Adding upstream version 13.4.upstream/13.4upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1624 insertions, 0 deletions

diff --git a/src/backend/access/transam/xlogreader.c b/src/backend/access/transam/xlogreader.c
new file mode 100644
index 0000000..aafd8e1
--- /dev/null
+++ b/src/backend/access/transam/xlogreader.c
@@ -0,0 +1,1624 @@
+/*-------------------------------------------------------------------------
+ *
+ * xlogreader.c
+ *		Generic XLog reading facility
+ *
+ * Portions Copyright (c) 2013-2020, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *		src/backend/access/transam/xlogreader.c
+ *
+ * NOTES
+ *		See xlogreader.h for more notes on this facility.
+ *
+ *		This file is compiled as both front-end and backend code, so it
+ *		may not use ereport, server-defined static variables, etc.
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include <unistd.h>
+
+#include "access/transam.h"
+#include "access/xlog_internal.h"
+#include "access/xlogreader.h"
+#include "access/xlogrecord.h"
+#include "catalog/pg_control.h"
+#include "common/pg_lzcompress.h"
+#include "replication/origin.h"
+
+#ifndef FRONTEND
+#include "miscadmin.h"
+#include "pgstat.h"
+#include "utils/memutils.h"
+#endif
+
+static void report_invalid_record(XLogReaderState *state, const char *fmt,...)
+			pg_attribute_printf(2, 3);
+static bool allocate_recordbuf(XLogReaderState *state, uint32 reclength);
+static int	ReadPageInternal(XLogReaderState *state, XLogRecPtr pageptr,
+							 int reqLen);
+static void XLogReaderInvalReadState(XLogReaderState *state);
+static bool ValidXLogRecordHeader(XLogReaderState *state, XLogRecPtr RecPtr,
+								  XLogRecPtr PrevRecPtr, XLogRecord *record, bool randAccess);
+static bool ValidXLogRecord(XLogReaderState *state, XLogRecord *record,
+							XLogRecPtr recptr);
+static void ResetDecoder(XLogReaderState *state);
+static void WALOpenSegmentInit(WALOpenSegment *seg, WALSegmentContext *segcxt,
+							   int segsize, const char *waldir);
+
+/* size of the buffer allocated for error message. */
+#define MAX_ERRORMSG_LEN 1000
+
+/*
+ * Construct a string in state->errormsg_buf explaining what's wrong with
+ * the current record being read.
+ */
+static void
+report_invalid_record(XLogReaderState *state, const char *fmt,...)
+{
+	va_list		args;
+
+	fmt = _(fmt);
+
+	va_start(args, fmt);
+	vsnprintf(state->errormsg_buf, MAX_ERRORMSG_LEN, fmt, args);
+	va_end(args);
+}
+
+/*
+ * Allocate and initialize a new XLogReader.
+ *
+ * Returns NULL if the xlogreader couldn't be allocated.
+ */
+XLogReaderState *
+XLogReaderAllocate(int wal_segment_size, const char *waldir,
+				   XLogReaderRoutine *routine, void *private_data)
+{
+	XLogReaderState *state;
+
+	state = (XLogReaderState *)
+		palloc_extended(sizeof(XLogReaderState),
+						MCXT_ALLOC_NO_OOM | MCXT_ALLOC_ZERO);
+	if (!state)
+		return NULL;
+
+	/* initialize caller-provided support functions */
+	state->routine = *routine;
+
+	state->max_block_id = -1;
+
+	/*
+	 * Permanently allocate readBuf.  We do it this way, rather than just
+	 * making a static array, for two reasons: (1) no need to waste the
+	 * storage in most instantiations of the backend; (2) a static char array
+	 * isn't guaranteed to have any particular alignment, whereas
+	 * palloc_extended() will provide MAXALIGN'd storage.
+	 */
+	state->readBuf = (char *) palloc_extended(XLOG_BLCKSZ,
+											  MCXT_ALLOC_NO_OOM);
+	if (!state->readBuf)
+	{
+		pfree(state);
+		return NULL;
+	}
+
+	/* Initialize segment info. */
+	WALOpenSegmentInit(&state->seg, &state->segcxt, wal_segment_size,
+					   waldir);
+
+	/* system_identifier initialized to zeroes above */
+	state->private_data = private_data;
+	/* ReadRecPtr, EndRecPtr and readLen initialized to zeroes above */
+	state->errormsg_buf = palloc_extended(MAX_ERRORMSG_LEN + 1,
+										  MCXT_ALLOC_NO_OOM);
+	if (!state->errormsg_buf)
+	{
+		pfree(state->readBuf);
+		pfree(state);
+		return NULL;
+	}
+	state->errormsg_buf[0] = '\0';
+
+	/*
+	 * Allocate an initial readRecordBuf of minimal size, which can later be
+	 * enlarged if necessary.
+	 */
+	if (!allocate_recordbuf(state, 0))
+	{
+		pfree(state->errormsg_buf);
+		pfree(state->readBuf);
+		pfree(state);
+		return NULL;
+	}
+
+	return state;
+}
+
+void
+XLogReaderFree(XLogReaderState *state)
+{
+	int			block_id;
+
+	if (state->seg.ws_file != -1)
+		state->routine.segment_close(state);
+
+	for (block_id = 0; block_id <= XLR_MAX_BLOCK_ID; block_id++)
+	{
+		if (state->blocks[block_id].data)
+			pfree(state->blocks[block_id].data);
+	}
+	if (state->main_data)
+		pfree(state->main_data);
+
+	pfree(state->errormsg_buf);
+	if (state->readRecordBuf)
+		pfree(state->readRecordBuf);
+	pfree(state->readBuf);
+	pfree(state);
+}
+
+/*
+ * Allocate readRecordBuf to fit a record of at least the given length.
+ * Returns true if successful, false if out of memory.
+ *
+ * readRecordBufSize is set to the new buffer size.
+ *
+ * To avoid useless small increases, round its size to a multiple of
+ * XLOG_BLCKSZ, and make sure it's at least 5*Max(BLCKSZ, XLOG_BLCKSZ) to start
+ * with.  (That is enough for all "normal" records, but very large commit or
+ * abort records might need more space.)
+ */
+static bool
+allocate_recordbuf(XLogReaderState *state, uint32 reclength)
+{
+	uint32		newSize = reclength;
+
+	newSize += XLOG_BLCKSZ - (newSize % XLOG_BLCKSZ);
+	newSize = Max(newSize, 5 * Max(BLCKSZ, XLOG_BLCKSZ));
+
+#ifndef FRONTEND
+
+	/*
+	 * Note that in much unlucky circumstances, the random data read from a
+	 * recycled segment can cause this routine to be called with a size
+	 * causing a hard failure at allocation.  For a standby, this would cause
+	 * the instance to stop suddenly with a hard failure, preventing it to
+	 * retry fetching WAL from one of its sources which could allow it to move
+	 * on with replay without a manual restart. If the data comes from a past
+	 * recycled segment and is still valid, then the allocation may succeed
+	 * but record checks are going to fail so this would be short-lived.  If
+	 * the allocation fails because of a memory shortage, then this is not a
+	 * hard failure either per the guarantee given by MCXT_ALLOC_NO_OOM.
+	 */
+	if (!AllocSizeIsValid(newSize))
+		return false;
+
+#endif
+
+	if (state->readRecordBuf)
+		pfree(state->readRecordBuf);
+	state->readRecordBuf =
+		(char *) palloc_extended(newSize, MCXT_ALLOC_NO_OOM);
+	if (state->readRecordBuf == NULL)
+	{
+		state->readRecordBufSize = 0;
+		return false;
+	}
+	state->readRecordBufSize = newSize;
+	return true;
+}
+
+/*
+ * Initialize the passed segment structs.
+ */
+static void
+WALOpenSegmentInit(WALOpenSegment *seg, WALSegmentContext *segcxt,
+				   int segsize, const char *waldir)
+{
+	seg->ws_file = -1;
+	seg->ws_segno = 0;
+	seg->ws_tli = 0;
+
+	segcxt->ws_segsize = segsize;
+	if (waldir)
+		snprintf(segcxt->ws_dir, MAXPGPATH, "%s", waldir);
+}
+
+/*
+ * Begin reading WAL at 'RecPtr'.
+ *
+ * 'RecPtr' should point to the beginnning of a valid WAL record.  Pointing at
+ * the beginning of a page is also OK, if there is a new record right after
+ * the page header, i.e. not a continuation.
+ *
+ * This does not make any attempt to read the WAL yet, and hence cannot fail.
+ * If the starting address is not correct, the first call to XLogReadRecord()
+ * will error out.
+ */
+void
+XLogBeginRead(XLogReaderState *state, XLogRecPtr RecPtr)
+{
+	Assert(!XLogRecPtrIsInvalid(RecPtr));
+
+	ResetDecoder(state);
+
+	/* Begin at the passed-in record pointer. */
+	state->EndRecPtr = RecPtr;
+	state->ReadRecPtr = InvalidXLogRecPtr;
+}
+
+/*
+ * Attempt to read an XLOG record.
+ *
+ * XLogBeginRead() or XLogFindNextRecord() must be called before the first call
+ * to XLogReadRecord().
+ *
+ * If the page_read callback fails to read the requested data, NULL is
+ * returned.  The callback is expected to have reported the error; errormsg
+ * is set to NULL.
+ *
+ * If the reading fails for some other reason, NULL is also returned, and
+ * *errormsg is set to a string with details of the failure.
+ *
+ * The returned pointer (or *errormsg) points to an internal buffer that's
+ * valid until the next call to XLogReadRecord.
+ */
+XLogRecord *
+XLogReadRecord(XLogReaderState *state, char **errormsg)
+{
+	XLogRecPtr	RecPtr;
+	XLogRecord *record;
+	XLogRecPtr	targetPagePtr;
+	bool		randAccess;
+	uint32		len,
+				total_len;
+	uint32		targetRecOff;
+	uint32		pageHeaderSize;
+	bool		gotheader;
+	int			readOff;
+
+	/*
+	 * randAccess indicates whether to verify the previous-record pointer of
+	 * the record we're reading.  We only do this if we're reading
+	 * sequentially, which is what we initially assume.
+	 */
+	randAccess = false;
+
+	/* reset error state */
+	*errormsg = NULL;
+	state->errormsg_buf[0] = '\0';
+
+	ResetDecoder(state);
+
+	RecPtr = state->EndRecPtr;
+
+	if (state->ReadRecPtr != InvalidXLogRecPtr)
+	{
+		/* read the record after the one we just read */
+
+		/*
+		 * EndRecPtr is pointing to end+1 of the previous WAL record.  If
+		 * we're at a page boundary, no more records can fit on the current
+		 * page. We must skip over the page header, but we can't do that until
+		 * we've read in the page, since the header size is variable.
+		 */
+	}
+	else
+	{
+		/*
+		 * Caller supplied a position to start at.
+		 *
+		 * In this case, EndRecPtr should already be pointing to a valid
+		 * record starting position.
+		 */
+		Assert(XRecOffIsValid(RecPtr));
+		randAccess = true;
+	}
+
+	state->currRecPtr = RecPtr;
+
+	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.
+	 */
+	readOff = ReadPageInternal(state, targetPagePtr,
+							   Min(targetRecOff + SizeOfXLogRecord, XLOG_BLCKSZ));
+	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)
+	{
+		/*
+		 * 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/%X",
+							  (uint32) (RecPtr >> 32), (uint32) RecPtr);
+		goto err;
+	}
+
+	if ((((XLogPageHeader) state->readBuf)->xlp_info & XLP_FIRST_IS_CONTRECORD) &&
+		targetRecOff == pageHeaderSize)
+	{
+		report_invalid_record(state, "contrecord is requested by %X/%X",
+							  (uint32) (RecPtr >> 32), (uint32) RecPtr);
+		goto err;
+	}
+
+	/* ReadPageInternal has verified the page header */
+	Assert(pageHeaderSize <= readOff);
+
+	/*
+	 * Read the record length.
+	 *
+	 * NB: Even though we use an XLogRecord pointer here, the whole record
+	 * header might not fit on this page. xl_tot_len is the first field of the
+	 * struct, so it must be on this page (the records are MAXALIGNed), but we
+	 * 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->ReadRecPtr, record,
+								   randAccess))
+			goto err;
+		gotheader = true;
+	}
+	else
+	{
+		/* XXX: more validation should be done here */
+		if (total_len < SizeOfXLogRecord)
+		{
+			report_invalid_record(state,
+								  "invalid record length at %X/%X: wanted %u, got %u",
+								  (uint32) (RecPtr >> 32), (uint32) RecPtr,
+								  (uint32) SizeOfXLogRecord, total_len);
+			goto err;
+		}
+		gotheader = false;
+	}
+
+	len = XLOG_BLCKSZ - RecPtr % XLOG_BLCKSZ;
+	if (total_len > len)
+	{
+		/* Need to reassemble record */
+		char	   *contdata;
+		XLogPageHeader pageHeader;
+		char	   *buffer;
+		uint32		gotlen;
+
+		/*
+		 * Enlarge readRecordBuf as needed.
+		 */
+		if (total_len > state->readRecordBufSize &&
+			!allocate_recordbuf(state, total_len))
+		{
+			/* We treat this as a "bogus data" condition */
+			report_invalid_record(state, "record length %u at %X/%X too long",
+								  total_len,
+								  (uint32) (RecPtr >> 32), (uint32) RecPtr);
+			goto err;
+		}
+
+		/* 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;
+
+		do
+		{
+			/* Calculate pointer to beginning of next page */
+			targetPagePtr += XLOG_BLCKSZ;
+
+			/* Wait for the next page to become available */
+			readOff = ReadPageInternal(state, targetPagePtr,
+									   Min(total_len - gotlen + SizeOfXLogShortPHD,
+										   XLOG_BLCKSZ));
+
+			if (readOff < 0)
+				goto err;
+
+			Assert(SizeOfXLogShortPHD <= readOff);
+
+			/* Check that the continuation on next page looks valid */
+			pageHeader = (XLogPageHeader) state->readBuf;
+			if (!(pageHeader->xlp_info & XLP_FIRST_IS_CONTRECORD))
+			{
+				report_invalid_record(state,
+									  "there is no contrecord flag at %X/%X",
+									  (uint32) (RecPtr >> 32), (uint32) RecPtr);
+				goto err;
+			}
+
+			/*
+			 * Cross-check that xlp_rem_len agrees with how much of the record
+			 * we expect there to be left.
+			 */
+			if (pageHeader->xlp_rem_len == 0 ||
+				total_len != (pageHeader->xlp_rem_len + gotlen))
+			{
+				report_invalid_record(state,
+									  "invalid contrecord length %u at %X/%X",
+									  pageHeader->xlp_rem_len,
+									  (uint32) (RecPtr >> 32), (uint32) RecPtr);
+				goto err;
+			}
+
+			/* Append the continuation from this page to the buffer */
+			pageHeaderSize = XLogPageHeaderSize(pageHeader);
+
+			if (readOff < pageHeaderSize)
+				readOff = ReadPageInternal(state, targetPagePtr,
+										   pageHeaderSize);
+
+			Assert(pageHeaderSize <= readOff);
+
+			contdata = (char *) state->readBuf + pageHeaderSize;
+			len = XLOG_BLCKSZ - pageHeaderSize;
+			if (pageHeader->xlp_rem_len < len)
+				len = pageHeader->xlp_rem_len;
+
+			if (readOff < pageHeaderSize + len)
+				readOff = ReadPageInternal(state, targetPagePtr,
+										   pageHeaderSize + len);
+
+			memcpy(buffer, (char *) contdata, len);
+			buffer += len;
+			gotlen += len;
+
+			/* If we just reassembled the record header, validate it. */
+			if (!gotheader)
+			{
+				record = (XLogRecord *) state->readRecordBuf;
+				if (!ValidXLogRecordHeader(state, RecPtr, state->ReadRecPtr,
+										   record, randAccess))
+					goto err;
+				gotheader = true;
+			}
+		} while (gotlen < total_len);
+
+		Assert(gotheader);
+
+		record = (XLogRecord *) state->readRecordBuf;
+		if (!ValidXLogRecord(state, record, RecPtr))
+			goto err;
+
+		pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) state->readBuf);
+		state->ReadRecPtr = RecPtr;
+		state->EndRecPtr = targetPagePtr + pageHeaderSize
+			+ MAXALIGN(pageHeader->xlp_rem_len);
+	}
+	else
+	{
+		/* Wait for the record data to become available */
+		readOff = ReadPageInternal(state, targetPagePtr,
+								   Min(targetRecOff + total_len, XLOG_BLCKSZ));
+		if (readOff < 0)
+			goto err;
+
+		/* Record does not cross a page boundary */
+		if (!ValidXLogRecord(state, record, RecPtr))
+			goto err;
+
+		state->EndRecPtr = RecPtr + MAXALIGN(total_len);
+
+		state->ReadRecPtr = RecPtr;
+	}
+
+	/*
+	 * Special processing if it's an XLOG SWITCH record
+	 */
+	if (record->xl_rmid == RM_XLOG_ID &&
+		(record->xl_info & ~XLR_INFO_MASK) == XLOG_SWITCH)
+	{
+		/* Pretend it extends to end of segment */
+		state->EndRecPtr += state->segcxt.ws_segsize - 1;
+		state->EndRecPtr -= XLogSegmentOffset(state->EndRecPtr, state->segcxt.ws_segsize);
+	}
+
+	if (DecodeXLogRecord(state, record, errormsg))
+		return record;
+	else
+		return NULL;
+
+err:
+
+	/*
+	 * Invalidate the read state. We might read from a different source after
+	 * failure.
+	 */
+	XLogReaderInvalReadState(state);
+
+	if (state->errormsg_buf[0] != '\0')
+		*errormsg = state->errormsg_buf;
+
+	return NULL;
+}
+
+/*
+ * Read a single xlog page including at least [pageptr, reqLen] of valid data
+ * via the page_read() callback.
+ *
+ * Returns -1 if the required page cannot be read for some reason; errormsg_buf
+ * is set in that case (unless the error occurs in the page_read callback).
+ *
+ * We fetch the page from a reader-local cache if we know we have the required
+ * data and if there hasn't been any error since caching the data.
+ */
+static int
+ReadPageInternal(XLogReaderState *state, XLogRecPtr pageptr, int reqLen)
+{
+	int			readLen;
+	uint32		targetPageOff;
+	XLogSegNo	targetSegNo;
+	XLogPageHeader hdr;
+
+	Assert((pageptr % XLOG_BLCKSZ) == 0);
+
+	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->seg.ws_segno &&
+		targetPageOff == state->segoff && reqLen <= state->readLen)
+		return state->readLen;
+
+	/*
+	 * Data is not in our buffer.
+	 *
+	 * Every time we actually read the segment, even if we looked at parts of
+	 * it before, we need to do verification as the page_read callback might
+	 * now be rereading data from a different source.
+	 *
+	 * Whenever switching to a new WAL segment, we read the first page of the
+	 * file and validate its header, even if that's not where the target
+	 * 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->seg.ws_segno && targetPageOff != 0)
+	{
+		XLogRecPtr	targetSegmentPtr = pageptr - targetPageOff;
+
+		readLen = state->routine.page_read(state, targetSegmentPtr, XLOG_BLCKSZ,
+										   state->currRecPtr,
+										   state->readBuf);
+		if (readLen < 0)
+			goto err;
+
+		/* we can be sure to have enough WAL available, we scrolled back */
+		Assert(readLen == XLOG_BLCKSZ);
+
+		if (!XLogReaderValidatePageHeader(state, targetSegmentPtr,
+										  state->readBuf))
+			goto err;
+	}
+
+	/*
+	 * First, read the requested data length, but at least a short page header
+	 * so that we can validate it.
+	 */
+	readLen = state->routine.page_read(state, pageptr, Max(reqLen, SizeOfXLogShortPHD),
+									   state->currRecPtr,
+									   state->readBuf);
+	if (readLen < 0)
+		goto err;
+
+	Assert(readLen <= XLOG_BLCKSZ);
+
+	/* Do we have enough data to check the header length? */
+	if (readLen <= SizeOfXLogShortPHD)
+		goto err;
+
+	Assert(readLen >= reqLen);
+
+	hdr = (XLogPageHeader) state->readBuf;
+
+	/* still not enough */
+	if (readLen < XLogPageHeaderSize(hdr))
+	{
+		readLen = state->routine.page_read(state, pageptr, XLogPageHeaderSize(hdr),
+										   state->currRecPtr,
+										   state->readBuf);
+		if (readLen < 0)
+			goto err;
+	}
+
+	/*
+	 * Now that we know we have the full header, validate it.
+	 */
+	if (!XLogReaderValidatePageHeader(state, pageptr, (char *) hdr))
+		goto err;
+
+	/* update read state information */
+	state->seg.ws_segno = targetSegNo;
+	state->segoff = targetPageOff;
+	state->readLen = readLen;
+
+	return readLen;
+
+err:
+	XLogReaderInvalReadState(state);
+	return -1;
+}
+
+/*
+ * Invalidate the xlogreader's read state to force a re-read.
+ */
+static void
+XLogReaderInvalReadState(XLogReaderState *state)
+{
+	state->seg.ws_segno = 0;
+	state->segoff = 0;
+	state->readLen = 0;
+}
+
+/*
+ * Validate an XLOG record header.
+ *
+ * This is just a convenience subroutine to avoid duplicated code in
+ * XLogReadRecord.  It's not intended for use from anywhere else.
+ */
+static bool
+ValidXLogRecordHeader(XLogReaderState *state, XLogRecPtr RecPtr,
+					  XLogRecPtr PrevRecPtr, XLogRecord *record,
+					  bool randAccess)
+{
+	if (record->xl_tot_len < SizeOfXLogRecord)
+	{
+		report_invalid_record(state,
+							  "invalid record length at %X/%X: wanted %u, got %u",
+							  (uint32) (RecPtr >> 32), (uint32) RecPtr,
+							  (uint32) SizeOfXLogRecord, record->xl_tot_len);
+		return false;
+	}
+	if (record->xl_rmid > RM_MAX_ID)
+	{
+		report_invalid_record(state,
+							  "invalid resource manager ID %u at %X/%X",
+							  record->xl_rmid, (uint32) (RecPtr >> 32),
+							  (uint32) RecPtr);
+		return false;
+	}
+	if (randAccess)
+	{
+		/*
+		 * We can't exactly verify the prev-link, but surely it should be less
+		 * than the record's own address.
+		 */
+		if (!(record->xl_prev < RecPtr))
+		{
+			report_invalid_record(state,
+								  "record with incorrect prev-link %X/%X at %X/%X",
+								  (uint32) (record->xl_prev >> 32),
+								  (uint32) record->xl_prev,
+								  (uint32) (RecPtr >> 32), (uint32) RecPtr);
+			return false;
+		}
+	}
+	else
+	{
+		/*
+		 * Record's prev-link should exactly match our previous location. This
+		 * check guards against torn WAL pages where a stale but valid-looking
+		 * WAL record starts on a sector boundary.
+		 */
+		if (record->xl_prev != PrevRecPtr)
+		{
+			report_invalid_record(state,
+								  "record with incorrect prev-link %X/%X at %X/%X",
+								  (uint32) (record->xl_prev >> 32),
+								  (uint32) record->xl_prev,
+								  (uint32) (RecPtr >> 32), (uint32) RecPtr);
+			return false;
+		}
+	}
+
+	return true;
+}
+
+
+/*
+ * CRC-check an XLOG record.  We do not believe the contents of an XLOG
+ * record (other than to the minimal extent of computing the amount of
+ * data to read in) until we've checked the CRCs.
+ *
+ * We assume all of the record (that is, xl_tot_len bytes) has been read
+ * into memory at *record.  Also, ValidXLogRecordHeader() has accepted the
+ * record's header, which means in particular that xl_tot_len is at least
+ * SizeOfXLogRecord.
+ */
+static bool
+ValidXLogRecord(XLogReaderState *state, XLogRecord *record, XLogRecPtr recptr)
+{
+	pg_crc32c	crc;
+
+	/* Calculate the CRC */
+	INIT_CRC32C(crc);
+	COMP_CRC32C(crc, ((char *) record) + SizeOfXLogRecord, record->xl_tot_len - SizeOfXLogRecord);
+	/* include the record header last */
+	COMP_CRC32C(crc, (char *) record, offsetof(XLogRecord, xl_crc));
+	FIN_CRC32C(crc);
+
+	if (!EQ_CRC32C(record->xl_crc, crc))
+	{
+		report_invalid_record(state,
+							  "incorrect resource manager data checksum in record at %X/%X",
+							  (uint32) (recptr >> 32), (uint32) recptr);
+		return false;
+	}
+
+	return true;
+}
+
+/*
+ * Validate a page header.
+ *
+ * Check if 'phdr' is valid as the header of the XLog page at position
+ * 'recptr'.
+ */
+bool
+XLogReaderValidatePageHeader(XLogReaderState *state, XLogRecPtr recptr,
+							 char *phdr)
+{
+	XLogRecPtr	recaddr;
+	XLogSegNo	segno;
+	int32		offset;
+	XLogPageHeader hdr = (XLogPageHeader) phdr;
+
+	Assert((recptr % XLOG_BLCKSZ) == 0);
+
+	XLByteToSeg(recptr, segno, state->segcxt.ws_segsize);
+	offset = XLogSegmentOffset(recptr, state->segcxt.ws_segsize);
+
+	XLogSegNoOffsetToRecPtr(segno, offset, state->segcxt.ws_segsize, recaddr);
+
+	if (hdr->xlp_magic != XLOG_PAGE_MAGIC)
+	{
+		char		fname[MAXFNAMELEN];
+
+		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",
+							  hdr->xlp_magic,
+							  fname,
+							  offset);
+		return false;
+	}
+
+	if ((hdr->xlp_info & ~XLP_ALL_FLAGS) != 0)
+	{
+		char		fname[MAXFNAMELEN];
+
+		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",
+							  hdr->xlp_info,
+							  fname,
+							  offset);
+		return false;
+	}
+
+	if (hdr->xlp_info & XLP_LONG_HEADER)
+	{
+		XLogLongPageHeader longhdr = (XLogLongPageHeader) hdr;
+
+		if (state->system_identifier &&
+			longhdr->xlp_sysid != state->system_identifier)
+		{
+			report_invalid_record(state,
+								  "WAL file is from different database system: WAL file database system identifier is %llu, pg_control database system identifier is %llu",
+								  (unsigned long long) longhdr->xlp_sysid,
+								  (unsigned long long) state->system_identifier);
+			return false;
+		}
+		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");
+			return false;
+		}
+		else if (longhdr->xlp_xlog_blcksz != XLOG_BLCKSZ)
+		{
+			report_invalid_record(state,
+								  "WAL file is from different database system: incorrect XLOG_BLCKSZ in page header");
+			return false;
+		}
+	}
+	else if (offset == 0)
+	{
+		char		fname[MAXFNAMELEN];
+
+		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,
+							  "invalid info bits %04X in log segment %s, offset %u",
+							  hdr->xlp_info,
+							  fname,
+							  offset);
+		return false;
+	}
+
+	/*
+	 * Check that the address on the page agrees with what we expected. This
+	 * check typically fails when an old WAL segment is recycled, and hasn't
+	 * yet been overwritten with new data yet.
+	 */
+	if (hdr->xlp_pageaddr != recaddr)
+	{
+		char		fname[MAXFNAMELEN];
+
+		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",
+							  (uint32) (hdr->xlp_pageaddr >> 32), (uint32) hdr->xlp_pageaddr,
+							  fname,
+							  offset);
+		return false;
+	}
+
+	/*
+	 * Since child timelines are always assigned a TLI greater than their
+	 * immediate parent's TLI, we should never see TLI go backwards across
+	 * successive pages of a consistent WAL sequence.
+	 *
+	 * Sometimes we re-read a segment that's already been (partially) read. So
+	 * we only verify TLIs for pages that are later than the last remembered
+	 * LSN.
+	 */
+	if (recptr > state->latestPagePtr)
+	{
+		if (hdr->xlp_tli < state->latestPageTLI)
+		{
+			char		fname[MAXFNAMELEN];
+
+			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",
+								  hdr->xlp_tli,
+								  state->latestPageTLI,
+								  fname,
+								  offset);
+			return false;
+		}
+	}
+	state->latestPagePtr = recptr;
+	state->latestPageTLI = hdr->xlp_tli;
+
+	return true;
+}
+
+#ifdef FRONTEND
+/*
+ * Functions that are currently not needed in the backend, but are better
+ * implemented inside xlogreader.c because of the internal facilities available
+ * here.
+ */
+
+/*
+ * Find the first record with an lsn >= RecPtr.
+ *
+ * This is different from XLogBeginRead() in that RecPtr doesn't need to point
+ * to a valid record boundary.  Useful for checking whether RecPtr is a valid
+ * xlog address for reading, and to find the first valid address after some
+ * address when dumping records for debugging purposes.
+ *
+ * This positions the reader, like XLogBeginRead(), so that the next call to
+ * XLogReadRecord() will read the next valid record.
+ */
+XLogRecPtr
+XLogFindNextRecord(XLogReaderState *state, XLogRecPtr RecPtr)
+{
+	XLogRecPtr	tmpRecPtr;
+	XLogRecPtr	found = InvalidXLogRecPtr;
+	XLogPageHeader header;
+	char	   *errormsg;
+
+	Assert(!XLogRecPtrIsInvalid(RecPtr));
+
+	/*
+	 * skip over potential continuation data, keeping in mind that it may span
+	 * multiple pages
+	 */
+	tmpRecPtr = RecPtr;
+	while (true)
+	{
+		XLogRecPtr	targetPagePtr;
+		int			targetRecOff;
+		uint32		pageHeaderSize;
+		int			readLen;
+
+		/*
+		 * Compute targetRecOff. It should typically be equal or greater than
+		 * short page-header since a valid record can't start anywhere before
+		 * that, except when caller has explicitly specified the offset that
+		 * falls somewhere there or when we are skipping multi-page
+		 * continuation record. It doesn't matter though because
+		 * ReadPageInternal() is prepared to handle that and will read at
+		 * least short page-header worth of data
+		 */
+		targetRecOff = tmpRecPtr % XLOG_BLCKSZ;
+
+		/* scroll back to page boundary */
+		targetPagePtr = tmpRecPtr - targetRecOff;
+
+		/* Read the page containing the record */
+		readLen = ReadPageInternal(state, targetPagePtr, targetRecOff);
+		if (readLen < 0)
+			goto err;
+
+		header = (XLogPageHeader) state->readBuf;
+
+		pageHeaderSize = XLogPageHeaderSize(header);
+
+		/* make sure we have enough data for the page header */
+		readLen = ReadPageInternal(state, targetPagePtr, pageHeaderSize);
+		if (readLen < 0)
+			goto err;
+
+		/* skip over potential continuation data */
+		if (header->xlp_info & XLP_FIRST_IS_CONTRECORD)
+		{
+			/*
+			 * If the length of the remaining continuation data is more than
+			 * what can fit in this page, the continuation record crosses over
+			 * this page. Read the next page and try again. xlp_rem_len in the
+			 * next page header will contain the remaining length of the
+			 * continuation data
+			 *
+			 * Note that record headers are MAXALIGN'ed
+			 */
+			if (MAXALIGN(header->xlp_rem_len) >= (XLOG_BLCKSZ - pageHeaderSize))
+				tmpRecPtr = targetPagePtr + XLOG_BLCKSZ;
+			else
+			{
+				/*
+				 * The previous continuation record ends in this page. Set
+				 * tmpRecPtr to point to the first valid record
+				 */
+				tmpRecPtr = targetPagePtr + pageHeaderSize
+					+ MAXALIGN(header->xlp_rem_len);
+				break;
+			}
+		}
+		else
+		{
+			tmpRecPtr = targetPagePtr + pageHeaderSize;
+			break;
+		}
+	}
+
+	/*
+	 * we know now that tmpRecPtr is an address pointing to a valid XLogRecord
+	 * because either we're at the first record after the beginning of a page
+	 * or we just jumped over the remaining data of a continuation.
+	 */
+	XLogBeginRead(state, tmpRecPtr);
+	while (XLogReadRecord(state, &errormsg) != NULL)
+	{
+		/* past the record we've found, break out */
+		if (RecPtr <= state->ReadRecPtr)
+		{
+			/* Rewind the reader to the beginning of the last record. */
+			found = state->ReadRecPtr;
+			XLogBeginRead(state, found);
+			return found;
+		}
+	}
+
+err:
+	XLogReaderInvalReadState(state);
+
+	return InvalidXLogRecPtr;
+}
+
+#endif							/* FRONTEND */
+
+/*
+ * Helper function to ease writing of XLogRoutine->page_read callbacks.
+ * If this function is used, caller must supply a segment_open callback in
+ * 'state', as that is used here.
+ *
+ * Read 'count' bytes into 'buf', starting at location 'startptr', from WAL
+ * fetched from timeline 'tli'.
+ *
+ * Returns true if succeeded, false if an error occurs, in which case
+ * 'errinfo' receives error details.
+ *
+ * XXX probably this should be improved to suck data directly from the
+ * WAL buffers when possible.
+ */
+bool
+WALRead(XLogReaderState *state,
+		char *buf, XLogRecPtr startptr, Size count, TimeLineID tli,
+		WALReadError *errinfo)
+{
+	char	   *p;
+	XLogRecPtr	recptr;
+	Size		nbytes;
+
+	p = buf;
+	recptr = startptr;
+	nbytes = count;
+
+	while (nbytes > 0)
+	{
+		uint32		startoff;
+		int			segbytes;
+		int			readbytes;
+
+		startoff = XLogSegmentOffset(recptr, state->segcxt.ws_segsize);
+
+		/*
+		 * If the data we want is not in a segment we have open, close what we
+		 * have (if anything) and open the next one, using the caller's
+		 * provided openSegment callback.
+		 */
+		if (state->seg.ws_file < 0 ||
+			!XLByteInSeg(recptr, state->seg.ws_segno, state->segcxt.ws_segsize) ||
+			tli != state->seg.ws_tli)
+		{
+			XLogSegNo	nextSegNo;
+
+			if (state->seg.ws_file >= 0)
+				state->routine.segment_close(state);
+
+			XLByteToSeg(recptr, nextSegNo, state->segcxt.ws_segsize);
+			state->routine.segment_open(state, nextSegNo, &tli);
+
+			/* This shouldn't happen -- indicates a bug in segment_open */
+			Assert(state->seg.ws_file >= 0);
+
+			/* Update the current segment info. */
+			state->seg.ws_tli = tli;
+			state->seg.ws_segno = nextSegNo;
+		}
+
+		/* How many bytes are within this segment? */
+		if (nbytes > (state->segcxt.ws_segsize - startoff))
+			segbytes = state->segcxt.ws_segsize - startoff;
+		else
+			segbytes = nbytes;
+
+#ifndef FRONTEND
+		pgstat_report_wait_start(WAIT_EVENT_WAL_READ);
+#endif
+
+		/* Reset errno first; eases reporting non-errno-affecting errors */
+		errno = 0;
+		readbytes = pg_pread(state->seg.ws_file, p, segbytes, (off_t) startoff);
+
+#ifndef FRONTEND
+		pgstat_report_wait_end();
+#endif
+
+		if (readbytes <= 0)
+		{
+			errinfo->wre_errno = errno;
+			errinfo->wre_req = segbytes;
+			errinfo->wre_read = readbytes;
+			errinfo->wre_off = startoff;
+			errinfo->wre_seg = state->seg;
+			return false;
+		}
+
+		/* Update state for read */
+		recptr += readbytes;
+		nbytes -= readbytes;
+		p += readbytes;
+	}
+
+	return true;
+}
+
+/* ----------------------------------------
+ * Functions for decoding the data and block references in a record.
+ * ----------------------------------------
+ */
+
+/* private function to reset the state between records */
+static void
+ResetDecoder(XLogReaderState *state)
+{
+	int			block_id;
+
+	state->decoded_record = NULL;
+
+	state->main_data_len = 0;
+
+	for (block_id = 0; block_id <= state->max_block_id; block_id++)
+	{
+		state->blocks[block_id].in_use = false;
+		state->blocks[block_id].has_image = false;
+		state->blocks[block_id].has_data = false;
+		state->blocks[block_id].apply_image = false;
+	}
+	state->max_block_id = -1;
+}
+
+/*
+ * Decode the previously read record.
+ *
+ * On error, a human-readable error message is returned in *errormsg, and
+ * the return value is false.
+ */
+bool
+DecodeXLogRecord(XLogReaderState *state, XLogRecord *record, char **errormsg)
+{
+	/*
+	 * read next _size bytes from record buffer, but check for overrun first.
+	 */
+#define COPY_HEADER_FIELD(_dst, _size)			\
+	do {										\
+		if (remaining < _size)					\
+			goto shortdata_err;					\
+		memcpy(_dst, ptr, _size);				\
+		ptr += _size;							\
+		remaining -= _size;						\
+	} while(0)
+
+	char	   *ptr;
+	uint32		remaining;
+	uint32		datatotal;
+	RelFileNode *rnode = NULL;
+	uint8		block_id;
+
+	ResetDecoder(state);
+
+	state->decoded_record = record;
+	state->record_origin = InvalidRepOriginId;
+
+	ptr = (char *) record;
+	ptr += SizeOfXLogRecord;
+	remaining = record->xl_tot_len - SizeOfXLogRecord;
+
+	/* Decode the headers */
+	datatotal = 0;
+	while (remaining > datatotal)
+	{
+		COPY_HEADER_FIELD(&block_id, sizeof(uint8));
+
+		if (block_id == XLR_BLOCK_ID_DATA_SHORT)
+		{
+			/* XLogRecordDataHeaderShort */
+			uint8		main_data_len;
+
+			COPY_HEADER_FIELD(&main_data_len, sizeof(uint8));
+
+			state->main_data_len = main_data_len;
+			datatotal += main_data_len;
+			break;				/* by convention, the main data fragment is
+								 * always last */
+		}
+		else if (block_id == XLR_BLOCK_ID_DATA_LONG)
+		{
+			/* XLogRecordDataHeaderLong */
+			uint32		main_data_len;
+
+			COPY_HEADER_FIELD(&main_data_len, sizeof(uint32));
+			state->main_data_len = main_data_len;
+			datatotal += main_data_len;
+			break;				/* by convention, the main data fragment is
+								 * always last */
+		}
+		else if (block_id == XLR_BLOCK_ID_ORIGIN)
+		{
+			COPY_HEADER_FIELD(&state->record_origin, sizeof(RepOriginId));
+		}
+		else if (block_id <= XLR_MAX_BLOCK_ID)
+		{
+			/* XLogRecordBlockHeader */
+			DecodedBkpBlock *blk;
+			uint8		fork_flags;
+
+			if (block_id <= state->max_block_id)
+			{
+				report_invalid_record(state,
+									  "out-of-order block_id %u at %X/%X",
+									  block_id,
+									  (uint32) (state->ReadRecPtr >> 32),
+									  (uint32) state->ReadRecPtr);
+				goto err;
+			}
+			state->max_block_id = block_id;
+
+			blk = &state->blocks[block_id];
+			blk->in_use = true;
+			blk->apply_image = false;
+
+			COPY_HEADER_FIELD(&fork_flags, sizeof(uint8));
+			blk->forknum = fork_flags & BKPBLOCK_FORK_MASK;
+			blk->flags = fork_flags;
+			blk->has_image = ((fork_flags & BKPBLOCK_HAS_IMAGE) != 0);
+			blk->has_data = ((fork_flags & BKPBLOCK_HAS_DATA) != 0);
+
+			COPY_HEADER_FIELD(&blk->data_len, sizeof(uint16));
+			/* cross-check that the HAS_DATA flag is set iff data_length > 0 */
+			if (blk->has_data && blk->data_len == 0)
+			{
+				report_invalid_record(state,
+									  "BKPBLOCK_HAS_DATA set, but no data included at %X/%X",
+									  (uint32) (state->ReadRecPtr >> 32), (uint32) state->ReadRecPtr);
+				goto err;
+			}
+			if (!blk->has_data && blk->data_len != 0)
+			{
+				report_invalid_record(state,
+									  "BKPBLOCK_HAS_DATA not set, but data length is %u at %X/%X",
+									  (unsigned int) blk->data_len,
+									  (uint32) (state->ReadRecPtr >> 32), (uint32) state->ReadRecPtr);
+				goto err;
+			}
+			datatotal += blk->data_len;
+
+			if (blk->has_image)
+			{
+				COPY_HEADER_FIELD(&blk->bimg_len, sizeof(uint16));
+				COPY_HEADER_FIELD(&blk->hole_offset, sizeof(uint16));
+				COPY_HEADER_FIELD(&blk->bimg_info, sizeof(uint8));
+
+				blk->apply_image = ((blk->bimg_info & BKPIMAGE_APPLY) != 0);
+
+				if (blk->bimg_info & BKPIMAGE_IS_COMPRESSED)
+				{
+					if (blk->bimg_info & BKPIMAGE_HAS_HOLE)
+						COPY_HEADER_FIELD(&blk->hole_length, sizeof(uint16));
+					else
+						blk->hole_length = 0;
+				}
+				else
+					blk->hole_length = BLCKSZ - blk->bimg_len;
+				datatotal += blk->bimg_len;
+
+				/*
+				 * cross-check that hole_offset > 0, hole_length > 0 and
+				 * bimg_len < BLCKSZ if the HAS_HOLE flag is set.
+				 */
+				if ((blk->bimg_info & BKPIMAGE_HAS_HOLE) &&
+					(blk->hole_offset == 0 ||
+					 blk->hole_length == 0 ||
+					 blk->bimg_len == BLCKSZ))
+				{
+					report_invalid_record(state,
+										  "BKPIMAGE_HAS_HOLE set, but hole offset %u length %u block image length %u at %X/%X",
+										  (unsigned int) blk->hole_offset,
+										  (unsigned int) blk->hole_length,
+										  (unsigned int) blk->bimg_len,
+										  (uint32) (state->ReadRecPtr >> 32), (uint32) state->ReadRecPtr);
+					goto err;
+				}
+
+				/*
+				 * cross-check that hole_offset == 0 and hole_length == 0 if
+				 * the HAS_HOLE flag is not set.
+				 */
+				if (!(blk->bimg_info & BKPIMAGE_HAS_HOLE) &&
+					(blk->hole_offset != 0 || blk->hole_length != 0))
+				{
+					report_invalid_record(state,
+										  "BKPIMAGE_HAS_HOLE not set, but hole offset %u length %u at %X/%X",
+										  (unsigned int) blk->hole_offset,
+										  (unsigned int) blk->hole_length,
+										  (uint32) (state->ReadRecPtr >> 32), (uint32) state->ReadRecPtr);
+					goto err;
+				}
+
+				/*
+				 * cross-check that bimg_len < BLCKSZ if the IS_COMPRESSED
+				 * flag is set.
+				 */
+				if ((blk->bimg_info & BKPIMAGE_IS_COMPRESSED) &&
+					blk->bimg_len == BLCKSZ)
+				{
+					report_invalid_record(state,
+										  "BKPIMAGE_IS_COMPRESSED set, but block image length %u at %X/%X",
+										  (unsigned int) blk->bimg_len,
+										  (uint32) (state->ReadRecPtr >> 32), (uint32) state->ReadRecPtr);
+					goto err;
+				}
+
+				/*
+				 * cross-check that bimg_len = BLCKSZ if neither HAS_HOLE nor
+				 * IS_COMPRESSED flag is set.
+				 */
+				if (!(blk->bimg_info & BKPIMAGE_HAS_HOLE) &&
+					!(blk->bimg_info & BKPIMAGE_IS_COMPRESSED) &&
+					blk->bimg_len != BLCKSZ)
+				{
+					report_invalid_record(state,
+										  "neither BKPIMAGE_HAS_HOLE nor BKPIMAGE_IS_COMPRESSED set, but block image length is %u at %X/%X",
+										  (unsigned int) blk->data_len,
+										  (uint32) (state->ReadRecPtr >> 32), (uint32) state->ReadRecPtr);
+					goto err;
+				}
+			}
+			if (!(fork_flags & BKPBLOCK_SAME_REL))
+			{
+				COPY_HEADER_FIELD(&blk->rnode, sizeof(RelFileNode));
+				rnode = &blk->rnode;
+			}
+			else
+			{
+				if (rnode == NULL)
+				{
+					report_invalid_record(state,
+										  "BKPBLOCK_SAME_REL set but no previous rel at %X/%X",
+										  (uint32) (state->ReadRecPtr >> 32), (uint32) state->ReadRecPtr);
+					goto err;
+				}
+
+				blk->rnode = *rnode;
+			}
+			COPY_HEADER_FIELD(&blk->blkno, sizeof(BlockNumber));
+		}
+		else
+		{
+			report_invalid_record(state,
+								  "invalid block_id %u at %X/%X",
+								  block_id,
+								  (uint32) (state->ReadRecPtr >> 32),
+								  (uint32) state->ReadRecPtr);
+			goto err;
+		}
+	}
+
+	if (remaining != datatotal)
+		goto shortdata_err;
+
+	/*
+	 * Ok, we've parsed the fragment headers, and verified that the total
+	 * length of the payload in the fragments is equal to the amount of data
+	 * left. Copy the data of each fragment to a separate buffer.
+	 *
+	 * We could just set up pointers into readRecordBuf, but we want to align
+	 * the data for the convenience of the callers. Backup images are not
+	 * copied, however; they don't need alignment.
+	 */
+
+	/* block data first */
+	for (block_id = 0; block_id <= state->max_block_id; block_id++)
+	{
+		DecodedBkpBlock *blk = &state->blocks[block_id];
+
+		if (!blk->in_use)
+			continue;
+
+		Assert(blk->has_image || !blk->apply_image);
+
+		if (blk->has_image)
+		{
+			blk->bkp_image = ptr;
+			ptr += blk->bimg_len;
+		}
+		if (blk->has_data)
+		{
+			if (!blk->data || blk->data_len > blk->data_bufsz)
+			{
+				if (blk->data)
+					pfree(blk->data);
+
+				/*
+				 * Force the initial request to be BLCKSZ so that we don't
+				 * waste time with lots of trips through this stanza as a
+				 * result of WAL compression.
+				 */
+				blk->data_bufsz = MAXALIGN(Max(blk->data_len, BLCKSZ));
+				blk->data = palloc(blk->data_bufsz);
+			}
+			memcpy(blk->data, ptr, blk->data_len);
+			ptr += blk->data_len;
+		}
+	}
+
+	/* and finally, the main data */
+	if (state->main_data_len > 0)
+	{
+		if (!state->main_data || state->main_data_len > state->main_data_bufsz)
+		{
+			if (state->main_data)
+				pfree(state->main_data);
+
+			/*
+			 * main_data_bufsz must be MAXALIGN'ed.  In many xlog record
+			 * types, we omit trailing struct padding on-disk to save a few
+			 * bytes; but compilers may generate accesses to the xlog struct
+			 * that assume that padding bytes are present.  If the palloc
+			 * request is not large enough to include such padding bytes then
+			 * we'll get valgrind complaints due to otherwise-harmless fetches
+			 * of the padding bytes.
+			 *
+			 * In addition, force the initial request to be reasonably large
+			 * so that we don't waste time with lots of trips through this
+			 * stanza.  BLCKSZ / 2 seems like a good compromise choice.
+			 */
+			state->main_data_bufsz = MAXALIGN(Max(state->main_data_len,
+												  BLCKSZ / 2));
+			state->main_data = palloc(state->main_data_bufsz);
+		}
+		memcpy(state->main_data, ptr, state->main_data_len);
+		ptr += state->main_data_len;
+	}
+
+	return true;
+
+shortdata_err:
+	report_invalid_record(state,
+						  "record with invalid length at %X/%X",
+						  (uint32) (state->ReadRecPtr >> 32), (uint32) state->ReadRecPtr);
+err:
+	*errormsg = state->errormsg_buf;
+
+	return false;
+}
+
+/*
+ * Returns information about the block that a block reference refers to.
+ *
+ * If the WAL record contains a block reference with the given ID, *rnode,
+ * *forknum, and *blknum are filled in (if not NULL), and returns true.
+ * Otherwise returns false.
+ */
+bool
+XLogRecGetBlockTag(XLogReaderState *record, uint8 block_id,
+				   RelFileNode *rnode, ForkNumber *forknum, BlockNumber *blknum)
+{
+	DecodedBkpBlock *bkpb;
+
+	if (!record->blocks[block_id].in_use)
+		return false;
+
+	bkpb = &record->blocks[block_id];
+	if (rnode)
+		*rnode = bkpb->rnode;
+	if (forknum)
+		*forknum = bkpb->forknum;
+	if (blknum)
+		*blknum = bkpb->blkno;
+	return true;
+}
+
+/*
+ * Returns the data associated with a block reference, or NULL if there is
+ * no data (e.g. because a full-page image was taken instead). The returned
+ * pointer points to a MAXALIGNed buffer.
+ */
+char *
+XLogRecGetBlockData(XLogReaderState *record, uint8 block_id, Size *len)
+{
+	DecodedBkpBlock *bkpb;
+
+	if (!record->blocks[block_id].in_use)
+		return NULL;
+
+	bkpb = &record->blocks[block_id];
+
+	if (!bkpb->has_data)
+	{
+		if (len)
+			*len = 0;
+		return NULL;
+	}
+	else
+	{
+		if (len)
+			*len = bkpb->data_len;
+		return bkpb->data;
+	}
+}
+
+/*
+ * Restore a full-page image from a backup block attached to an XLOG record.
+ *
+ * Returns true if a full-page image is restored.
+ */
+bool
+RestoreBlockImage(XLogReaderState *record, uint8 block_id, char *page)
+{
+	DecodedBkpBlock *bkpb;
+	char	   *ptr;
+	PGAlignedBlock tmp;
+
+	if (!record->blocks[block_id].in_use)
+		return false;
+	if (!record->blocks[block_id].has_image)
+		return false;
+
+	bkpb = &record->blocks[block_id];
+	ptr = bkpb->bkp_image;
+
+	if (bkpb->bimg_info & BKPIMAGE_IS_COMPRESSED)
+	{
+		/* If a backup block image is compressed, decompress it */
+		if (pglz_decompress(ptr, bkpb->bimg_len, tmp.data,
+							BLCKSZ - bkpb->hole_length, true) < 0)
+		{
+			report_invalid_record(record, "invalid compressed image at %X/%X, block %d",
+								  (uint32) (record->ReadRecPtr >> 32),
+								  (uint32) record->ReadRecPtr,
+								  block_id);
+			return false;
+		}
+		ptr = tmp.data;
+	}
+
+	/* generate page, taking into account hole if necessary */
+	if (bkpb->hole_length == 0)
+	{
+		memcpy(page, ptr, BLCKSZ);
+	}
+	else
+	{
+		memcpy(page, ptr, bkpb->hole_offset);
+		/* must zero-fill the hole */
+		MemSet(page + bkpb->hole_offset, 0, bkpb->hole_length);
+		memcpy(page + (bkpb->hole_offset + bkpb->hole_length),
+			   ptr + bkpb->hole_offset,
+			   BLCKSZ - (bkpb->hole_offset + bkpb->hole_length));
+	}
+
+	return true;
+}
+
+#ifndef FRONTEND
+
+/*
+ * Extract the FullTransactionId from a WAL record.
+ */
+FullTransactionId
+XLogRecGetFullXid(XLogReaderState *record)
+{
+	TransactionId xid,
+				next_xid;
+	uint32		epoch;
+
+	/*
+	 * This function is only safe during replay, because it depends on the
+	 * replay state.  See AdvanceNextFullTransactionIdPastXid() for more.
+	 */
+	Assert(AmStartupProcess() || !IsUnderPostmaster);
+
+	xid = XLogRecGetXid(record);
+	next_xid = XidFromFullTransactionId(ShmemVariableCache->nextFullXid);
+	epoch = EpochFromFullTransactionId(ShmemVariableCache->nextFullXid);
+
+	/*
+	 * If xid is numerically greater than next_xid, it has to be from the last
+	 * epoch.
+	 */
+	if (unlikely(xid > next_xid))
+		--epoch;
+
+	return FullTransactionIdFromEpochAndXid(epoch, xid);
+}
+
+#endif