From be11fa26e37f798be7d948b5ebbd3adf5e9aeb73 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Mon, 26 Jan 2004 19:15:59 +0000
Subject: [PATCH] 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 | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/src/backend/access/transam/varsup.c b/src/backend/access/transam/varsup.c
index d09a9d504d6..617c7d19c43 100644
--- a/src/backend/access/transam/varsup.c
+++ b/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)
-- 
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