v8-0001-pg_walinspect.patch

From 69408f5e8493c490b96c65fed46fc7f4d4954ea5 Mon Sep 17 00:00:00 2001
From: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Date: Thu, 10 Mar 2022 16:26:04 +0000
Subject: [PATCH v8] pg_walinspect

---
 contrib/Makefile                             |   1 +
 contrib/pg_walinspect/.gitignore             |   4 +
 contrib/pg_walinspect/Makefile               |  26 +
 contrib/pg_walinspect/pg_walinspect--1.0.sql |  96 ++
 contrib/pg_walinspect/pg_walinspect.c        | 949 +++++++++++++++++++
 contrib/pg_walinspect/pg_walinspect.control  |   5 +
 src/backend/access/transam/xlogreader.c      |  14 +-
 src/bin/pg_waldump/pg_waldump.c              |   5 +
 src/common/relpath.c                         |  18 +
 src/include/access/xlog.h                    |   2 +-
 src/include/access/xlog_internal.h           |   2 +-
 src/include/access/xlogreader.h              |   2 -
 src/include/common/relpath.h                 |   1 +
 13 files changed, 1114 insertions(+), 11 deletions(-)
 create mode 100644 contrib/pg_walinspect/.gitignore
 create mode 100644 contrib/pg_walinspect/Makefile
 create mode 100644 contrib/pg_walinspect/pg_walinspect--1.0.sql
 create mode 100644 contrib/pg_walinspect/pg_walinspect.c
 create mode 100644 contrib/pg_walinspect/pg_walinspect.control

diff --git a/contrib/Makefile b/contrib/Makefile
index e3e221308b..705c6fc36b 100644
--- a/contrib/Makefile
+++ b/contrib/Makefile
@@ -40,6 +40,7 @@ SUBDIRS = \
 		pgrowlocks	\
 		pgstattuple	\
 		pg_visibility	\
+		pg_walinspect	\
 		postgres_fdw	\
 		seg		\
 		spi		\
diff --git a/contrib/pg_walinspect/.gitignore b/contrib/pg_walinspect/.gitignore
new file mode 100644
index 0000000000..5dcb3ff972
--- /dev/null
+++ b/contrib/pg_walinspect/.gitignore
@@ -0,0 +1,4 @@
+# Generated subdirectories
+/log/
+/results/
+/tmp_check/
diff --git a/contrib/pg_walinspect/Makefile b/contrib/pg_walinspect/Makefile
new file mode 100644
index 0000000000..c92a97447f
--- /dev/null
+++ b/contrib/pg_walinspect/Makefile
@@ -0,0 +1,26 @@
+# contrib/pg_walinspect/Makefile
+
+MODULE_big = pg_walinspect
+OBJS = \
+	$(WIN32RES) \
+	pg_walinspect.o
+PGFILEDESC = "pg_walinspect - functions to inspect contents of PostgreSQL Write-Ahead Log"
+
+PG_CPPFLAGS = -I$(libpq_srcdir)
+SHLIB_LINK_INTERNAL = $(libpq)
+
+EXTENSION = pg_walinspect
+DATA = pg_walinspect--1.0.sql
+
+REGRESS = pg_walinspect
+
+ifdef USE_PGXS
+PG_CONFIG = pg_config
+PGXS := $(shell $(PG_CONFIG) --pgxs)
+include $(PGXS)
+else
+subdir = contrib/pg_walinspect
+top_builddir = ../..
+include $(top_builddir)/src/Makefile.global
+include $(top_srcdir)/contrib/contrib-global.mk
+endif
diff --git a/contrib/pg_walinspect/pg_walinspect--1.0.sql b/contrib/pg_walinspect/pg_walinspect--1.0.sql
new file mode 100644
index 0000000000..8720d3bb5e
--- /dev/null
+++ b/contrib/pg_walinspect/pg_walinspect--1.0.sql
@@ -0,0 +1,96 @@
+/* contrib/pg_walinspect/pg_walinspect--1.0.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION pg_walinspect" to load this file. \quit
+
+--
+-- pg_get_raw_wal_record()
+--
+CREATE FUNCTION pg_get_raw_wal_record(IN in_lsn pg_lsn,
+    OUT lsn pg_lsn,
+    OUT record bytea
+)
+AS 'MODULE_PATHNAME', 'pg_get_raw_wal_record'
+LANGUAGE C CALLED ON NULL INPUT PARALLEL SAFE;
+
+REVOKE EXECUTE ON FUNCTION pg_get_raw_wal_record(pg_lsn) FROM PUBLIC;
+GRANT EXECUTE ON FUNCTION pg_get_raw_wal_record(pg_lsn) TO pg_monitor;
+
+--
+-- pg_get_first_valid_wal_record_lsn()
+--
+CREATE FUNCTION pg_get_first_valid_wal_record_lsn(IN in_lsn pg_lsn,
+    OUT lsn pg_lsn,
+    OUT prev_lsn pg_lsn
+)
+AS 'MODULE_PATHNAME', 'pg_get_first_valid_wal_record_lsn'
+LANGUAGE C CALLED ON NULL INPUT PARALLEL SAFE;
+
+REVOKE EXECUTE ON FUNCTION pg_get_first_valid_wal_record_lsn(pg_lsn) FROM PUBLIC;
+GRANT EXECUTE ON FUNCTION pg_get_first_valid_wal_record_lsn(pg_lsn) TO pg_monitor;
+
+--
+-- pg_get_wal_record_info()
+--
+CREATE FUNCTION pg_get_wal_record_info(IN in_lsn pg_lsn,
+    OUT lsn pg_lsn,
+    OUT prev_lsn pg_lsn,
+    OUT xid xid,
+    OUT resource_manager text,
+    OUT length int4,
+    OUT total_length int4,
+	OUT description text,
+    OUT block_ref text,
+    OUT data bytea,
+    OUT data_len int4
+)
+AS 'MODULE_PATHNAME', 'pg_get_wal_record_info'
+LANGUAGE C CALLED ON NULL INPUT PARALLEL SAFE;
+
+REVOKE EXECUTE ON FUNCTION pg_get_wal_record_info(pg_lsn) FROM PUBLIC;
+GRANT EXECUTE ON FUNCTION pg_get_wal_record_info(pg_lsn) TO pg_monitor;
+
+--
+-- pg_get_wal_records_info()
+--
+CREATE FUNCTION pg_get_wal_records_info(IN start_lsn pg_lsn,
+    IN end_lsn pg_lsn DEFAULT NULL,
+    OUT lsn pg_lsn,
+    OUT prev_lsn pg_lsn,
+    OUT xid xid,
+    OUT resource_manager text,
+    OUT length int4,
+    OUT total_length int4,
+	OUT description text,
+    OUT block_ref text,
+    OUT data bytea,
+    OUT data_len int4
+)
+RETURNS SETOF record
+AS 'MODULE_PATHNAME', 'pg_get_wal_records_info'
+LANGUAGE C CALLED ON NULL INPUT PARALLEL SAFE;
+
+REVOKE EXECUTE ON FUNCTION pg_get_wal_records_info(pg_lsn, pg_lsn) FROM PUBLIC;
+GRANT EXECUTE ON FUNCTION pg_get_wal_records_info(pg_lsn, pg_lsn) TO pg_monitor;
+
+--
+-- pg_get_wal_stats()
+--
+CREATE FUNCTION pg_get_wal_stats(IN start_lsn pg_lsn,
+    IN end_lsn pg_lsn DEFAULT NULL,
+    OUT resource_manager text,
+    OUT count int8,
+    OUT count_percentage float4,
+    OUT record_size int8,
+    OUT record_size_percentage float4,
+    OUT fpi_size int8,
+    OUT fpi_size_percentage float4,
+    OUT combined_size int8,
+    OUT combined_size_percentage float4
+)
+RETURNS SETOF record
+AS 'MODULE_PATHNAME', 'pg_get_wal_stats'
+LANGUAGE C CALLED ON NULL INPUT PARALLEL SAFE;
+
+REVOKE EXECUTE ON FUNCTION pg_get_wal_stats(pg_lsn, pg_lsn) FROM PUBLIC;
+GRANT EXECUTE ON FUNCTION pg_get_wal_stats(pg_lsn, pg_lsn) TO pg_monitor;
diff --git a/contrib/pg_walinspect/pg_walinspect.c b/contrib/pg_walinspect/pg_walinspect.c
new file mode 100644
index 0000000000..99f2f43c42
--- /dev/null
+++ b/contrib/pg_walinspect/pg_walinspect.c
@@ -0,0 +1,949 @@
+/*-------------------------------------------------------------------------
+ *
+ * pg_walinspect.c
+ *		  Functions to inspect contents of PostgreSQL Write-Ahead Log
+ *
+ * Copyright (c) 2022, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *		  contrib/pg_walinspect/pg_walinspect.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "access/xlog.h"
+#include "access/xlog_internal.h"
+#include "access/xlogreader.h"
+#include "access/xlogrecovery.h"
+#include "access/xlogutils.h"
+#include "funcapi.h"
+#include "miscadmin.h"
+#include "utils/builtins.h"
+#include "utils/pg_lsn.h"
+
+/*
+ * NOTE: For any code change or issue fix here, it is highly recommended to
+ * give a thought about doing the same in pg_waldump tool as well.
+ */
+
+PG_MODULE_MAGIC;
+
+#define MAX_XLINFO_TYPES 16
+
+typedef struct Stats
+{
+	uint64		count;
+	uint64		rec_len;
+	uint64		fpi_len;
+}	Stats;
+
+typedef struct XLogRecStats
+{
+	uint64		count;
+	Stats		rmgr_stats[RM_NEXT_ID];
+	Stats		record_stats[RM_NEXT_ID][MAX_XLINFO_TYPES];
+}	XLogRecStats;
+
+extern void _PG_init(void);
+extern void _PG_fini(void);
+
+PG_FUNCTION_INFO_V1(pg_get_raw_wal_record);
+PG_FUNCTION_INFO_V1(pg_get_first_valid_wal_record_lsn);
+PG_FUNCTION_INFO_V1(pg_get_wal_record_info);
+PG_FUNCTION_INFO_V1(pg_get_wal_records_info);
+PG_FUNCTION_INFO_V1(pg_get_wal_stats);
+
+static XLogRecPtr ValidateInputLSN(XLogRecPtr lsn);
+static XLogRecPtr ValidateStartAndEndLSNs(XLogRecPtr start_lsn,
+										  XLogRecPtr end_lsn);
+static XLogReaderState *InitXLogReaderState(XLogRecPtr lsn,
+											XLogRecPtr *first_record);
+static XLogRecord *ReadNextXLogRecord(XLogReaderState *xlogreader,
+									  XLogRecPtr first_record);
+static void GetXLogRecordInfo(XLogReaderState *record, XLogRecPtr lsn,
+							  Datum *values, bool *nulls, uint32 ncols);
+static void StoreXLogRecordStats(XLogRecStats * stats,
+								 XLogReaderState *record);
+static void GetXLogSummaryStats(XLogRecStats * stats,
+								Tuplestorestate *tupstore, TupleDesc tupdesc,
+								Datum *values, bool *nulls, uint32 ncols);
+static void FillXLogStatsRow(const char *name, uint64 n, uint64 total_count,
+							 uint64 rec_len, uint64 total_rec_len,
+							 uint64 fpi_len, uint64 total_fpi_len,
+							 uint64 tot_len, uint64 total_len,
+							 Datum *values, bool *nulls, uint32 ncols);
+static void GetWalStatsInternal(FunctionCallInfo fcinfo, XLogRecPtr start_lsn,
+								XLogRecPtr end_lsn);
+static void GetWALRecordsInfoInternal(FunctionCallInfo fcinfo,
+									  XLogRecPtr start_lsn,
+									  XLogRecPtr end_lsn);
+
+/*
+ * Module load callback.
+ */
+void
+_PG_init(void)
+{
+	/* Define custom GUCs and install hooks here, if any. */
+
+	/*
+	 * Have EmitWarningsOnPlaceholders("pg_walinspect"); if custom GUCs are
+	 * defined.
+	 */
+}
+
+/*
+ * Module unload callback.
+ */
+void
+_PG_fini(void)
+{
+	/* Uninstall hooks, if any. */
+}
+
+/*
+ * Validate given LSN and return the LSN up to which the server has WAL.
+ */
+static XLogRecPtr
+ValidateInputLSN(XLogRecPtr lsn)
+{
+	XLogRecPtr curr_lsn;
+
+	/* Validate input WAL LSN. */
+	if (XLogRecPtrIsInvalid(lsn))
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("invalid WAL LSN")));
+
+	if (!RecoveryInProgress())
+		curr_lsn = GetFlushRecPtr(NULL);
+	else
+		curr_lsn = GetXLogReplayRecPtr(NULL);
+
+	Assert(!XLogRecPtrIsInvalid(curr_lsn));
+
+	if (lsn >= curr_lsn)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("cannot get information of WAL at future LSN"),
+				 errdetail("Last known WAL LSN on the database system is %X/%X.",
+						   LSN_FORMAT_ARGS(curr_lsn))));
+
+	return curr_lsn;
+}
+
+/*
+ * Validate given start LSN and end LSN, return the new end LSN in case user
+ * hasn't specified one.
+ */
+static XLogRecPtr
+ValidateStartAndEndLSNs(XLogRecPtr start_lsn, XLogRecPtr end_lsn)
+{
+	XLogRecPtr curr_lsn;
+
+	/* Validate WAL start LSN. */
+	if (XLogRecPtrIsInvalid(start_lsn))
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("invalid WAL start LSN")));
+
+	if (!RecoveryInProgress())
+		curr_lsn = GetFlushRecPtr(NULL);
+	else
+		curr_lsn = GetXLogReplayRecPtr(NULL);
+
+	if (start_lsn >= curr_lsn)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("WAL start LSN cannot be a future WAL LSN"),
+				 errdetail("Last known WAL LSN on the database system is %X/%X.",
+						   LSN_FORMAT_ARGS(curr_lsn))));
+
+	/*
+	 * If end_lsn is specified, let's ensure that it's not a future LSN i.e.
+	 * something the database system doesn't know about.
+	 */
+	if (!XLogRecPtrIsInvalid(end_lsn) &&
+		(end_lsn >= curr_lsn))
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("WAL end LSN cannot be a future WAL LSN"),
+				 errdetail("Last known WAL LSN on the database system is %X/%X.",
+						   LSN_FORMAT_ARGS(curr_lsn))));
+
+	/*
+	 * When end_lsn is not specified let's read up to the last WAL position
+	 * known to be on the server.
+	 */
+	if (XLogRecPtrIsInvalid(end_lsn))
+		end_lsn = curr_lsn;
+
+	Assert(!XLogRecPtrIsInvalid(end_lsn));
+
+	if (start_lsn >= end_lsn)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("WAL start LSN must be less than end LSN")));
+
+	return end_lsn;
+}
+
+/*
+ * Intialize WAL reader and identify first valid LSN.
+ */
+static XLogReaderState *
+InitXLogReaderState(XLogRecPtr lsn, XLogRecPtr *first_record)
+{
+	XLogReaderState *xlogreader;
+
+	/*
+	 * Reading WAL below the first page of the first sgements isn't allowed.
+	 * This is a bootstrap WAL page and the page_read callback fails to read
+	 * it.
+	 */
+	if (lsn < XLOG_BLCKSZ)
+		ereport(ERROR,
+				(errmsg("could not read WAL at LSN %X/%X",
+						LSN_FORMAT_ARGS(lsn))));
+
+	xlogreader = XLogReaderAllocate(wal_segment_size, NULL,
+									XL_ROUTINE(.page_read = &read_local_xlog_page,
+											   .segment_open = &wal_segment_open,
+											   .segment_close = &wal_segment_close),
+									NULL);
+
+	if (xlogreader == NULL)
+		ereport(ERROR,
+				(errcode(ERRCODE_OUT_OF_MEMORY),
+				 errmsg("out of memory"),
+				 errdetail("Failed while allocating a WAL reading processor.")));
+
+	/* First find a valid recptr to start from. */
+	*first_record = XLogFindNextRecord(xlogreader, lsn);
+
+	if (XLogRecPtrIsInvalid(*first_record))
+		ereport(ERROR,
+				(errmsg("could not find a valid record after %X/%X",
+						LSN_FORMAT_ARGS(lsn))));
+
+	/*
+	 * Display a message that we're skipping data if the given LSN wasn't a
+	 * pointer to the start of a record and also wasn't a pointer to the
+	 * beginning of a segment (e.g. we were used in file mode).
+	 */
+	if (*first_record != lsn && XLogSegmentOffset(lsn, wal_segment_size) != 0)
+		ereport(WARNING,
+				(errmsg_plural("first record is after %X/%X, at %X/%X, skipping over %u byte",
+							   "first record is after %X/%X, at %X/%X, skipping over %u bytes",
+							   (*first_record - lsn),
+							   LSN_FORMAT_ARGS(lsn),
+							   LSN_FORMAT_ARGS(*first_record),
+							   (uint32) (*first_record - lsn))));
+
+	return xlogreader;
+}
+
+/*
+ * Read next WAL record.
+ */
+static XLogRecord *
+ReadNextXLogRecord(XLogReaderState *xlogreader, XLogRecPtr first_record)
+{
+	XLogRecord *record;
+	char	*errormsg;
+
+	record = XLogReadRecord(xlogreader, &errormsg);
+
+	if (record == NULL)
+	{
+		if (errormsg)
+			ereport(ERROR,
+					(errcode_for_file_access(),
+					 errmsg("could not read WAL at %X/%X: %s",
+							LSN_FORMAT_ARGS(first_record), errormsg)));
+		else
+			ereport(ERROR,
+					(errcode_for_file_access(),
+					 errmsg("could not read WAL at %X/%X",
+							LSN_FORMAT_ARGS(first_record))));
+	}
+
+	return record;
+}
+
+/*
+ * Get raw WAL record.
+ *
+ * This function emits an error if a future WAL LSN i.e. WAL LSN the database
+ * system doesn't know about is specified.
+ */
+Datum
+pg_get_raw_wal_record(PG_FUNCTION_ARGS)
+{
+#define PG_GET_RAW_WAL_RECORD_COLS 2
+	XLogRecPtr	lsn;
+	XLogRecord *record;
+	XLogRecPtr	first_record;
+	XLogReaderState *xlogreader;
+	bytea	*raw_record;
+	uint32	rec_len;
+	char	*raw_record_data;
+	TupleDesc	tupdesc;
+	Datum	result;
+	HeapTuple	tuple;
+	Datum	values[PG_GET_RAW_WAL_RECORD_COLS];
+	bool	nulls[PG_GET_RAW_WAL_RECORD_COLS];
+	int	i = 0;
+
+	lsn = PG_GETARG_LSN(0);
+
+	(void) ValidateInputLSN(lsn);
+
+	/* Build a tuple descriptor for our result type. */
+	if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
+		elog(ERROR, "return type must be a row type");
+
+	xlogreader = InitXLogReaderState(lsn, &first_record);
+
+	Assert(xlogreader);
+
+	record = ReadNextXLogRecord(xlogreader, first_record);
+
+	rec_len = XLogRecGetTotalLen(xlogreader);
+
+	Assert(rec_len > 0);
+
+	raw_record = (bytea *) palloc(rec_len + VARHDRSZ);
+	SET_VARSIZE(raw_record, rec_len + VARHDRSZ);
+	raw_record_data = VARDATA(raw_record);
+
+	memcpy(raw_record_data, record, rec_len);
+
+	XLogReaderFree(xlogreader);
+
+	MemSet(values, 0, sizeof(values));
+	MemSet(nulls, 0, sizeof(nulls));
+
+	values[i++] = LSNGetDatum(first_record);
+	values[i++] = PointerGetDatum(raw_record);
+
+	Assert(i == PG_GET_RAW_WAL_RECORD_COLS);
+
+	tuple = heap_form_tuple(tupdesc, values, nulls);
+	result = HeapTupleGetDatum(tuple);
+
+	PG_RETURN_DATUM(result);
+#undef PG_GET_RAW_WAL_RECORD_COLS
+}
+
+/*
+ * Get first valid raw WAL record LSN following the given LSN.
+ *
+ * This function emits an error if a future WAL LSN i.e. WAL LSN the database
+ * system doesn't know about is specified.
+ */
+Datum
+pg_get_first_valid_wal_record_lsn(PG_FUNCTION_ARGS)
+{
+#define PG_GET_FIRST_VALID_WAL_RECORD_LSN_COLS 2
+	XLogRecPtr	lsn;
+	XLogRecPtr	first_record;
+	XLogRecPtr	prev_record;
+	XLogReaderState *xlogreader;
+	TupleDesc	tupdesc;
+	Datum	result;
+	HeapTuple	tuple;
+	Datum	values[PG_GET_FIRST_VALID_WAL_RECORD_LSN_COLS];
+	bool	nulls[PG_GET_FIRST_VALID_WAL_RECORD_LSN_COLS];
+	int	i = 0;
+
+	lsn = PG_GETARG_LSN(0);
+
+	(void) ValidateInputLSN(lsn);
+
+	/* Build a tuple descriptor for our result type. */
+	if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
+		elog(ERROR, "return type must be a row type");
+
+	xlogreader = InitXLogReaderState(lsn, &first_record);
+
+	Assert(xlogreader);
+
+	(void) ReadNextXLogRecord(xlogreader, first_record);
+
+	prev_record = XLogRecGetPrev(xlogreader);
+
+	XLogReaderFree(xlogreader);
+
+	/*
+	 * Previous valid WAL record must be at an LSN lower than next valid WAL
+	 * record LSN. Otherwise, it is an indication of something wrong, so error
+	 * out.
+	 */
+	if (prev_record >= first_record)
+		ereport(ERROR,
+				(errcode(ERRCODE_DATA_CORRUPTED),
+				 errmsg("record with incorrect prev-link %X/%X at %X/%X",
+				 LSN_FORMAT_ARGS(prev_record),
+				 LSN_FORMAT_ARGS(first_record))));
+
+	MemSet(values, 0, sizeof(values));
+	MemSet(nulls, 0, sizeof(nulls));
+
+	values[i++] = LSNGetDatum(first_record);
+	values[i++] = LSNGetDatum(prev_record);
+
+	Assert(i == PG_GET_FIRST_VALID_WAL_RECORD_LSN_COLS);
+
+	tuple = heap_form_tuple(tupdesc, values, nulls);
+	result = HeapTupleGetDatum(tuple);
+
+	PG_RETURN_DATUM(result);
+#undef PG_GET_FIRST_VALID_WAL_RECORD_LSN_COLS
+}
+
+/*
+ * Calculate size of a record, split into !FPI and FPI parts.
+ */
+static void
+GetXLogRecordLen(XLogReaderState *record, uint32 *rec_len, uint32 *fpi_len)
+{
+	int	block_id;
+
+	/*
+	 * Calculate the amount of FPI data in the record.
+	 *
+	 * XXX: We peek into xlogreader's private decoded backup blocks for the
+	 * bimg_len indicating the length of FPI data. It doesn't seem worth it to
+	 * add an accessor macro for this.
+	 */
+	*fpi_len = 0;
+	for (block_id = 0; block_id <= record->max_block_id; block_id++)
+	{
+		if (XLogRecHasBlockImage(record, block_id))
+			*fpi_len += record->blocks[block_id].bimg_len;
+	}
+
+	/*
+	 * Calculate the length of the record as the total length - the length of
+	 * all the block images.
+	 */
+	*rec_len = XLogRecGetTotalLen(record) - *fpi_len;
+}
+
+/*
+ * Get WAL record info.
+ */
+static void
+GetXLogRecordInfo(XLogReaderState *record, XLogRecPtr lsn,
+				  Datum *values, bool *nulls, uint32 ncols)
+{
+	const char *id;
+	const RmgrData *desc;
+	uint32	rec_len;
+	uint32	fpi_len;
+	RelFileNode rnode;
+	ForkNumber	forknum;
+	BlockNumber blk;
+	int	block_id;
+	StringInfoData rec_desc;
+	StringInfoData rec_blk_ref;
+	StringInfoData temp;
+	bytea	*data;
+	char	*main_data;
+	uint32	main_data_len;
+	int	i = 0;
+
+	desc = &RmgrTable[XLogRecGetRmid(record)];
+
+	GetXLogRecordLen(record, &rec_len, &fpi_len);
+
+	values[i++] = LSNGetDatum(lsn);
+	values[i++] = LSNGetDatum(XLogRecGetPrev(record));
+	values[i++] = TransactionIdGetDatum(XLogRecGetXid(record));
+	values[i++] = CStringGetTextDatum(desc->rm_name);
+	values[i++] = UInt32GetDatum(rec_len);
+	values[i++] = UInt32GetDatum(XLogRecGetTotalLen(record));
+
+	initStringInfo(&rec_desc);
+
+	id = desc->rm_identify(XLogRecGetInfo(record));
+
+	if (id == NULL)
+		appendStringInfo(&rec_desc, "UNKNOWN (%x) ", XLogRecGetInfo(record) & ~XLR_INFO_MASK);
+	else
+		appendStringInfo(&rec_desc, "%s ", id);
+
+	initStringInfo(&temp);
+
+	desc->rm_desc(&temp, record);
+	appendStringInfo(&rec_desc, "%s", temp.data);
+
+	values[i++] = CStringGetTextDatum(rec_desc.data);
+
+	pfree(temp.data);
+
+	initStringInfo(&rec_blk_ref);
+
+	/* Block references (detailed format). */
+	for (block_id = 0; block_id <= record->max_block_id; block_id++)
+	{
+		if (!XLogRecHasBlockRef(record, block_id))
+			continue;
+
+		XLogRecGetBlockTag(record, block_id, &rnode, &forknum, &blk);
+
+		if (forknum != MAIN_FORKNUM)
+		{
+			appendStringInfo(&rec_blk_ref,
+							"blkref #%u: rel %u/%u/%u fork %s blk %u",
+							block_id, rnode.spcNode, rnode.dbNode,
+							rnode.relNode, get_forkname(forknum), blk);
+		}
+		else
+		{
+			appendStringInfo(&rec_blk_ref,
+							"blkref #%u: rel %u/%u/%u blk %u",
+							block_id, rnode.spcNode, rnode.dbNode,
+							rnode.relNode, blk);
+		}
+
+		if (XLogRecHasBlockImage(record, block_id))
+		{
+			uint8		bimg_info = record->blocks[block_id].bimg_info;
+
+			if (BKPIMAGE_COMPRESSED(bimg_info))
+			{
+				const char *method;
+
+				if ((bimg_info & BKPIMAGE_COMPRESS_PGLZ) != 0)
+					method = "pglz";
+				else if ((bimg_info & BKPIMAGE_COMPRESS_LZ4) != 0)
+					method = "lz4";
+				else
+					method = "unknown";
+
+				appendStringInfo(&rec_blk_ref, " (FPW%s); hole: offset: %u, length: %u, "
+								 "compression saved: %u, method: %s",
+								 XLogRecBlockImageApply(record, block_id) ?
+								 "" : " for WAL verification",
+								 record->blocks[block_id].hole_offset,
+								 record->blocks[block_id].hole_length,
+								 BLCKSZ -
+								 record->blocks[block_id].hole_length -
+								 record->blocks[block_id].bimg_len,
+								 method);
+			}
+			else
+			{
+				appendStringInfo(&rec_blk_ref, " (FPW%s); hole: offset: %u, length: %u",
+								 XLogRecBlockImageApply(record, block_id) ?
+								 "" : " for WAL verification",
+								 record->blocks[block_id].hole_offset,
+								 record->blocks[block_id].hole_length);
+			}
+		}
+	}
+
+	values[i++] = CStringGetTextDatum(rec_blk_ref.data);
+
+	main_data_len = XLogRecGetDataLen(record);
+
+	data = (bytea *) palloc(main_data_len + VARHDRSZ);
+	SET_VARSIZE(data, main_data_len + VARHDRSZ);
+	main_data = VARDATA(data);
+
+	memcpy(main_data, XLogRecGetData(record), main_data_len);
+
+	values[i++] = PointerGetDatum(data);
+	values[i++] = UInt32GetDatum(main_data_len);
+
+	Assert(i == ncols);
+}
+
+/*
+ * Get WAL record info and data.
+ *
+ * This function emits an error if a future WAL LSN i.e. WAL LSN the database
+ * system doesn't know about is specified.
+ */
+Datum
+pg_get_wal_record_info(PG_FUNCTION_ARGS)
+{
+#define PG_GET_WAL_RECORD_INFO_COLS 10
+	XLogRecPtr	lsn;
+	XLogRecPtr	first_record;
+	XLogReaderState *xlogreader;
+	TupleDesc	tupdesc;
+	Datum	result;
+	HeapTuple	tuple;
+	Datum	values[PG_GET_WAL_RECORD_INFO_COLS];
+	bool	nulls[PG_GET_WAL_RECORD_INFO_COLS];
+
+	lsn = PG_GETARG_LSN(0);
+
+	(void) ValidateInputLSN(lsn);
+
+	/* Build a tuple descriptor for our result type. */
+	if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
+		elog(ERROR, "return type must be a row type");
+
+	xlogreader = InitXLogReaderState(lsn, &first_record);
+
+	Assert(xlogreader);
+
+	(void) ReadNextXLogRecord(xlogreader, first_record);
+
+	MemSet(values, 0, sizeof(values));
+	MemSet(nulls, 0, sizeof(nulls));
+
+	GetXLogRecordInfo(xlogreader, first_record, values, nulls,
+					  PG_GET_WAL_RECORD_INFO_COLS);
+
+	XLogReaderFree(xlogreader);
+
+	tuple = heap_form_tuple(tupdesc, values, nulls);
+	result = HeapTupleGetDatum(tuple);
+
+	PG_RETURN_DATUM(result);
+#undef PG_GET_WAL_RECORD_INFO_COLS
+}
+
+/*
+ * Get info and data of all WAL records between start LSN and end LSN.
+ */
+static void
+GetWALRecordsInfoInternal(FunctionCallInfo fcinfo, XLogRecPtr start_lsn,
+						  XLogRecPtr end_lsn)
+{
+#define PG_GET_WAL_RECORDS_INFO_COLS 10
+	XLogRecPtr	first_record;
+	XLogReaderState *xlogreader;
+	ReturnSetInfo *rsinfo;
+	TupleDesc	tupdesc;
+	Tuplestorestate *tupstore;
+	MemoryContext per_query_ctx;
+	MemoryContext oldcontext;
+	Datum	values[PG_GET_WAL_RECORDS_INFO_COLS];
+	bool	nulls[PG_GET_WAL_RECORDS_INFO_COLS];
+
+	rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
+
+	/* Check to see if caller supports us returning a tuplestore. */
+	if (rsinfo == NULL || !IsA(rsinfo, ReturnSetInfo))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("set-valued function called in context that cannot accept a set")));
+	if (!(rsinfo->allowedModes & SFRM_Materialize))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("materialize mode required, but it is not allowed in this context")));
+
+	/* Build a tuple descriptor for our result type. */
+	if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
+		elog(ERROR, "return type must be a row type");
+
+	/* Build tuplestore to hold the result rows. */
+	per_query_ctx = rsinfo->econtext->ecxt_per_query_memory;
+	oldcontext = MemoryContextSwitchTo(per_query_ctx);
+
+	tupstore = tuplestore_begin_heap(true, false, work_mem);
+	rsinfo->returnMode = SFRM_Materialize;
+	rsinfo->setResult = tupstore;
+	rsinfo->setDesc = tupdesc;
+
+	MemoryContextSwitchTo(oldcontext);
+
+	xlogreader = InitXLogReaderState(start_lsn, &first_record);
+
+	Assert(xlogreader);
+
+	MemSet(values, 0, sizeof(values));
+	MemSet(nulls, 0, sizeof(nulls));
+
+	for (;;)
+	{
+		/* Exit loop if read up to end_lsn. */
+		if (!XLogRecPtrIsInvalid(end_lsn) &&
+			xlogreader->EndRecPtr >= end_lsn)
+			break;
+
+		(void) ReadNextXLogRecord(xlogreader, first_record);
+
+		GetXLogRecordInfo(xlogreader, xlogreader->currRecPtr, values, nulls,
+						  PG_GET_WAL_RECORDS_INFO_COLS);
+
+		tuplestore_putvalues(tupstore, tupdesc, values, nulls);
+
+		CHECK_FOR_INTERRUPTS();
+	}
+
+	XLogReaderFree(xlogreader);
+
+	/* Clean up and return the tuplestore. */
+	tuplestore_donestoring(tupstore);
+#undef PG_GET_WAL_RECORDS_INFO_COLS
+}
+
+/*
+ * Get info and data of all WAL records between start LSN and end LSN.
+ *
+ * This function emits an error if a future end WAL LSN i.e. WAL LSN the
+ * database system doesn't know about is specified.
+ *
+ * This function will figure out the end LSN if it's not specified.
+ */
+Datum
+pg_get_wal_records_info(PG_FUNCTION_ARGS)
+{
+	XLogRecPtr	start_lsn;
+	XLogRecPtr	end_lsn;
+
+	start_lsn = PG_GETARG_LSN(0);
+	end_lsn = PG_GETARG_LSN(1);
+
+	end_lsn = ValidateStartAndEndLSNs(start_lsn, end_lsn);
+
+	GetWALRecordsInfoInternal(fcinfo, start_lsn, end_lsn);
+
+	PG_RETURN_VOID();
+}
+
+/*
+ * Store per-rmgr and per-record statistics for a given record.
+ */
+static void
+StoreXLogRecordStats(XLogRecStats * stats, XLogReaderState *record)
+{
+	RmgrId	rmid;
+	uint8	recid;
+	uint32	rec_len;
+	uint32	fpi_len;
+
+	stats->count++;
+
+	rmid = XLogRecGetRmid(record);
+
+	GetXLogRecordLen(record, &rec_len, &fpi_len);
+
+	/* Update per-rmgr statistics. */
+	stats->rmgr_stats[rmid].count++;
+	stats->rmgr_stats[rmid].rec_len += rec_len;
+	stats->rmgr_stats[rmid].fpi_len += fpi_len;
+
+	/*
+	 * Update per-record statistics, where the record is identified by a
+	 * combination of the RmgrId and the four bits of the xl_info field that
+	 * are the rmgr's domain (resulting in sixteen possible entries per
+	 * RmgrId).
+	 */
+	recid = XLogRecGetInfo(record) >> 4;
+
+	/*
+	 * XACT records need to be handled differently. Those records use the
+	 * first bit of those four bits for an optional flag variable and the
+	 * following three bits for the opcode. We filter opcode out of xl_info
+	 * and use it as the identifier of the record.
+	 */
+	if (rmid == RM_XACT_ID)
+		recid &= 0x07;
+
+	stats->record_stats[rmid][recid].count++;
+	stats->record_stats[rmid][recid].rec_len += rec_len;
+	stats->record_stats[rmid][recid].fpi_len += fpi_len;
+}
+
+/*
+ * Fill single row of record counts and sizes for an rmgr or record.
+ */
+static void
+FillXLogStatsRow(const char *name,
+				 uint64 n, uint64 total_count,
+				 uint64 rec_len, uint64 total_rec_len,
+				 uint64 fpi_len, uint64 total_fpi_len,
+				 uint64 tot_len, uint64 total_len,
+				 Datum *values, bool *nulls, uint32 ncols)
+{
+	double	n_pct;
+	double	rec_len_pct;
+	double	fpi_len_pct;
+	double	tot_len_pct;
+	int	i = 0;
+
+	n_pct = 0;
+	if (total_count != 0)
+		n_pct = 100 * (double) n / total_count;
+
+	rec_len_pct = 0;
+	if (total_rec_len != 0)
+		rec_len_pct = 100 * (double) rec_len / total_rec_len;
+
+	fpi_len_pct = 0;
+	if (total_fpi_len != 0)
+		fpi_len_pct = 100 * (double) fpi_len / total_fpi_len;
+
+	tot_len_pct = 0;
+	if (total_len != 0)
+		tot_len_pct = 100 * (double) tot_len / total_len;
+
+	values[i++] = CStringGetTextDatum(name);
+	values[i++] = Int64GetDatum(n);
+	values[i++] = Float4GetDatum(n_pct);
+	values[i++] = Int64GetDatum(rec_len);
+	values[i++] = Float4GetDatum(rec_len_pct);
+	values[i++] = Int64GetDatum(fpi_len);
+	values[i++] = Float4GetDatum(fpi_len_pct);
+	values[i++] = Int64GetDatum(tot_len);
+	values[i++] = Float4GetDatum(tot_len_pct);
+
+	Assert(i == ncols);
+}
+
+/*
+ * Get summary statistics about the records seen so far.
+ */
+static void
+GetXLogSummaryStats(XLogRecStats * stats, Tuplestorestate *tupstore,
+					TupleDesc tupdesc, Datum *values, bool *nulls,
+					uint32 ncols)
+{
+	uint64	total_count = 0;
+	uint64	total_rec_len = 0;
+	uint64	total_fpi_len = 0;
+	uint64	total_len = 0;
+	int	ri;
+
+	/*
+	 * Each row shows its percentages of the total, so make a first pass to
+	 * calculate column totals.
+	 */
+	for (ri = 0; ri < RM_NEXT_ID; ri++)
+	{
+		total_count += stats->rmgr_stats[ri].count;
+		total_rec_len += stats->rmgr_stats[ri].rec_len;
+		total_fpi_len += stats->rmgr_stats[ri].fpi_len;
+	}
+	total_len = total_rec_len + total_fpi_len;
+
+	for (ri = 0; ri < RM_NEXT_ID; ri++)
+	{
+		uint64		count;
+		uint64		rec_len;
+		uint64		fpi_len;
+		uint64		tot_len;
+		const RmgrData *desc = &RmgrTable[ri];
+
+		count = stats->rmgr_stats[ri].count;
+		rec_len = stats->rmgr_stats[ri].rec_len;
+		fpi_len = stats->rmgr_stats[ri].fpi_len;
+		tot_len = rec_len + fpi_len;
+
+		FillXLogStatsRow(desc->rm_name, count, total_count, rec_len,
+						 total_rec_len, fpi_len, total_fpi_len, tot_len,
+						 total_len, values, nulls, ncols);
+
+		tuplestore_putvalues(tupstore, tupdesc, values, nulls);
+	}
+}
+
+/*
+ * Get WAL stats between start LSN and end LSN.
+ */
+static void
+GetWalStatsInternal(FunctionCallInfo fcinfo, XLogRecPtr start_lsn,
+					XLogRecPtr end_lsn)
+{
+#define PG_GET_WAL_STATS_COLS 9
+	XLogRecPtr	first_record;
+	XLogReaderState *xlogreader;
+	XLogRecStats stats;
+	ReturnSetInfo *rsinfo;
+	TupleDesc	tupdesc;
+	Tuplestorestate *tupstore;
+	MemoryContext per_query_ctx;
+	MemoryContext oldcontext;
+	Datum	values[PG_GET_WAL_STATS_COLS];
+	bool	nulls[PG_GET_WAL_STATS_COLS];
+
+	rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
+
+	/* Check to see if caller supports us returning a tuplestore. */
+	if (rsinfo == NULL || !IsA(rsinfo, ReturnSetInfo))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("set-valued function called in context that cannot accept a set")));
+	if (!(rsinfo->allowedModes & SFRM_Materialize))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("materialize mode required, but it is not allowed in this context")));
+
+	/* Build a tuple descriptor for our result type. */
+	if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
+		elog(ERROR, "return type must be a row type");
+
+	/* Build tuplestore to hold the result rows. */
+	per_query_ctx = rsinfo->econtext->ecxt_per_query_memory;
+	oldcontext = MemoryContextSwitchTo(per_query_ctx);
+
+	tupstore = tuplestore_begin_heap(true, false, work_mem);
+	rsinfo->returnMode = SFRM_Materialize;
+	rsinfo->setResult = tupstore;
+	rsinfo->setDesc = tupdesc;
+
+	MemoryContextSwitchTo(oldcontext);
+
+	xlogreader = InitXLogReaderState(start_lsn, &first_record);
+
+	MemSet(&stats, 0, sizeof(stats));
+
+	for (;;)
+	{
+		/* Exit loop if read up to end_lsn. */
+		if (!XLogRecPtrIsInvalid(end_lsn) &&
+			xlogreader->EndRecPtr >= end_lsn)
+			break;
+
+		(void) ReadNextXLogRecord(xlogreader, first_record);
+
+		StoreXLogRecordStats(&stats, xlogreader);
+
+		CHECK_FOR_INTERRUPTS();
+	}
+
+	XLogReaderFree(xlogreader);
+
+	MemSet(values, 0, sizeof(values));
+	MemSet(nulls, 0, sizeof(nulls));
+
+	GetXLogSummaryStats(&stats, tupstore, tupdesc, values, nulls,
+						PG_GET_WAL_STATS_COLS);
+
+	/* Clean up and return the tuplestore. */
+	tuplestore_donestoring(tupstore);
+#undef PG_GET_WAL_STATS_COLS
+}
+
+/*
+ * Get WAL stats between start LSN and end LSN.
+ *
+ * This function emits an error if a future end WAL LSN i.e. WAL LSN the
+ * database system doesn't know about is specified.
+ *
+ * This function will figure out the end LSN if it's not specified.
+ */
+Datum
+pg_get_wal_stats(PG_FUNCTION_ARGS)
+{
+	XLogRecPtr	start_lsn;
+	XLogRecPtr	end_lsn;
+
+	start_lsn = PG_GETARG_LSN(0);
+	end_lsn = PG_GETARG_LSN(1);
+
+	end_lsn = ValidateStartAndEndLSNs(start_lsn, end_lsn);
+
+	GetWalStatsInternal(fcinfo, start_lsn, end_lsn);
+
+	PG_RETURN_VOID();
+}
diff --git a/contrib/pg_walinspect/pg_walinspect.control b/contrib/pg_walinspect/pg_walinspect.control
new file mode 100644
index 0000000000..017e56a2bb
--- /dev/null
+++ b/contrib/pg_walinspect/pg_walinspect.control
@@ -0,0 +1,5 @@
+# pg_walinspect extension
+comment = 'functions to inspect contents of PostgreSQL Write-Ahead Log'
+default_version = '1.0'
+module_pathname = '$libdir/pg_walinspect'
+relocatable = true
diff --git a/src/backend/access/transam/xlogreader.c b/src/backend/access/transam/xlogreader.c
index 35029cf97d..4d217cabbe 100644
--- a/src/backend/access/transam/xlogreader.c
+++ b/src/backend/access/transam/xlogreader.c
@@ -956,13 +956,6 @@ XLogReaderValidatePageHeader(XLogReaderState *state, XLogRecPtr recptr,
 	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.
  *
@@ -1080,6 +1073,13 @@ err:
 	return InvalidXLogRecPtr;
 }
 
+#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.
+ */
+
 #endif							/* FRONTEND */
 
 /*
diff --git a/src/bin/pg_waldump/pg_waldump.c b/src/bin/pg_waldump/pg_waldump.c
index 2340dc247b..60940d30c2 100644
--- a/src/bin/pg_waldump/pg_waldump.c
+++ b/src/bin/pg_waldump/pg_waldump.c
@@ -26,6 +26,11 @@
 #include "getopt_long.h"
 #include "rmgrdesc.h"
 
+/*
+ * NOTE: For any code change or issue fix here, it is highly recommended to
+ * give a thought about doing the same in pg_walinspect contrib module as well.
+ */
+
 static const char *progname;
 
 static int	WalSegSz;
diff --git a/src/common/relpath.c b/src/common/relpath.c
index 636c96efd3..e8e3c44eae 100644
--- a/src/common/relpath.c
+++ b/src/common/relpath.c
@@ -40,6 +40,24 @@ const char *const forkNames[] = {
 StaticAssertDecl(lengthof(forkNames) == (MAX_FORKNUM + 1),
 				 "array length mismatch");
 
+/*
+ * get_forkname - return fork name given fork number
+ *
+ * This function is defined with "extern PGDLLIMPORT ..." in the core here so
+ * that the loadable modules can access it.
+ */
+const char *const
+get_forkname(ForkNumber num)
+{
+	/*
+	 * As this function gets called by external modules, let's ensure that the
+	 * fork number passed in is valid.
+	 */
+	Assert(num > InvalidForkNumber && num <= MAX_FORKNUM);
+
+	return forkNames[num];
+}
+
 /*
  * forkname_to_number - look up fork number by name
  *
diff --git a/src/include/access/xlog.h b/src/include/access/xlog.h
index 4b45ac64db..f34f228563 100644
--- a/src/include/access/xlog.h
+++ b/src/include/access/xlog.h
@@ -31,7 +31,7 @@ extern XLogRecPtr XactLastRecEnd;
 extern PGDLLIMPORT XLogRecPtr XactLastCommitEnd;
 
 /* these variables are GUC parameters related to XLOG */
-extern int	wal_segment_size;
+extern PGDLLIMPORT int	wal_segment_size;
 extern int	min_wal_size_mb;
 extern int	max_wal_size_mb;
 extern int	wal_keep_size_mb;
diff --git a/src/include/access/xlog_internal.h b/src/include/access/xlog_internal.h
index 849954a8e5..38fd51bafa 100644
--- a/src/include/access/xlog_internal.h
+++ b/src/include/access/xlog_internal.h
@@ -319,7 +319,7 @@ typedef struct RmgrData
 							  struct XLogRecordBuffer *buf);
 } RmgrData;
 
-extern const RmgrData RmgrTable[];
+extern PGDLLIMPORT const RmgrData RmgrTable[];
 
 /*
  * Exported to support xlog switching from checkpointer
diff --git a/src/include/access/xlogreader.h b/src/include/access/xlogreader.h
index 477f0efe26..d62d6ce7f8 100644
--- a/src/include/access/xlogreader.h
+++ b/src/include/access/xlogreader.h
@@ -276,9 +276,7 @@ extern void XLogReaderFree(XLogReaderState *state);
 
 /* Position the XLogReader to given record */
 extern void XLogBeginRead(XLogReaderState *state, XLogRecPtr RecPtr);
-#ifdef FRONTEND
 extern XLogRecPtr XLogFindNextRecord(XLogReaderState *state, XLogRecPtr RecPtr);
-#endif							/* FRONTEND */
 
 /* Read the next XLog record. Returns NULL on end-of-WAL or failure */
 extern struct XLogRecord *XLogReadRecord(XLogReaderState *state,
diff --git a/src/include/common/relpath.h b/src/include/common/relpath.h
index a4b5dc853b..3743f2e505 100644
--- a/src/include/common/relpath.h
+++ b/src/include/common/relpath.h
@@ -57,6 +57,7 @@ typedef enum ForkNumber
 #define FORKNAMECHARS	4		/* max chars for a fork name */
 
 extern const char *const forkNames[];
+extern PGDLLIMPORT const char *const get_forkname(ForkNumber num);
 
 extern ForkNumber forkname_to_number(const char *forkName);
 extern int	forkname_chars(const char *str, ForkNumber *fork);
-- 
2.25.1