Rework WAL-reading supporting structs

The state-tracking of WAL reading in various places was pretty messy, mostly because the ancient physical-replication WAL reading code wasn't using the XLogReader abstraction. This led to some untidy code. Make it prettier by creating two additional supporting structs, WALSegmentContext and WALOpenSegment which keep track of WAL-reading state. This makes code cleaner, as well as supports more future cleanup. Author: Antonin Houska Reviewed-by: Álvaro Herrera and (older versions) Robert Haas Discussion: https://postgr.es/m/14984.1554998742@spoje.net
2019-09-24 16:08:31 -03:00 · 2019-09-24 16:08:31 -03:00 · 709d003fbd
parent a9ae99d019
commit 709d003fbd
12 changed files with 189 additions and 179 deletions
--- a/src/backend/access/transam/twophase.c
+++ b/src/backend/access/transam/twophase.c
@ -1377,7 +1377,6 @@ ParsePrepareRecord(uint8 info, char *xlrec, xl_xact_parsed_prepare *parsed)
 *
 * Note clearly that this function can access WAL during normal operation,
 * similarly to the way WALSender or Logical Decoding would do.
- *
 */
 static void
 XlogReadTwoPhaseData(XLogRecPtr lsn, char **buf, int *len)
@ -1386,8 +1385,8 @@ XlogReadTwoPhaseData(XLogRecPtr lsn, char **buf, int *len)
 	XLogReaderState *xlogreader;
 	char	   *errormsg;

-	xlogreader = XLogReaderAllocate(wal_segment_size, &read_local_xlog_page,
-									NULL);
+	xlogreader = XLogReaderAllocate(wal_segment_size, NULL,
+									&read_local_xlog_page, NULL);
 	if (!xlogreader)
 		ereport(ERROR,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@ -885,8 +885,7 @@ static int	XLogFileRead(XLogSegNo segno, int emode, TimeLineID tli,
 						 int source, bool notfoundOk);
 static int	XLogFileReadAnyTLI(XLogSegNo segno, int emode, int source);
 static int	XLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr,
-						 int reqLen, XLogRecPtr targetRecPtr, char *readBuf,
-						 TimeLineID *readTLI);
+						 int reqLen, XLogRecPtr targetRecPtr, char *readBuf);
 static bool WaitForWALToBecomeAvailable(XLogRecPtr RecPtr, bool randAccess,
 										bool fetching_ckpt, XLogRecPtr tliRecPtr);
 static int	emode_for_corrupt_record(int emode, XLogRecPtr RecPtr);
@ -1195,7 +1194,8 @@ XLogInsertRecord(XLogRecData *rdata,
 			appendBinaryStringInfo(&recordBuf, rdata->data, rdata->len);

 		if (!debug_reader)
-			debug_reader = XLogReaderAllocate(wal_segment_size, NULL, NULL);
+			debug_reader = XLogReaderAllocate(wal_segment_size, NULL,
+											  NULL, NULL);

 		if (!debug_reader)
 		{
@ -4296,7 +4296,7 @@ ReadRecord(XLogReaderState *xlogreader, XLogRecPtr RecPtr, int emode,
 			XLByteToSeg(xlogreader->latestPagePtr, segno, wal_segment_size);
 			offset = XLogSegmentOffset(xlogreader->latestPagePtr,
 									   wal_segment_size);
-			XLogFileName(fname, xlogreader->readPageTLI, segno,
+			XLogFileName(fname, xlogreader->seg.ws_tli, segno,
 						 wal_segment_size);
 			ereport(emode_for_corrupt_record(emode,
 											 RecPtr ? RecPtr : EndRecPtr),
@ -6353,7 +6353,8 @@ StartupXLOG(void)

 	/* Set up XLOG reader facility */
 	MemSet(&private, 0, sizeof(XLogPageReadPrivate));
-	xlogreader = XLogReaderAllocate(wal_segment_size, &XLogPageRead, &private);
+	xlogreader = XLogReaderAllocate(wal_segment_size, NULL,
+									&XLogPageRead, &private);
 	if (!xlogreader)
 		ereport(ERROR,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
@ -7355,7 +7356,7 @@ StartupXLOG(void)
 	 * and we were reading the old WAL from a segment belonging to a higher
 	 * timeline.
 	 */
-	EndOfLogTLI = xlogreader->readPageTLI;
+	EndOfLogTLI = xlogreader->seg.ws_tli;

 	/*
 	 * Complain if we did not roll forward far enough to render the backup
@ -11523,7 +11524,7 @@ CancelBackup(void)
 */
 static int
 XLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr, int reqLen,
-			 XLogRecPtr targetRecPtr, char *readBuf, TimeLineID *readTLI)
+			 XLogRecPtr targetRecPtr, char *readBuf)
 {
 	XLogPageReadPrivate *private =
 	(XLogPageReadPrivate *) xlogreader->private_data;
@ -11640,7 +11641,7 @@ retry:
 	Assert(targetPageOff == readOff);
 	Assert(reqLen <= readLen);

-	*readTLI = curFileTLI;
+	xlogreader->seg.ws_tli = curFileTLI;

 	/*
 	 * Check the page header immediately, so that we can retry immediately if
--- a/src/backend/access/transam/xlogreader.c
+++ b/src/backend/access/transam/xlogreader.c
@ -68,8 +68,8 @@ report_invalid_record(XLogReaderState *state, const char *fmt,...)
 * Returns NULL if the xlogreader couldn't be allocated.
 */
 XLogReaderState *
-XLogReaderAllocate(int wal_segment_size, XLogPageReadCB pagereadfunc,
-				   void *private_data)
+XLogReaderAllocate(int wal_segment_size, const char *waldir,
+				   XLogPageReadCB pagereadfunc, void *private_data)
 {
 	XLogReaderState *state;

@ -96,7 +96,10 @@ XLogReaderAllocate(int wal_segment_size, XLogPageReadCB pagereadfunc,
 		return NULL;
 	}

-	state->wal_segment_size = wal_segment_size;
+	/* Initialize segment info. */
+	WALOpenSegmentInit(&state->seg, &state->segcxt, wal_segment_size,
+					   waldir);
+
 	state->read_page = pagereadfunc;
 	/* system_identifier initialized to zeroes above */
 	state->private_data = private_data;
@ -198,6 +201,23 @@ allocate_recordbuf(XLogReaderState *state, uint32 reclength)
 	return true;
 }

+/*
+ * Initialize the passed segment structs.
+ */
+void
+WALOpenSegmentInit(WALOpenSegment *seg, WALSegmentContext *segcxt,
+				   int segsize, const char *waldir)
+{
+	seg->ws_file = -1;
+	seg->ws_segno = 0;
+	seg->ws_off = 0;
+	seg->ws_tli = 0;
+
+	segcxt->ws_segsize = segsize;
+	if (waldir)
+		snprintf(segcxt->ws_dir, MAXPGPATH, "%s", waldir);
+}
+
 /*
 * Attempt to read an XLOG record.
 *
@ -490,8 +510,8 @@ XLogReadRecord(XLogReaderState *state, XLogRecPtr RecPtr, char **errormsg)
 		(record->xl_info & ~XLR_INFO_MASK) == XLOG_SWITCH)
 	{
 		/* Pretend it extends to end of segment */
-		state->EndRecPtr += state->wal_segment_size - 1;
-		state->EndRecPtr -= XLogSegmentOffset(state->EndRecPtr, state->wal_segment_size);
+		state->EndRecPtr += state->segcxt.ws_segsize - 1;
+		state->EndRecPtr -= XLogSegmentOffset(state->EndRecPtr, state->segcxt.ws_segsize);
 	}

 	if (DecodeXLogRecord(state, record, errormsg))
@ -533,12 +553,12 @@ ReadPageInternal(XLogReaderState *state, XLogRecPtr pageptr, int reqLen)

 	Assert((pageptr % XLOG_BLCKSZ) == 0);

-	XLByteToSeg(pageptr, targetSegNo, state->wal_segment_size);
-	targetPageOff = XLogSegmentOffset(pageptr, state->wal_segment_size);
+	XLByteToSeg(pageptr, targetSegNo, state->segcxt.ws_segsize);
+	targetPageOff = XLogSegmentOffset(pageptr, state->segcxt.ws_segsize);

 	/* check whether we have all the requested data already */
-	if (targetSegNo == state->readSegNo && targetPageOff == state->readOff &&
-		reqLen <= state->readLen)
+	if (targetSegNo == state->seg.ws_segno &&
+		targetPageOff == state->seg.ws_off && reqLen <= state->readLen)
 		return state->readLen;

 	/*
@ -553,13 +573,13 @@ ReadPageInternal(XLogReaderState *state, XLogRecPtr pageptr, int reqLen)
 	 * record is.  This is so that we can check the additional identification
 	 * info that is present in the first page's "long" header.
 	 */
-	if (targetSegNo != state->readSegNo && targetPageOff != 0)
+	if (targetSegNo != state->seg.ws_segno && targetPageOff != 0)
 	{
 		XLogRecPtr	targetSegmentPtr = pageptr - targetPageOff;

 		readLen = state->read_page(state, targetSegmentPtr, XLOG_BLCKSZ,
 								   state->currRecPtr,
-								   state->readBuf, &state->readPageTLI);
+								   state->readBuf);
 		if (readLen < 0)
 			goto err;

@ -577,7 +597,7 @@ ReadPageInternal(XLogReaderState *state, XLogRecPtr pageptr, int reqLen)
 	 */
 	readLen = state->read_page(state, pageptr, Max(reqLen, SizeOfXLogShortPHD),
 							   state->currRecPtr,
-							   state->readBuf, &state->readPageTLI);
+							   state->readBuf);
 	if (readLen < 0)
 		goto err;

@ -596,7 +616,7 @@ ReadPageInternal(XLogReaderState *state, XLogRecPtr pageptr, int reqLen)
 	{
 		readLen = state->read_page(state, pageptr, XLogPageHeaderSize(hdr),
 								   state->currRecPtr,
-								   state->readBuf, &state->readPageTLI);
+								   state->readBuf);
 		if (readLen < 0)
 			goto err;
 	}
@ -608,8 +628,8 @@ ReadPageInternal(XLogReaderState *state, XLogRecPtr pageptr, int reqLen)
 		goto err;

 	/* update read state information */
-	state->readSegNo = targetSegNo;
-	state->readOff = targetPageOff;
+	state->seg.ws_segno = targetSegNo;
+	state->seg.ws_off = targetPageOff;
 	state->readLen = readLen;

 	return readLen;
@ -625,8 +645,8 @@ err:
 static void
 XLogReaderInvalReadState(XLogReaderState *state)
 {
-	state->readSegNo = 0;
-	state->readOff = 0;
+	state->seg.ws_segno = 0;
+	state->seg.ws_off = 0;
 	state->readLen = 0;
 }

@ -745,16 +765,16 @@ XLogReaderValidatePageHeader(XLogReaderState *state, XLogRecPtr recptr,

 	Assert((recptr % XLOG_BLCKSZ) == 0);

-	XLByteToSeg(recptr, segno, state->wal_segment_size);
-	offset = XLogSegmentOffset(recptr, state->wal_segment_size);
+	XLByteToSeg(recptr, segno, state->segcxt.ws_segsize);
+	offset = XLogSegmentOffset(recptr, state->segcxt.ws_segsize);

-	XLogSegNoOffsetToRecPtr(segno, offset, state->wal_segment_size, recaddr);
+	XLogSegNoOffsetToRecPtr(segno, offset, state->segcxt.ws_segsize, recaddr);

 	if (hdr->xlp_magic != XLOG_PAGE_MAGIC)
 	{
 		char		fname[MAXFNAMELEN];

-		XLogFileName(fname, state->readPageTLI, segno, state->wal_segment_size);
+		XLogFileName(fname, state->seg.ws_tli, segno, state->segcxt.ws_segsize);

 		report_invalid_record(state,
 							  "invalid magic number %04X in log segment %s, offset %u",
@ -768,7 +788,7 @@ XLogReaderValidatePageHeader(XLogReaderState *state, XLogRecPtr recptr,
 	{
 		char		fname[MAXFNAMELEN];

-		XLogFileName(fname, state->readPageTLI, segno, state->wal_segment_size);
+		XLogFileName(fname, state->seg.ws_tli, segno, state->segcxt.ws_segsize);

 		report_invalid_record(state,
 							  "invalid info bits %04X in log segment %s, offset %u",
@ -791,7 +811,7 @@ XLogReaderValidatePageHeader(XLogReaderState *state, XLogRecPtr recptr,
 								  (unsigned long long) state->system_identifier);
 			return false;
 		}
-		else if (longhdr->xlp_seg_size != state->wal_segment_size)
+		else if (longhdr->xlp_seg_size != state->segcxt.ws_segsize)
 		{
 			report_invalid_record(state,
 								  "WAL file is from different database system: incorrect segment size in page header");
@ -808,7 +828,7 @@ XLogReaderValidatePageHeader(XLogReaderState *state, XLogRecPtr recptr,
 	{
 		char		fname[MAXFNAMELEN];

-		XLogFileName(fname, state->readPageTLI, segno, state->wal_segment_size);
+		XLogFileName(fname, state->seg.ws_tli, segno, state->segcxt.ws_segsize);

 		/* hmm, first page of file doesn't have a long header? */
 		report_invalid_record(state,
@ -828,7 +848,7 @@ XLogReaderValidatePageHeader(XLogReaderState *state, XLogRecPtr recptr,
 	{
 		char		fname[MAXFNAMELEN];

-		XLogFileName(fname, state->readPageTLI, segno, state->wal_segment_size);
+		XLogFileName(fname, state->seg.ws_tli, segno, state->segcxt.ws_segsize);

 		report_invalid_record(state,
 							  "unexpected pageaddr %X/%X in log segment %s, offset %u",
@ -853,7 +873,7 @@ XLogReaderValidatePageHeader(XLogReaderState *state, XLogRecPtr recptr,
 		{
 			char		fname[MAXFNAMELEN];

-			XLogFileName(fname, state->readPageTLI, segno, state->wal_segment_size);
+			XLogFileName(fname, state->seg.ws_tli, segno, state->segcxt.ws_segsize);

 			report_invalid_record(state,
 								  "out-of-sequence timeline ID %u (after %u) in log segment %s, offset %u",
@ -997,7 +1017,6 @@ out:

 #endif							/* FRONTEND */

-
 /* ----------------------------------------
 * Functions for decoding the data and block references in a record.
 * ----------------------------------------
--- a/src/backend/access/transam/xlogutils.c
+++ b/src/backend/access/transam/xlogutils.c
@ -802,8 +802,8 @@ XLogRead(char *buf, int segsize, TimeLineID tli, XLogRecPtr startptr,
 void
 XLogReadDetermineTimeline(XLogReaderState *state, XLogRecPtr wantPage, uint32 wantLength)
 {
-	const XLogRecPtr lastReadPage = state->readSegNo *
-	state->wal_segment_size + state->readOff;
+	const XLogRecPtr lastReadPage = state->seg.ws_segno *
+	state->segcxt.ws_segsize + state->seg.ws_off;

 	Assert(wantPage != InvalidXLogRecPtr && wantPage % XLOG_BLCKSZ == 0);
 	Assert(wantLength <= XLOG_BLCKSZ);
@ -847,8 +847,8 @@ XLogReadDetermineTimeline(XLogReaderState *state, XLogRecPtr wantPage, uint32 wa
 	if (state->currTLIValidUntil != InvalidXLogRecPtr &&
 		state->currTLI != ThisTimeLineID &&
 		state->currTLI != 0 &&
-		((wantPage + wantLength) / state->wal_segment_size) <
-		(state->currTLIValidUntil / state->wal_segment_size))
+		((wantPage + wantLength) / state->segcxt.ws_segsize) <
+		(state->currTLIValidUntil / state->segcxt.ws_segsize))
 		return;

 	/*
@ -869,12 +869,12 @@ XLogReadDetermineTimeline(XLogReaderState *state, XLogRecPtr wantPage, uint32 wa
 		 * by a promotion or replay from a cascaded replica.
 		 */
 		List	   *timelineHistory = readTimeLineHistory(ThisTimeLineID);
+		XLogRecPtr	endOfSegment;

-		XLogRecPtr	endOfSegment = (((wantPage / state->wal_segment_size) + 1)
-									* state->wal_segment_size) - 1;
-
-		Assert(wantPage / state->wal_segment_size ==
-			   endOfSegment / state->wal_segment_size);
+		endOfSegment = ((wantPage / state->segcxt.ws_segsize) + 1) *
+			state->segcxt.ws_segsize - 1;
+		Assert(wantPage / state->segcxt.ws_segsize ==
+			   endOfSegment / state->segcxt.ws_segsize);

 		/*
 		 * Find the timeline of the last LSN on the segment containing
@ -909,8 +909,7 @@ XLogReadDetermineTimeline(XLogReaderState *state, XLogRecPtr wantPage, uint32 wa
 */
 int
 read_local_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr,
-					 int reqLen, XLogRecPtr targetRecPtr, char *cur_page,
-					 TimeLineID *pageTLI)
+					 int reqLen, XLogRecPtr targetRecPtr, char *cur_page)
 {
 	XLogRecPtr	read_upto,
 				loc;
@ -933,8 +932,7 @@ read_local_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr,
 			read_upto = GetFlushRecPtr();
 		else
 			read_upto = GetXLogReplayRecPtr(&ThisTimeLineID);
-
-		*pageTLI = ThisTimeLineID;
+		state->seg.ws_tli = ThisTimeLineID;

 		/*
 		 * Check which timeline to get the record from.
@ -984,14 +982,14 @@ read_local_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr,
 			read_upto = state->currTLIValidUntil;

 			/*
-			 * Setting pageTLI to our wanted record's TLI is slightly wrong;
+			 * Setting ws_tli to our wanted record's TLI is slightly wrong;
 			 * the page might begin on an older timeline if it contains a
 			 * timeline switch, since its xlog segment will have been copied
 			 * from the prior timeline. This is pretty harmless though, as
 			 * nothing cares so long as the timeline doesn't go backwards.  We
 			 * should read the page header instead; FIXME someday.
 			 */
-			*pageTLI = state->currTLI;
+			state->seg.ws_tli = state->currTLI;

 			/* No need to wait on a historical timeline */
 			break;
@ -1022,7 +1020,7 @@ read_local_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr,
 	 * as 'count', read the whole page anyway. It's guaranteed to be
 	 * zero-padded up to the page boundary if it's incomplete.
 	 */
-	XLogRead(cur_page, state->wal_segment_size, *pageTLI, targetPagePtr,
+	XLogRead(cur_page, state->segcxt.ws_segsize, state->seg.ws_tli, targetPagePtr,
 			 XLOG_BLCKSZ);

 	/* number of valid bytes in the buffer */
--- a/src/backend/replication/logical/logical.c
+++ b/src/backend/replication/logical/logical.c
@ -173,7 +173,7 @@ StartupDecodingContext(List *output_plugin_options,

 	ctx->slot = slot;

-	ctx->reader = XLogReaderAllocate(wal_segment_size, read_page, ctx);
+	ctx->reader = XLogReaderAllocate(wal_segment_size, NULL, read_page, ctx);
 	if (!ctx->reader)
 		ereport(ERROR,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
--- a/src/backend/replication/logical/logicalfuncs.c
+++ b/src/backend/replication/logical/logicalfuncs.c
@ -116,10 +116,10 @@ check_permissions(void)

 int
 logical_read_local_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr,
-							 int reqLen, XLogRecPtr targetRecPtr, char *cur_page, TimeLineID *pageTLI)
+							 int reqLen, XLogRecPtr targetRecPtr, char *cur_page)
 {
 	return read_local_xlog_page(state, targetPagePtr, reqLen,
-								targetRecPtr, cur_page, pageTLI);
+								targetRecPtr, cur_page);
 }

 /*
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@ -128,16 +128,8 @@ bool		log_replication_commands = false;
 */
 bool		wake_wal_senders = false;

-/*
- * These variables are used similarly to openLogFile/SegNo/Off,
- * but for walsender to read the XLOG.
- */
-static int	sendFile = -1;
-static XLogSegNo sendSegNo = 0;
-static uint32 sendOff = 0;
-
-/* Timeline ID of the currently open file */
-static TimeLineID curFileTimeLine = 0;
+static WALOpenSegment *sendSeg = NULL;
+static WALSegmentContext *sendCxt = NULL;

 /*
 * These variables keep track of the state of the timeline we're currently
@ -256,7 +248,7 @@ static void LagTrackerWrite(XLogRecPtr lsn, TimestampTz local_flush_time);
 static TimeOffset LagTrackerRead(int head, XLogRecPtr lsn, TimestampTz now);
 static bool TransactionIdInRecentPast(TransactionId xid, uint32 epoch);

-static void XLogRead(char *buf, XLogRecPtr startptr, Size count);
+static void XLogRead(WALSegmentContext *segcxt, char *buf, XLogRecPtr startptr, Size count);


 /* Initialize walsender process before entering the main command loop */
@ -285,6 +277,13 @@ InitWalSender(void)

 	/* Initialize empty timestamp buffer for lag tracking. */
 	lag_tracker = MemoryContextAllocZero(TopMemoryContext, sizeof(LagTracker));
+
+	/* Make sure we can remember the current read position in XLOG. */
+	sendSeg = (WALOpenSegment *)
+		MemoryContextAlloc(TopMemoryContext, sizeof(WALOpenSegment));
+	sendCxt = (WALSegmentContext *)
+		MemoryContextAlloc(TopMemoryContext, sizeof(WALSegmentContext));
+	WALOpenSegmentInit(sendSeg, sendCxt, wal_segment_size, NULL);
 }

 /*
@ -301,10 +300,10 @@ WalSndErrorCleanup(void)
 	ConditionVariableCancelSleep();
 	pgstat_report_wait_end();

-	if (sendFile >= 0)
+	if (sendSeg->ws_file >= 0)
 	{
-		close(sendFile);
-		sendFile = -1;
+		close(sendSeg->ws_file);
+		sendSeg->ws_file = -1;
 	}

 	if (MyReplicationSlot != NULL)
@ -763,7 +762,7 @@ StartReplication(StartReplicationCmd *cmd)
 */
 static int
 logical_read_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr, int reqLen,
-					   XLogRecPtr targetRecPtr, char *cur_page, TimeLineID *pageTLI)
+					   XLogRecPtr targetRecPtr, char *cur_page)
 {
 	XLogRecPtr	flushptr;
 	int			count;
@ -787,7 +786,7 @@ logical_read_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr, int req
 		count = flushptr - targetPagePtr;	/* part of the page available */

 	/* now actually read the data, we know it's there */
-	XLogRead(cur_page, targetPagePtr, XLOG_BLCKSZ);
+	XLogRead(sendCxt, cur_page, targetPagePtr, XLOG_BLCKSZ);

 	return count;
 }
@ -2364,7 +2363,7 @@ WalSndKill(int code, Datum arg)
 * more than one.
 */
 static void
-XLogRead(char *buf, XLogRecPtr startptr, Size count)
+XLogRead(WALSegmentContext *segcxt, char *buf, XLogRecPtr startptr, Size count)
 {
 	char	   *p;
 	XLogRecPtr	recptr;
@ -2382,17 +2381,18 @@ retry:
 		int			segbytes;
 		int			readbytes;

-		startoff = XLogSegmentOffset(recptr, wal_segment_size);
+		startoff = XLogSegmentOffset(recptr, segcxt->ws_segsize);

-		if (sendFile < 0 || !XLByteInSeg(recptr, sendSegNo, wal_segment_size))
+		if (sendSeg->ws_file < 0 ||
+			!XLByteInSeg(recptr, sendSeg->ws_segno, segcxt->ws_segsize))
 		{
 			char		path[MAXPGPATH];

 			/* Switch to another logfile segment */
-			if (sendFile >= 0)
-				close(sendFile);
+			if (sendSeg->ws_file >= 0)
+				close(sendSeg->ws_file);

-			XLByteToSeg(recptr, sendSegNo, wal_segment_size);
+			XLByteToSeg(recptr, sendSeg->ws_segno, segcxt->ws_segsize);

 			/*-------
 			 * When reading from a historic timeline, and there is a timeline
@ -2420,20 +2420,20 @@ retry:
 			 * used portion of the old segment is copied to the new file.
 			 *-------
 			 */
-			curFileTimeLine = sendTimeLine;
+			sendSeg->ws_tli = sendTimeLine;
 			if (sendTimeLineIsHistoric)
 			{
 				XLogSegNo	endSegNo;

-				XLByteToSeg(sendTimeLineValidUpto, endSegNo, wal_segment_size);
-				if (sendSegNo == endSegNo)
-					curFileTimeLine = sendTimeLineNextTLI;
+				XLByteToSeg(sendTimeLineValidUpto, endSegNo, segcxt->ws_segsize);
+				if (sendSeg->ws_segno == endSegNo)
+					sendSeg->ws_tli = sendTimeLineNextTLI;
 			}

-			XLogFilePath(path, curFileTimeLine, sendSegNo, wal_segment_size);
+			XLogFilePath(path, sendSeg->ws_tli, sendSeg->ws_segno, segcxt->ws_segsize);

-			sendFile = BasicOpenFile(path, O_RDONLY | PG_BINARY);
-			if (sendFile < 0)
+			sendSeg->ws_file = BasicOpenFile(path, O_RDONLY | PG_BINARY);
+			if (sendSeg->ws_file < 0)
 			{
 				/*
 				 * If the file is not found, assume it's because the standby
@ -2444,58 +2444,58 @@ retry:
 					ereport(ERROR,
 							(errcode_for_file_access(),
 							 errmsg("requested WAL segment %s has already been removed",
-									XLogFileNameP(curFileTimeLine, sendSegNo))));
+									XLogFileNameP(sendSeg->ws_tli, sendSeg->ws_segno))));
 				else
 					ereport(ERROR,
 							(errcode_for_file_access(),
 							 errmsg("could not open file \"%s\": %m",
 									path)));
 			}
-			sendOff = 0;
+			sendSeg->ws_off = 0;
 		}

 		/* Need to seek in the file? */
-		if (sendOff != startoff)
+		if (sendSeg->ws_off != startoff)
 		{
-			if (lseek(sendFile, (off_t) startoff, SEEK_SET) < 0)
+			if (lseek(sendSeg->ws_file, (off_t) startoff, SEEK_SET) < 0)
 				ereport(ERROR,
 						(errcode_for_file_access(),
 						 errmsg("could not seek in log segment %s to offset %u: %m",
-								XLogFileNameP(curFileTimeLine, sendSegNo),
+								XLogFileNameP(sendSeg->ws_tli, sendSeg->ws_segno),
 								startoff)));
-			sendOff = startoff;
+			sendSeg->ws_off = startoff;
 		}

 		/* How many bytes are within this segment? */
-		if (nbytes > (wal_segment_size - startoff))
-			segbytes = wal_segment_size - startoff;
+		if (nbytes > (segcxt->ws_segsize - startoff))
+			segbytes = segcxt->ws_segsize - startoff;
 		else
 			segbytes = nbytes;

 		pgstat_report_wait_start(WAIT_EVENT_WAL_READ);
-		readbytes = read(sendFile, p, segbytes);
+		readbytes = read(sendSeg->ws_file, p, segbytes);
 		pgstat_report_wait_end();
 		if (readbytes < 0)
 		{
 			ereport(ERROR,
 					(errcode_for_file_access(),
 					 errmsg("could not read from log segment %s, offset %u, length %zu: %m",
-							XLogFileNameP(curFileTimeLine, sendSegNo),
-							sendOff, (Size) segbytes)));
+							XLogFileNameP(sendSeg->ws_tli, sendSeg->ws_segno),
+							sendSeg->ws_off, (Size) segbytes)));
 		}
 		else if (readbytes == 0)
 		{
 			ereport(ERROR,
 					(errcode(ERRCODE_DATA_CORRUPTED),
 					 errmsg("could not read from log segment %s, offset %u: read %d of %zu",
-							XLogFileNameP(curFileTimeLine, sendSegNo),
-							sendOff, readbytes, (Size) segbytes)));
+							XLogFileNameP(sendSeg->ws_tli, sendSeg->ws_segno),
+							sendSeg->ws_off, readbytes, (Size) segbytes)));
 		}

 		/* Update state for read */
 		recptr += readbytes;

-		sendOff += readbytes;
+		sendSeg->ws_off += readbytes;
 		nbytes -= readbytes;
 		p += readbytes;
 	}
@ -2507,7 +2507,7 @@ retry:
 	 * read() succeeds in that case, but the data we tried to read might
 	 * already have been overwritten with new WAL records.
 	 */
-	XLByteToSeg(startptr, segno, wal_segment_size);
+	XLByteToSeg(startptr, segno, segcxt->ws_segsize);
 	CheckXLogRemoved(segno, ThisTimeLineID);

 	/*
@ -2526,10 +2526,10 @@ retry:
 		walsnd->needreload = false;
 		SpinLockRelease(&walsnd->mutex);

-		if (reload && sendFile >= 0)
+		if (reload && sendSeg->ws_file >= 0)
 		{
-			close(sendFile);
-			sendFile = -1;
+			close(sendSeg->ws_file);
+			sendSeg->ws_file = -1;

 			goto retry;
 		}
@ -2695,9 +2695,9 @@ XLogSendPhysical(void)
 	if (sendTimeLineIsHistoric && sendTimeLineValidUpto <= sentPtr)
 	{
 		/* close the current file. */
-		if (sendFile >= 0)
-			close(sendFile);
-		sendFile = -1;
+		if (sendSeg->ws_file >= 0)
+			close(sendSeg->ws_file);
+		sendSeg->ws_file = -1;

 		/* Send CopyDone */
 		pq_putmessage_noblock('c', NULL, 0);
@ -2768,7 +2768,7 @@ XLogSendPhysical(void)
 	 * calls.
 	 */
 	enlargeStringInfo(&output_message, nbytes);
-	XLogRead(&output_message.data[output_message.len], startptr, nbytes);
+	XLogRead(sendCxt, &output_message.data[output_message.len], startptr, nbytes);
 	output_message.len += nbytes;
 	output_message.data[output_message.len] = '\0';

--- a/src/bin/pg_rewind/parsexlog.c
+++ b/src/bin/pg_rewind/parsexlog.c
@ -43,14 +43,12 @@ static char xlogfpath[MAXPGPATH];

 typedef struct XLogPageReadPrivate
 {
-	const char *datadir;
 	int			tliIndex;
 } XLogPageReadPrivate;

 static int	SimpleXLogPageRead(XLogReaderState *xlogreader,
 							   XLogRecPtr targetPagePtr,
-							   int reqLen, XLogRecPtr targetRecPtr, char *readBuf,
-							   TimeLineID *pageTLI);
+							   int reqLen, XLogRecPtr targetRecPtr, char *readBuf);

 /*
 * Read WAL from the datadir/pg_wal, starting from 'startpoint' on timeline
@ -66,9 +64,8 @@ extractPageMap(const char *datadir, XLogRecPtr startpoint, int tliIndex,
 	char	   *errormsg;
 	XLogPageReadPrivate private;

-	private.datadir = datadir;
 	private.tliIndex = tliIndex;
-	xlogreader = XLogReaderAllocate(WalSegSz, &SimpleXLogPageRead,
+	xlogreader = XLogReaderAllocate(WalSegSz, datadir, &SimpleXLogPageRead,
 									&private);
 	if (xlogreader == NULL)
 		pg_fatal("out of memory");
@ -119,9 +116,8 @@ readOneRecord(const char *datadir, XLogRecPtr ptr, int tliIndex)
 	XLogPageReadPrivate private;
 	XLogRecPtr	endptr;

-	private.datadir = datadir;
 	private.tliIndex = tliIndex;
-	xlogreader = XLogReaderAllocate(WalSegSz, &SimpleXLogPageRead,
+	xlogreader = XLogReaderAllocate(WalSegSz, datadir, &SimpleXLogPageRead,
 									&private);
 	if (xlogreader == NULL)
 		pg_fatal("out of memory");
@ -177,9 +173,8 @@ findLastCheckpoint(const char *datadir, XLogRecPtr forkptr, int tliIndex,
 			forkptr += SizeOfXLogShortPHD;
 	}

-	private.datadir = datadir;
 	private.tliIndex = tliIndex;
-	xlogreader = XLogReaderAllocate(WalSegSz, &SimpleXLogPageRead,
+	xlogreader = XLogReaderAllocate(WalSegSz, datadir, &SimpleXLogPageRead,
 									&private);
 	if (xlogreader == NULL)
 		pg_fatal("out of memory");
@ -237,8 +232,7 @@ findLastCheckpoint(const char *datadir, XLogRecPtr forkptr, int tliIndex,
 /* XLogReader callback function, to read a WAL page */
 static int
 SimpleXLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr,
-				   int reqLen, XLogRecPtr targetRecPtr, char *readBuf,
-				   TimeLineID *pageTLI)
+				   int reqLen, XLogRecPtr targetRecPtr, char *readBuf)
 {
 	XLogPageReadPrivate *private = (XLogPageReadPrivate *) xlogreader->private_data;
 	uint32		targetPageOff;
@ -283,7 +277,8 @@ SimpleXLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr,
 		XLogFileName(xlogfname, targetHistory[private->tliIndex].tli,
 					 xlogreadsegno, WalSegSz);

-		snprintf(xlogfpath, MAXPGPATH, "%s/" XLOGDIR "/%s", private->datadir, xlogfname);
+		snprintf(xlogfpath, MAXPGPATH, "%s/" XLOGDIR "/%s",
+				 xlogreader->segcxt.ws_dir, xlogfname);

 		xlogreadfd = open(xlogfpath, O_RDONLY | PG_BINARY, 0);

@ -321,7 +316,7 @@ SimpleXLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr,

 	Assert(targetSegNo == xlogreadsegno);

-	*pageTLI = targetHistory[private->tliIndex].tli;
+	xlogreader->seg.ws_tli = targetHistory[private->tliIndex].tli;
 	return XLOG_BLCKSZ;
 }

--- a/src/bin/pg_waldump/pg_waldump.c
+++ b/src/bin/pg_waldump/pg_waldump.c
@ -33,7 +33,6 @@ static int	WalSegSz;
 typedef struct XLogDumpPrivate
 {
 	TimeLineID	timeline;
-	char	   *inpath;
 	XLogRecPtr	startptr;
 	XLogRecPtr	endptr;
 	bool		endptr_reached;
@ -224,7 +223,7 @@ search_directory(const char *directory, const char *fname)
 }

 /*
- * Identify the target directory and set WalSegSz.
+ * Identify the target directory.
 *
 * Try to find the file in several places:
 * if directory != NULL:
@ -235,29 +234,22 @@ search_directory(const char *directory, const char *fname)
 *	 XLOGDIR /
 *	 $PGDATA / XLOGDIR /
 *
- * Set the valid target directory in private->inpath.
+ * The valid target directory is returned.
 */
-static void
-identify_target_directory(XLogDumpPrivate *private, char *directory,
-						  char *fname)
+static char *
+identify_target_directory(char *directory, char *fname)
 {
 	char		fpath[MAXPGPATH];

 	if (directory != NULL)
 	{
 		if (search_directory(directory, fname))
-		{
-			private->inpath = pg_strdup(directory);
-			return;
-		}
+			return pg_strdup(directory);

 		/* directory / XLOGDIR */
 		snprintf(fpath, MAXPGPATH, "%s/%s", directory, XLOGDIR);
 		if (search_directory(fpath, fname))
-		{
-			private->inpath = pg_strdup(fpath);
-			return;
-		}
+			return pg_strdup(fpath);
 	}
 	else
 	{
@ -265,16 +257,10 @@ identify_target_directory(XLogDumpPrivate *private, char *directory,

 		/* current directory */
 		if (search_directory(".", fname))
-		{
-			private->inpath = pg_strdup(".");
-			return;
-		}
+			return pg_strdup(".");
 		/* XLOGDIR */
 		if (search_directory(XLOGDIR, fname))
-		{
-			private->inpath = pg_strdup(XLOGDIR);
-			return;
-		}
+			return pg_strdup(XLOGDIR);

 		datadir = getenv("PGDATA");
 		/* $PGDATA / XLOGDIR */
@ -282,10 +268,7 @@ identify_target_directory(XLogDumpPrivate *private, char *directory,
 		{
 			snprintf(fpath, MAXPGPATH, "%s/%s", datadir, XLOGDIR);
 			if (search_directory(fpath, fname))
-			{
-				private->inpath = pg_strdup(fpath);
-				return;
-			}
+				return pg_strdup(fpath);
 		}
 	}

@ -294,6 +277,8 @@ identify_target_directory(XLogDumpPrivate *private, char *directory,
 		fatal_error("could not locate WAL file \"%s\"", fname);
 	else
 		fatal_error("could not find any WAL file");
+
+	return NULL;				/* not reached */
 }

 /*
@ -423,7 +408,7 @@ XLogDumpXLogRead(const char *directory, TimeLineID timeline_id,
 */
 static int
 XLogDumpReadPage(XLogReaderState *state, XLogRecPtr targetPagePtr, int reqLen,
-				 XLogRecPtr targetPtr, char *readBuff, TimeLineID *curFileTLI)
+				 XLogRecPtr targetPtr, char *readBuff)
 {
 	XLogDumpPrivate *private = state->private_data;
 	int			count = XLOG_BLCKSZ;
@ -441,7 +426,7 @@ XLogDumpReadPage(XLogReaderState *state, XLogRecPtr targetPagePtr, int reqLen,
 		}
 	}

-	XLogDumpXLogRead(private->inpath, private->timeline, targetPagePtr,
+	XLogDumpXLogRead(state->segcxt.ws_dir, private->timeline, targetPagePtr,
 					 readBuff, count);

 	return count;
@ -820,6 +805,7 @@ main(int argc, char **argv)
 	XLogDumpStats stats;
 	XLogRecord *record;
 	XLogRecPtr	first_record;
+	char	   *waldir = NULL;
 	char	   *errormsg;

 	static struct option long_options[] = {
@ -912,7 +898,7 @@ main(int argc, char **argv)
 				}
 				break;
 			case 'p':
-				private.inpath = pg_strdup(optarg);
+				waldir = pg_strdup(optarg);
 				break;
 			case 'r':
 				{
@ -994,13 +980,13 @@ main(int argc, char **argv)
 		goto bad_argument;
 	}

-	if (private.inpath != NULL)
+	if (waldir != NULL)
 	{
 		/* validate path points to directory */
-		if (!verify_directory(private.inpath))
+		if (!verify_directory(waldir))
 		{
 			pg_log_error("path \"%s\" could not be opened: %s",
-						 private.inpath, strerror(errno));
+						 waldir, strerror(errno));
 			goto bad_argument;
 		}
 	}
@ -1015,17 +1001,17 @@ main(int argc, char **argv)

 		split_path(argv[optind], &directory, &fname);

-		if (private.inpath == NULL && directory != NULL)
+		if (waldir == NULL && directory != NULL)
 		{
-			private.inpath = directory;
+			waldir = directory;

-			if (!verify_directory(private.inpath))
+			if (!verify_directory(waldir))
 				fatal_error("could not open directory \"%s\": %s",
-							private.inpath, strerror(errno));
+							waldir, strerror(errno));
 		}

-		identify_target_directory(&private, private.inpath, fname);
-		fd = open_file_in_directory(private.inpath, fname);
+		waldir = identify_target_directory(waldir, fname);
+		fd = open_file_in_directory(waldir, fname);
 		if (fd < 0)
 			fatal_error("could not open file \"%s\"", fname);
 		close(fd);
@ -1056,7 +1042,7 @@ main(int argc, char **argv)
 			/* ignore directory, already have that */
 			split_path(argv[optind + 1], &directory, &fname);

-			fd = open_file_in_directory(private.inpath, fname);
+			fd = open_file_in_directory(waldir, fname);
 			if (fd < 0)
 				fatal_error("could not open file \"%s\"", fname);
 			close(fd);
@ -1088,7 +1074,7 @@ main(int argc, char **argv)
 		}
 	}
 	else
-		identify_target_directory(&private, private.inpath, NULL);
+		waldir = identify_target_directory(waldir, NULL);

 	/* we don't know what to print */
 	if (XLogRecPtrIsInvalid(private.startptr))
@ -1100,7 +1086,7 @@ main(int argc, char **argv)
 	/* done with argument parsing, do the actual work */

 	/* we have everything we need, start reading */
-	xlogreader_state = XLogReaderAllocate(WalSegSz, XLogDumpReadPage,
+	xlogreader_state = XLogReaderAllocate(WalSegSz, waldir, XLogDumpReadPage,
 										  &private);
 	if (!xlogreader_state)
 		fatal_error("out of memory");
--- a/src/include/access/xlogreader.h
+++ b/src/include/access/xlogreader.h
@ -31,6 +31,22 @@

 #include "access/xlogrecord.h"

+/* WALOpenSegment represents a WAL segment being read. */
+typedef struct WALOpenSegment
+{
+	int			ws_file;		/* segment file descriptor */
+	XLogSegNo	ws_segno;		/* segment number */
+	uint32		ws_off;			/* offset in the segment */
+	TimeLineID	ws_tli;			/* timeline ID of the currently open file */
+} WALOpenSegment;
+
+/* WALSegmentContext carries context information about WAL segments to read */
+typedef struct WALSegmentContext
+{
+	char		ws_dir[MAXPGPATH];
+	int			ws_segsize;
+} WALSegmentContext;
+
 typedef struct XLogReaderState XLogReaderState;

 /* Function type definition for the read_page callback */
@ -38,8 +54,7 @@ typedef int (*XLogPageReadCB) (XLogReaderState *xlogreader,
 							   XLogRecPtr targetPagePtr,
 							   int reqLen,
 							   XLogRecPtr targetRecPtr,
-							   char *readBuf,
-							   TimeLineID *pageTLI);
+							   char *readBuf);

 typedef struct
 {
@ -77,11 +92,6 @@ struct XLogReaderState
 	 * ----------------------------------------
 	 */

-	/*
-	 * Segment size of the to-be-parsed data (mandatory).
-	 */
-	int			wal_segment_size;
-
 	/*
 	 * Data input callback (mandatory).
 	 *
@ -99,9 +109,8 @@ struct XLogReaderState
 	 * actual WAL record it's interested in.  In that case, targetRecPtr can
 	 * be used to determine which timeline to read the page from.
 	 *
-	 * The callback shall set *pageTLI to the TLI of the file the page was
-	 * read from.  It is currently used only for error reporting purposes, to
-	 * reconstruct the name of the WAL file where an error occurred.
+	 * The callback shall set ->seg.ws_tli to the TLI of the file the page was
+	 * read from.
 	 */
 	XLogPageReadCB read_page;

@ -156,10 +165,9 @@ struct XLogReaderState
 	char	   *readBuf;
 	uint32		readLen;

-	/* last read segment, segment offset, TLI for data currently in readBuf */
-	XLogSegNo	readSegNo;
-	uint32		readOff;
-	TimeLineID	readPageTLI;
+	/* last read XLOG position for data currently in readBuf */
+	WALSegmentContext segcxt;
+	WALOpenSegment seg;

 	/*
 	 * beginning of prior page read, and its TLI.  Doesn't necessarily
@ -202,12 +210,17 @@ struct XLogReaderState

 /* Get a new XLogReader */
 extern XLogReaderState *XLogReaderAllocate(int wal_segment_size,
+										   const char *waldir,
 										   XLogPageReadCB pagereadfunc,
 										   void *private_data);

 /* Free an XLogReader */
 extern void XLogReaderFree(XLogReaderState *state);

+/* Initialize supporting structures */
+extern void WALOpenSegmentInit(WALOpenSegment *seg, WALSegmentContext *segcxt,
+							   int segsize, const char *waldir);
+
 /* Read the next XLog record. Returns NULL on end-of-WAL or failure */
 extern struct XLogRecord *XLogReadRecord(XLogReaderState *state,
 										 XLogRecPtr recptr, char **errormsg);
--- a/src/include/access/xlogutils.h
+++ b/src/include/access/xlogutils.h
@ -49,8 +49,7 @@ extern void FreeFakeRelcacheEntry(Relation fakerel);

 extern int	read_local_xlog_page(XLogReaderState *state,
 								 XLogRecPtr targetPagePtr, int reqLen,
-								 XLogRecPtr targetRecPtr, char *cur_page,
-								 TimeLineID *pageTLI);
+								 XLogRecPtr targetRecPtr, char *cur_page);

 extern void XLogReadDetermineTimeline(XLogReaderState *state,
 									  XLogRecPtr wantPage, uint32 wantLength);
--- a/src/include/replication/logicalfuncs.h
+++ b/src/include/replication/logicalfuncs.h
@ -14,6 +14,6 @@
 extern int	logical_read_local_xlog_page(XLogReaderState *state,
 										 XLogRecPtr targetPagePtr,
 										 int reqLen, XLogRecPtr targetRecPtr,
-										 char *cur_page, TimeLineID *pageTLI);
+										 char *cur_page);

 #endif