Fix timing of Startup CLOG and MultiXact during Hot Standby

Patch by me, bug report by Chris Redekop, analysis by Florian Pflug

src/backend/access/transam/clog.c

@@ -490,6 +490,25 @@ StartupCLOG(void)
 	 */
 	ClogCtl->shared->latest_page_number = pageno;
 
+	LWLockRelease(CLogControlLock);
+}
+
+/*
+ * This must be called ONCE at the end of startup/recovery.
+ */
+void
+TrimCLOG(void)
+{
+	TransactionId xid = ShmemVariableCache->nextXid;
+	int			pageno = TransactionIdToPage(xid);
+
+	LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
+
+	/*
+	 * Re-Initialize our idea of the latest page number.
+	 */
+	ClogCtl->shared->latest_page_number = pageno;
+
 	/*
 	 * Zero out the remainder of the current clog page.  Under normal
 	 * circumstances it should be zeroes already, but it seems at least

src/backend/access/transam/multixact.c

@@ -1568,7 +1568,7 @@ StartupMultiXact(void)
 
 	/*
 	 * Zero out the remainder of the current members page.	See notes in
-	 * StartupCLOG() for motivation.
+	 * TrimCLOG() for motivation.
 	 */
 	entryno = MXOffsetToMemberEntry(offset);
 	if (entryno != 0)

src/backend/access/transam/xlog.c

@@ -6384,10 +6384,12 @@ StartupXLOG(void)
 				oldestActiveXID = checkPoint.oldestActiveXid;
 			Assert(TransactionIdIsValid(oldestActiveXID));
 
-			/* Startup commit log and related stuff */
+			/*
+			 * Startup commit log and subtrans only. Other SLRUs are not
+			 * maintained during recovery and need not be started yet.
+			 */
 			StartupCLOG();
 			StartupSUBTRANS(oldestActiveXID);
-			StartupMultiXact();
 
 			/*
 			 * If we're beginning at a shutdown checkpoint, we know that
@@ -6876,16 +6878,21 @@ StartupXLOG(void)
 	TransactionIdRetreat(ShmemVariableCache->latestCompletedXid);
 
 	/*
-	 * Start up the commit log and related stuff, too. In hot standby mode we
+	 * Start up the commit log and subtrans, if not already done for
-	 * did this already before WAL replay.
+	 * hot standby.
 	 */
 	if (standbyState == STANDBY_DISABLED)
 	{
 		StartupCLOG();
 		StartupSUBTRANS(oldestActiveXID);
-		StartupMultiXact();
 	}
 
+	/*
+	 * Perform end of recovery actions for any SLRUs that need it.
+	 */
+	StartupMultiXact();
+	TrimCLOG();
+
 	/* Reload shared-memory state for prepared transactions */
 	RecoverPreparedTransactions();
 

src/include/access/clog.h

@@ -40,6 +40,7 @@ extern Size CLOGShmemSize(void);
 extern void CLOGShmemInit(void);
 extern void BootStrapCLOG(void);
 extern void StartupCLOG(void);
+extern void TrimCLOG(void);
 extern void ShutdownCLOG(void);
 extern void CheckPointCLOG(void);
 extern void ExtendCLOG(TransactionId newestXact);