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

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),

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

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.
  * ----------------------------------------

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 */