From 6da05dd2c7efd5900905277d34172812cd59bde4 Mon Sep 17 00:00:00 2001
From: Simon Riggs <simon@2ndQuadrant.com>
Date: Fri, 2 Oct 2009 21:28:14 +0100
Subject: [PATCH] Resolve a race condition on update of
 ShmemVariableCache->latestCompletedXid, plus use XidGenLock during
 xact_redo_abort() as in xact_redo_commit(). Some changes direct from Heikki,
 others address code review comments from Heikki.

---
 src/backend/access/transam/xact.c   | 21 +++++++--------------
 src/backend/storage/ipc/procarray.c |  5 +++++
 2 files changed, 12 insertions(+), 14 deletions(-)

diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 8c45738f14..d5a95c63c8 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -4607,10 +4607,7 @@ xact_redo_commit(xl_xact_commit *xlrec, TransactionId xid,
 		/*
 		 * Release locks, if any. We do this for both two phase and normal
 		 * one phase transactions. In effect we are ignoring the prepare
-		 * phase and just going straight to lock release. This explains
-		 * why the twophase_postcommit_standby_callbacks[] do not invoke
-		 * a special routine to handle locks - that is performed here
-		 * instead.
+		 * phase and just going straight to lock release.
 		 */
 		RelationReleaseRecoveryLockTree(xid, xlrec->nsubxacts, sub_xids);
 	}
@@ -4629,15 +4626,6 @@ xact_redo_commit(xl_xact_commit *xlrec, TransactionId xid,
 		LWLockRelease(XidGenLock);
 	}
 
-	/* Same here, don't use lock to test, but need one to modify */
-	if (TransactionIdFollowsOrEquals(max_xid,
-									 ShmemVariableCache->latestCompletedXid))
-	{
-		LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
-		ShmemVariableCache->latestCompletedXid = max_xid;
-		LWLockRelease(ProcArrayLock);
-	}
-
 	/* Make sure files supposed to be dropped are dropped */
 	for (i = 0; i < xlrec->nrels; i++)
 	{
@@ -4706,12 +4694,17 @@ xact_redo_abort(xl_xact_abort *xlrec, TransactionId xid)
 	}
 
 	/* Make sure nextXid is beyond any XID mentioned in the record */
+	/* We don't expect anyone else to modify nextXid, hence we
+	 * don't need to hold a lock while checking this. We still acquire
+	 * the lock to modify it, though.
+	 */
 	if (TransactionIdFollowsOrEquals(max_xid,
 									 ShmemVariableCache->nextXid))
 	{
+		LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
 		ShmemVariableCache->nextXid = max_xid;
-		ShmemVariableCache->latestCompletedXid = ShmemVariableCache->nextXid;
 		TransactionIdAdvance(ShmemVariableCache->nextXid);
+		LWLockRelease(XidGenLock);
 	}
 
 	/* Make sure files supposed to be dropped are dropped */
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 0145ab6448..daee07b71f 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -2345,6 +2345,11 @@ ExpireTreeKnownAssignedTransactionIds(TransactionId xid, int nsubxids,
 		TransactionIdPrecedes(procArray->lastOverflowedXid, max_xid))
 		procArray->lastOverflowedXid = max_xid;
 
+	/* Same here, don't use lock to test, but need one to modify */
+	if (TransactionIdFollowsOrEquals(max_xid,
+									 ShmemVariableCache->latestCompletedXid))
+		ShmemVariableCache->latestCompletedXid = max_xid;
+
 	LWLockRelease(ProcArrayLock);
 }
 
-- 
2.39.5