Add new 'old_snapshot' contrib module.

You can use this to view the contents of the time to XID mapping
which the server maintains when old_snapshot_threshold != -1.
Being able to view that information may be interesting for users,
and it's definitely useful for figuring out whether the mapping
is being maintained correctly. It isn't, so that will need to be
fixed in a subsequent commit.

Patch by me, reviewed by Thomas Munro, Dilip Kumar, Hamid Akhtar.

Discussion: http://postgr.es/m/CA+TgmoY=aqf0zjTD+3dUWYkgMiNDegDLFjo+6ze=Wtpik+3XqA@mail.gmail.com

contrib/Makefile

@@ -27,6 +27,7 @@ SUBDIRS = \
 		lo		\
 		ltree		\
 		oid2name	\
+		old_snapshot	\
 		pageinspect	\
 		passwordcheck	\
 		pg_buffercache	\

contrib/old_snapshot/Makefile

@@ -0,0 +1,22 @@
+# contrib/old_snapshot/Makefile
+
+MODULE_big = old_snapshot
+OBJS = \
+	$(WIN32RES) \
+	time_mapping.o
+PG_CPPFLAGS = -I$(libpq_srcdir)
+
+EXTENSION = old_snapshot
+DATA = old_snapshot--1.0.sql
+PGFILEDESC = "old_snapshot - utilities in support of old_snapshot_threshold"
+
+ifdef USE_PGXS
+PG_CONFIG = pg_config
+PGXS := $(shell $(PG_CONFIG) --pgxs)
+include $(PGXS)
+else
+subdir = contrib/old_snapshot
+top_builddir = ../..
+include $(top_builddir)/src/Makefile.global
+include $(top_srcdir)/contrib/contrib-global.mk
+endif

contrib/old_snapshot/old_snapshot--1.0.sql

@@ -0,0 +1,14 @@
+/* contrib/old_snapshot/old_snapshot--1.0.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION old_snapshot" to load this file. \quit
+
+-- Show visibility map and page-level visibility information for each block.
+CREATE FUNCTION pg_old_snapshot_time_mapping(array_offset OUT int4,
+											 end_timestamp OUT timestamptz,
+											 newest_xmin OUT xid)
+RETURNS SETOF record
+AS 'MODULE_PATHNAME', 'pg_old_snapshot_time_mapping'
+LANGUAGE C STRICT;
+
+-- XXX. Do we want REVOKE commands here?

contrib/old_snapshot/old_snapshot.control

@@ -0,0 +1,5 @@
+# old_snapshot extension
+comment = 'utilities in support of old_snapshot_threshold'
+default_version = '1.0'
+module_pathname = '$libdir/old_snapshot'
+relocatable = true

contrib/old_snapshot/time_mapping.c

@@ -0,0 +1,159 @@
+/*-------------------------------------------------------------------------
+ *
+ * time_mapping.c
+ *	  time to XID mapping information
+ *
+ * Copyright (c) 2020, PostgreSQL Global Development Group
+ *
+ *	  contrib/old_snapshot/time_mapping.c
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "funcapi.h"
+#include "storage/lwlock.h"
+#include "utils/old_snapshot.h"
+#include "utils/snapmgr.h"
+#include "utils/timestamp.h"
+
+/*
+ * Backend-private copy of the information from oldSnapshotControl which relates
+ * to the time to XID mapping, plus an index so that we can iterate.
+ *
+ * Note that the length of the xid_by_minute array is given by
+ * OLD_SNAPSHOT_TIME_MAP_ENTRIES (which is not a compile-time constant).
+ */
+typedef struct
+{
+	int				current_index;
+	int				head_offset;
+	TimestampTz		head_timestamp;
+	int				count_used;
+	TransactionId	xid_by_minute[FLEXIBLE_ARRAY_MEMBER];
+} OldSnapshotTimeMapping;
+
+#define NUM_TIME_MAPPING_COLUMNS 3
+
+PG_MODULE_MAGIC;
+PG_FUNCTION_INFO_V1(pg_old_snapshot_time_mapping);
+
+static OldSnapshotTimeMapping *GetOldSnapshotTimeMapping(void);
+static TupleDesc MakeOldSnapshotTimeMappingTupleDesc(void);
+static HeapTuple MakeOldSnapshotTimeMappingTuple(TupleDesc tupdesc,
+												 OldSnapshotTimeMapping *mapping);
+
+/*
+ * SQL-callable set-returning function.
+ */
+Datum
+pg_old_snapshot_time_mapping(PG_FUNCTION_ARGS)
+{
+	FuncCallContext *funcctx;
+	OldSnapshotTimeMapping *mapping;
+
+	if (SRF_IS_FIRSTCALL())
+	{
+		MemoryContext	oldcontext;
+
+		funcctx = SRF_FIRSTCALL_INIT();
+		oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
+		mapping = GetOldSnapshotTimeMapping();
+		funcctx->user_fctx = mapping;
+		funcctx->tuple_desc = MakeOldSnapshotTimeMappingTupleDesc();
+		MemoryContextSwitchTo(oldcontext);
+	}
+
+	funcctx = SRF_PERCALL_SETUP();
+	mapping = (OldSnapshotTimeMapping *) funcctx->user_fctx;
+
+	while (mapping->current_index < mapping->count_used)
+	{
+		HeapTuple	tuple;
+
+		tuple = MakeOldSnapshotTimeMappingTuple(funcctx->tuple_desc, mapping);
+		++mapping->current_index;
+		SRF_RETURN_NEXT(funcctx, HeapTupleGetDatum(tuple));
+	}
+
+	SRF_RETURN_DONE(funcctx);
+}
+
+/*
+ * Get the old snapshot time mapping data from shared memory.
+ */
+static OldSnapshotTimeMapping *
+GetOldSnapshotTimeMapping(void)
+{
+	OldSnapshotTimeMapping *mapping;
+
+	mapping = palloc(offsetof(OldSnapshotTimeMapping, xid_by_minute)
+					 + sizeof(TransactionId) * OLD_SNAPSHOT_TIME_MAP_ENTRIES);
+	mapping->current_index = 0;
+
+	LWLockAcquire(OldSnapshotTimeMapLock, LW_SHARED);
+	mapping->head_offset = oldSnapshotControl->head_offset;
+	mapping->head_timestamp = oldSnapshotControl->head_timestamp;
+	mapping->count_used = oldSnapshotControl->count_used;
+	for (int i = 0; i < OLD_SNAPSHOT_TIME_MAP_ENTRIES; ++i)
+		mapping->xid_by_minute[i] = oldSnapshotControl->xid_by_minute[i];
+	LWLockRelease(OldSnapshotTimeMapLock);
+
+	return mapping;
+}
+
+/*
+ * Build a tuple descriptor for the pg_old_snapshot_time_mapping() SRF.
+ */
+static TupleDesc
+MakeOldSnapshotTimeMappingTupleDesc(void)
+{
+	TupleDesc	tupdesc;
+
+	tupdesc = CreateTemplateTupleDesc(NUM_TIME_MAPPING_COLUMNS);
+
+	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "array_offset",
+					   INT4OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "end_timestamp",
+					   TIMESTAMPTZOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 3, "newest_xmin",
+					   XIDOID, -1, 0);
+
+	return BlessTupleDesc(tupdesc);
+}
+
+/*
+ * Convert one entry from the old snapshot time mapping to a HeapTuple.
+ */
+static HeapTuple
+MakeOldSnapshotTimeMappingTuple(TupleDesc tupdesc, OldSnapshotTimeMapping *mapping)
+{
+	Datum	values[NUM_TIME_MAPPING_COLUMNS];
+	bool	nulls[NUM_TIME_MAPPING_COLUMNS];
+	int		array_position;
+	TimestampTz	timestamp;
+
+	/*
+	 * Figure out the array position corresponding to the current index.
+	 *
+	 * Index 0 means the oldest entry in the mapping, which is stored at
+	 * mapping->head_offset. Index 1 means the next-oldest entry, which is a the
+	 * following index, and so on. We wrap around when we reach the end of the array.
+	 */
+	array_position = (mapping->head_offset + mapping->current_index)
+		% OLD_SNAPSHOT_TIME_MAP_ENTRIES;
+
+	/*
+	 * No explicit timestamp is stored for any entry other than the oldest one,
+	 * but each entry corresponds to 1-minute period, so we can just add.
+	 */
+	timestamp = TimestampTzPlusMilliseconds(mapping->head_timestamp,
+											mapping->current_index * 60000);
+
+	/* Initialize nulls and values arrays. */
+	memset(nulls, 0, sizeof(nulls));
+	values[0] = Int32GetDatum(array_position);
+	values[1] = TimestampTzGetDatum(timestamp);
+	values[2] = TransactionIdGetDatum(mapping->xid_by_minute[array_position]);
+
+	return heap_form_tuple(tupdesc, values, nulls);
+}

doc/src/sgml/contrib.sgml

@@ -116,6 +116,7 @@ CREATE EXTENSION <replaceable>module_name</replaceable>;
  &isn;
  &lo;
  &ltree;
+ &oldsnapshot;
  &pageinspect;
  &passwordcheck;
  &pgbuffercache;

doc/src/sgml/filelist.sgml

@@ -129,6 +129,7 @@
 <!ENTITY lo              SYSTEM "lo.sgml">
 <!ENTITY ltree           SYSTEM "ltree.sgml">
 <!ENTITY oid2name        SYSTEM "oid2name.sgml">
+<!ENTITY oldsnapshot     SYSTEM "oldsnapshot.sgml">
 <!ENTITY pageinspect     SYSTEM "pageinspect.sgml">
 <!ENTITY passwordcheck   SYSTEM "passwordcheck.sgml">
 <!ENTITY pgbuffercache   SYSTEM "pgbuffercache.sgml">

doc/src/sgml/oldsnapshot.sgml

@@ -0,0 +1,33 @@
+<!-- doc/src/sgml/oldsnapshot.sgml -->
+
+<sect1 id="oldsnapshot" xreflabel="old_snapshot">
+ <title>old_snapshot</title>
+
+ <indexterm zone="oldsnapshot">
+  <primary>old_snapshot</primary>
+ </indexterm>
+
+ <para>
+  The <filename>old_snapshot</filename> module allows inspection
+  of the server state that is used to implement
+  <xref linkend="guc-old-snapshot-threshold" />.
+ </para>
+
+ <sect2>
+  <title>Functions</title>
+
+  <variablelist>
+   <varlistentry>
+    <term><function>pg_old_snapshot_time_mapping(array_offset OUT int4, end_timestamp OUT timestamptz, newest_xmin OUT xid) returns setof record</function></term>
+    <listitem>
+     <para>
+      Returns all of the entries in the server's timestamp to XID mapping.
+      Each entry represents the newest xmin of any snapshot taken in the
+      corresponding minute.
+     </para>
+    </listitem>
+   </varlistentry>
+  </variablelist>
+ </sect2>
+
+</sect1>