0001-Add-GetOldestXminNoLock-as-a-variant-and-implementat.patch

>From 660b6995c0260eb112d7b6f7158e3b4654c3d9bf Mon Sep 17 00:00:00 2001
From: Andres Freund <andres@anarazel.de>
Date: Thu, 13 Dec 2012 20:47:57 +0100
Subject: [PATCH 1/2] Add GetOldestXminNoLock as a variant (and
 implementation) of GetOldestXmin

This is useful because it allows to compute the current OldestXmin while
already holding the procarray lock which enables setting the own xmin horizon
safely.
---
 src/backend/storage/ipc/procarray.c | 30 +++++++++++++++++++++---------
 src/include/storage/procarray.h     |  1 +
 2 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 985350e..a704c9c 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -1104,6 +1104,27 @@ TransactionIdIsActive(TransactionId xid)
 TransactionId
 GetOldestXmin(bool allDbs, bool ignoreVacuum)
 {
+	TransactionId res;
+
+	/* Cannot look for individual databases during recovery */
+	Assert(allDbs || !RecoveryInProgress());
+
+	LWLockAcquire(ProcArrayLock, LW_SHARED);
+	res = GetOldestXminNoLock(allDbs, ignoreVacuum);
+	LWLockRelease(ProcArrayLock);
+	return res;
+}
+
+/*
+ * GetOldestXminNoLock -- worker routine for GetOldestXmin and others
+ *
+ * Requires ProcArrayLock to be already locked!
+ *
+ * Check GetOldestXmin for the semantics of this.
+ */
+TransactionId
+GetOldestXminNoLock(bool allDbs, bool ignoreVacuum)
+{
 	ProcArrayStruct *arrayP = procArray;
 	TransactionId result;
 	int			index;
@@ -1111,8 +1132,6 @@ GetOldestXmin(bool allDbs, bool ignoreVacuum)
 	/* Cannot look for individual databases during recovery */
 	Assert(allDbs || !RecoveryInProgress());
 
-	LWLockAcquire(ProcArrayLock, LW_SHARED);
-
 	/*
 	 * We initialize the MIN() calculation with latestCompletedXid + 1. This
 	 * is a lower bound for the XIDs that might appear in the ProcArray later,
@@ -1174,8 +1193,6 @@ GetOldestXmin(bool allDbs, bool ignoreVacuum)
 		 */
 		TransactionId kaxmin = KnownAssignedXidsGetOldestXmin();
 
-		LWLockRelease(ProcArrayLock);
-
 		if (TransactionIdIsNormal(kaxmin) &&
 			TransactionIdPrecedes(kaxmin, result))
 			result = kaxmin;
@@ -1183,11 +1200,6 @@ GetOldestXmin(bool allDbs, bool ignoreVacuum)
 	else
 	{
 		/*
-		 * No other information needed, so release the lock immediately.
-		 */
-		LWLockRelease(ProcArrayLock);
-
-		/*
 		 * Compute the cutoff XID by subtracting vacuum_defer_cleanup_age,
 		 * being careful not to generate a "permanent" XID.
 		 *
diff --git a/src/include/storage/procarray.h b/src/include/storage/procarray.h
index 9933dad..ce8d98b 100644
--- a/src/include/storage/procarray.h
+++ b/src/include/storage/procarray.h
@@ -50,6 +50,7 @@ extern RunningTransactions GetRunningTransactionData(void);
 extern bool TransactionIdIsInProgress(TransactionId xid);
 extern bool TransactionIdIsActive(TransactionId xid);
 extern TransactionId GetOldestXmin(bool allDbs, bool ignoreVacuum);
+extern TransactionId GetOldestXminNoLock(bool allDbs, bool ignoreVacuum);
 extern TransactionId GetOldestActiveTransactionId(void);
 
 extern VirtualTransactionId *GetVirtualXIDsDelayingChkpt(int *nvxids);
-- 
1.7.12.289.g0ce9864.dirty