Correct epoch of txid_current() when executed on a Hot Standby server.

Initialise ckptXidEpoch from starting checkpoint and maintain the correct
value as we roll forwards. This allows GetNextXidAndEpoch() to return the
correct epoch when executed during recovery. Backpatch to 9.0 when the
problem is first observable by a user.

Bug report from Daniel Farina

src/backend/access/transam/xlog.c

@@ -6320,6 +6320,10 @@ StartupXLOG(void)
 		/* No need to hold ControlFileLock yet, we aren't up far enough */
 		UpdateControlFile();
 
+		/* initialize shared-memory copy of latest checkpoint XID/epoch */
+		XLogCtl->ckptXidEpoch = ControlFile->checkPointCopy.nextXidEpoch;
+		XLogCtl->ckptXid = ControlFile->checkPointCopy.nextXid;
+
 		/* initialize our local copy of minRecoveryPoint */
 		minRecoveryPoint = ControlFile->minRecoveryPoint;
 
@@ -6915,10 +6919,6 @@ StartupXLOG(void)
 	/* start the archive_timeout timer running */
 	XLogCtl->Write.lastSegSwitchTime = (pg_time_t) time(NULL);
 
-	/* initialize shared-memory copy of latest checkpoint XID/epoch */
-	XLogCtl->ckptXidEpoch = ControlFile->checkPointCopy.nextXidEpoch;
-	XLogCtl->ckptXid = ControlFile->checkPointCopy.nextXid;
-
 	/* also initialize latestCompletedXid, to nextXid - 1 */
 	LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
 	ShmemVariableCache->latestCompletedXid = ShmemVariableCache->nextXid;
@@ -8601,6 +8601,17 @@ xlog_redo(XLogRecPtr lsn, XLogRecord *record)
 		ControlFile->checkPointCopy.nextXidEpoch = checkPoint.nextXidEpoch;
 		ControlFile->checkPointCopy.nextXid = checkPoint.nextXid;
 
+		/* Update shared-memory copy of checkpoint XID/epoch */
+		{
+			/* use volatile pointer to prevent code rearrangement */
+			volatile XLogCtlData *xlogctl = XLogCtl;
+
+			SpinLockAcquire(&xlogctl->info_lck);
+			xlogctl->ckptXidEpoch = checkPoint.nextXidEpoch;
+			xlogctl->ckptXid = checkPoint.nextXid;
+			SpinLockRelease(&xlogctl->info_lck);
+		}
+
 		/*
 		 * TLI may change in a shutdown checkpoint, but it shouldn't decrease
 		 */
@@ -8645,6 +8656,17 @@ xlog_redo(XLogRecPtr lsn, XLogRecord *record)
 		ControlFile->checkPointCopy.nextXidEpoch = checkPoint.nextXidEpoch;
 		ControlFile->checkPointCopy.nextXid = checkPoint.nextXid;
 
+		/* Update shared-memory copy of checkpoint XID/epoch */
+		{
+			/* use volatile pointer to prevent code rearrangement */
+			volatile XLogCtlData *xlogctl = XLogCtl;
+
+			SpinLockAcquire(&xlogctl->info_lck);
+			xlogctl->ckptXidEpoch = checkPoint.nextXidEpoch;
+			xlogctl->ckptXid = checkPoint.nextXid;
+			SpinLockRelease(&xlogctl->info_lck);
+		}
+
 		/* TLI should not change in an on-line checkpoint */
 		if (checkPoint.ThisTimeLineID != ThisTimeLineID)
 			ereport(PANIC,