From 8829af9a646444b2588c268cf1a2912b39e0c645 Mon Sep 17 00:00:00 2001
From: Robert Haas <rhaas@postgresql.org>
Date: Thu, 16 Apr 2020 12:14:32 -0400
Subject: [PATCH v1 2/3] contrib/old_snapshot: time->xid mapping.

---
 contrib/Makefile                           |   1 +
 contrib/old_snapshot/Makefile              |  24 ++++
 contrib/old_snapshot/old_snapshot--1.0.sql |  14 ++
 contrib/old_snapshot/old_snapshot.control  |   5 +
 contrib/old_snapshot/time_mapping.c        | 159 +++++++++++++++++++++
 5 files changed, 203 insertions(+)
 create mode 100644 contrib/old_snapshot/Makefile
 create mode 100644 contrib/old_snapshot/old_snapshot--1.0.sql
 create mode 100644 contrib/old_snapshot/old_snapshot.control
 create mode 100644 contrib/old_snapshot/time_mapping.c

diff --git a/contrib/Makefile b/contrib/Makefile
index 1846d415b6..452ade0782 100644
--- a/contrib/Makefile
+++ b/contrib/Makefile
@@ -27,6 +27,7 @@ SUBDIRS = \
 		lo		\
 		ltree		\
 		oid2name	\
+		old_snapshot	\
 		pageinspect	\
 		passwordcheck	\
 		pg_buffercache	\
diff --git a/contrib/old_snapshot/Makefile b/contrib/old_snapshot/Makefile
new file mode 100644
index 0000000000..091231f25f
--- /dev/null
+++ b/contrib/old_snapshot/Makefile
@@ -0,0 +1,24 @@
+# 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"
+
+REGRESS = old_snapshot
+
+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 $(top_srcdir)/contrib/contrib-global.mk
+endif
diff --git a/contrib/old_snapshot/old_snapshot--1.0.sql b/contrib/old_snapshot/old_snapshot--1.0.sql
new file mode 100644
index 0000000000..9ebb8829e3
--- /dev/null
+++ b/contrib/old_snapshot/old_snapshot--1.0.sql
@@ -0,0 +1,14 @@
+/* contrib/old_snapshot/old_snapshot--1.0.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION old_snapshot" to load this file. \quit
+
+-- Show visibility map and page-level visibility information for each block.
+CREATE FUNCTION pg_old_snapshot_time_mapping(array_offset OUT int4,
+											 end_timestamp OUT timestamptz,
+											 newest_xmin OUT xid)
+RETURNS SETOF record
+AS 'MODULE_PATHNAME', 'pg_old_snapshot_time_mapping'
+LANGUAGE C STRICT;
+
+-- XXX. Do we want REVOKE commands here?
diff --git a/contrib/old_snapshot/old_snapshot.control b/contrib/old_snapshot/old_snapshot.control
new file mode 100644
index 0000000000..491eec536c
--- /dev/null
+++ b/contrib/old_snapshot/old_snapshot.control
@@ -0,0 +1,5 @@
+# old_snapshot extension
+comment = 'utilities in support of old_snapshot_threshold'
+default_version = '1.0'
+module_pathname = '$libdir/old_snapshot'
+relocatable = true
diff --git a/contrib/old_snapshot/time_mapping.c b/contrib/old_snapshot/time_mapping.c
new file mode 100644
index 0000000000..37e0055a00
--- /dev/null
+++ b/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);
+}
-- 
2.17.2 (Apple Git-113)

