SInvalLock would then mean primarily that PROCs couldn't be added/
+	 * (SInvalLock would then mean primarily that PGPROCs couldn't be added/
 	 * removed while holding the lock.)
 	 */
 	if (MyProc != NULL)