v1-xlogreader-reuse-readrecordbuf.patch

diff --git a/src/backend/access/transam/xlogreader.c b/src/backend/access/transam/xlogreader.c
index 5e5001b..fe337e2 100644
--- a/src/backend/access/transam/xlogreader.c
+++ b/src/backend/access/transam/xlogreader.c
@@ -586,53 +586,72 @@ restart:
 	state->nonblocking = nonblocking;
 	state->currRecPtr = RecPtr;
 	assembled = false;
+	gotheader = false;
 
 	targetPagePtr = RecPtr - (RecPtr % XLOG_BLCKSZ);
 	targetRecOff = RecPtr % XLOG_BLCKSZ;
 
 	/*
-	 * Read the page containing the record into state->readBuf. Request enough
-	 * byte to cover the whole record header, or at least the part of it that
-	 * fits on the same page.
+	 * If this record spans multiple pages, it's possible that the first
+	 * ReadPageInternal call succeeded while one of the following returned
+	 * XLREAD_WOUDBLOCK. In that case, we already have the data from some
+	 * of the calls in state->readRecordBuf, so skip re-reading it.
+	 * This is especially important if the pages are in different WALs,
+	 * in which case going back to a previous WAL might require re-fetching
+	 * it from an archive - a potentially slow operation.
 	 */
-	readOff = ReadPageInternal(state, targetPagePtr,
-							   Min(targetRecOff + SizeOfXLogRecord, XLOG_BLCKSZ));
-	if (readOff == XLREAD_WOULDBLOCK)
-		return XLREAD_WOULDBLOCK;
-	else if (readOff < 0)
-		goto err;
-
-	/*
-	 * ReadPageInternal always returns at least the page header, so we can
-	 * examine it now.
-	 */
-	pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) state->readBuf);
-	if (targetRecOff == 0)
-	{
+	if (state->readRecordBufUsed == 0) {
 		/*
-		 * At page start, so skip over page header.
+		 * Read the page containing the record into state->readBuf. Request
+		 * enough bytes to cover the whole record header, or at least the part
+		 * of it that fits on the same page.
 		 */
-		RecPtr += pageHeaderSize;
-		targetRecOff = pageHeaderSize;
-	}
-	else if (targetRecOff < pageHeaderSize)
-	{
-		report_invalid_record(state, "invalid record offset at %X/%08X: expected at least %u, got %u",
-							  LSN_FORMAT_ARGS(RecPtr),
-							  pageHeaderSize, targetRecOff);
-		goto err;
-	}
+		readOff = ReadPageInternal(state, targetPagePtr,
+								   Min(targetRecOff + SizeOfXLogRecord, XLOG_BLCKSZ));
+		if (readOff == XLREAD_WOULDBLOCK)
+			return XLREAD_WOULDBLOCK;
+		else if (readOff < 0)
+			goto err;
 
-	if ((((XLogPageHeader) state->readBuf)->xlp_info & XLP_FIRST_IS_CONTRECORD) &&
-		targetRecOff == pageHeaderSize)
-	{
-		report_invalid_record(state, "contrecord is requested by %X/%08X",
-							  LSN_FORMAT_ARGS(RecPtr));
-		goto err;
-	}
+		/*
+		 * ReadPageInternal always returns at least the page header, so we can
+		 * examine it now.
+		 */
+		pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) state->readBuf);
+		if (targetRecOff == 0)
+		{
+			/*
+			 * At page start, so skip over page header.
+			 */
+			RecPtr += pageHeaderSize;
+			targetRecOff = pageHeaderSize;
+		}
+		else if (targetRecOff < pageHeaderSize)
+		{
+			report_invalid_record(state,
+								  "invalid record offset at %X/%08X: expected at least %u, got %u",
+								  LSN_FORMAT_ARGS(RecPtr),
+								  pageHeaderSize, targetRecOff);
+			goto err;
+		}
 
-	/* ReadPageInternal has verified the page header */
-	Assert(pageHeaderSize <= readOff);
+		if ((((XLogPageHeader) state->readBuf)->xlp_info & XLP_FIRST_IS_CONTRECORD) &&
+			targetRecOff == pageHeaderSize)
+		{
+			report_invalid_record(state, "contrecord is requested by %X/%08X",
+								  LSN_FORMAT_ARGS(RecPtr));
+			goto err;
+		}
+
+		/* ReadPageInternal has verified the page header */
+		Assert(pageHeaderSize <= readOff);
+
+		record = (XLogRecord *) (state->readBuf + RecPtr % XLOG_BLCKSZ);
+	} else {
+		record = (XLogRecord *) state->readRecordBuf;
+		/* If we have the header, then it has already been validated. */
+		gotheader = state->readRecordBufUsed >= SizeOfXLogRecord;
+	}
 
 	/*
 	 * Read the record length.
@@ -643,28 +662,25 @@ restart:
 	 * cannot access any other fields until we've verified that we got the
 	 * whole header.
 	 */
-	record = (XLogRecord *) (state->readBuf + RecPtr % XLOG_BLCKSZ);
 	total_len = record->xl_tot_len;
 
-	/*
-	 * If the whole record header is on this page, validate it immediately.
-	 * Otherwise do just a basic sanity check on xl_tot_len, and validate the
-	 * rest of the header after reading it from the next page.  The xl_tot_len
-	 * check is necessary here to ensure that we enter the "Need to reassemble
-	 * record" code path below; otherwise we might fail to apply
-	 * ValidXLogRecordHeader at all.
-	 */
-	if (targetRecOff <= XLOG_BLCKSZ - SizeOfXLogRecord)
-	{
-		if (!ValidXLogRecordHeader(state, RecPtr, state->DecodeRecPtr, record,
-								   randAccess))
-			goto err;
-		gotheader = true;
-	}
-	else
-	{
-		/* There may be no next page if it's too small. */
-		if (total_len < SizeOfXLogRecord)
+	if (!gotheader) {
+		/*
+		* If the whole record header is on this page, validate it immediately.
+		* Otherwise do just a basic sanity check on xl_tot_len, and validate
+		* the rest of the header after reading it from the next page.  The
+		* xl_tot_len check is necessary here to ensure that we enter the "Need
+		* to reassemble record" code path below; otherwise we might fail to
+		* apply ValidXLogRecordHeader at all.
+		*/
+		if (targetRecOff <= XLOG_BLCKSZ - SizeOfXLogRecord)
+		{
+			if (!ValidXLogRecordHeader(state, RecPtr, state->DecodeRecPtr,
+									   record, randAccess))
+				goto err;
+			gotheader = true;
+		}
+		else if (total_len < SizeOfXLogRecord)
 		{
 			report_invalid_record(state,
 								  "invalid record length at %X/%08X: expected at least %u, got %u",
@@ -672,8 +688,6 @@ restart:
 								  (uint32) SizeOfXLogRecord, total_len);
 			goto err;
 		}
-		/* We'll validate the header once we have the next page. */
-		gotheader = false;
 	}
 
 	/*
@@ -712,11 +726,20 @@ restart:
 		Assert(state->readRecordBufSize >= XLOG_BLCKSZ * 2);
 		Assert(state->readRecordBufSize >= len);
 
-		/* Copy the first fragment of the record from the first page. */
-		memcpy(state->readRecordBuf,
-			   state->readBuf + RecPtr % XLOG_BLCKSZ, len);
-		buffer = state->readRecordBuf + len;
-		gotlen = len;
+		if (state->readRecordBufUsed == 0) {
+			/* Copy the first fragment of the record from the first page. */
+			memcpy(state->readRecordBuf,
+				   state->readBuf + RecPtr % XLOG_BLCKSZ, len);
+			state->readRecordBufUsed = len;
+			state->readRecordBufPage = targetPagePtr;
+			buffer = state->readRecordBuf + len;
+			gotlen = len;
+			record = (XLogRecord *) state->readRecordBuf;
+		} else {
+			gotlen = state->readRecordBufUsed;
+			buffer = state->readRecordBuf + gotlen;
+			targetPagePtr = state->readRecordBufPage;
+		}
 
 		do
 		{
@@ -749,6 +772,7 @@ restart:
 			if (pageHeader->xlp_info & XLP_FIRST_IS_OVERWRITE_CONTRECORD)
 			{
 				state->overwrittenRecPtr = RecPtr;
+				state->readRecordBufUsed = 0;
 				RecPtr = targetPagePtr;
 				goto restart;
 			}
@@ -839,10 +863,12 @@ restart:
 				memcpy(state->readRecordBuf, save_copy, gotlen);
 				buffer = state->readRecordBuf + gotlen;
 			}
+
+			state->readRecordBufUsed = gotlen;
+			state->readRecordBufPage = targetPagePtr;
 		} while (gotlen < total_len);
 		Assert(gotheader);
 
-		record = (XLogRecord *) state->readRecordBuf;
 		if (!ValidXLogRecord(state, record, RecPtr))
 			goto err;
 
@@ -870,6 +896,8 @@ restart:
 		state->DecodeRecPtr = RecPtr;
 	}
 
+	state->readRecordBufUsed = 0;
+
 	/*
 	 * Special processing if it's an XLOG SWITCH record
 	 */
@@ -1126,6 +1154,7 @@ XLogReaderInvalReadState(XLogReaderState *state)
 	state->seg.ws_segno = 0;
 	state->segoff = 0;
 	state->readLen = 0;
+	state->readRecordBufUsed = 0;
 }
 
 /*
@@ -1626,6 +1655,7 @@ ResetDecoder(XLogReaderState *state)
 	state->decode_queue_tail = NULL;
 	state->decode_queue_head = NULL;
 	state->record = NULL;
+	state->readRecordBufUsed = 0;
 
 	/* Reset the decode buffer to empty. */
 	state->decode_buffer_tail = state->decode_buffer;
diff --git a/src/backend/access/transam/xlogrecovery.c b/src/backend/access/transam/xlogrecovery.c
index ae2398d..d4e6820 100644
--- a/src/backend/access/transam/xlogrecovery.c
+++ b/src/backend/access/transam/xlogrecovery.c
@@ -3167,9 +3167,6 @@ ReadRecord(XLogPrefetcher *xlogprefetcher, int emode,
 	private->randAccess = !XLogRecPtrIsValid(xlogreader->ReadRecPtr);
 	private->replayTLI = replayTLI;
 
-	/* This is the first attempt to read this page. */
-	lastSourceFailed = false;
-
 	for (;;)
 	{
 		char	   *errormsg;
diff --git a/src/include/access/xlogreader.h b/src/include/access/xlogreader.h
index dfabbbd..1b30be4 100644
--- a/src/include/access/xlogreader.h
+++ b/src/include/access/xlogreader.h
@@ -304,7 +304,9 @@ struct XLogReaderState
 	 * crosses a page boundary.
 	 */
 	char	   *readRecordBuf;
-	uint32		readRecordBufSize;
+	uint32	    readRecordBufSize;
+	uint32	    readRecordBufUsed;
+	XLogRecPtr  readRecordBufPage;
 
 	/* Buffer to hold error message */
 	char	   *errormsg_buf;