Re: Pluggable cumulative statistics

Michael Paquier Wed, 17 Jul 2024 22:56:59 -0700

On Tue, Jul 16, 2024 at 10:27:25AM +0900, Michael Paquier wrote:
> Perhaps we should have a few more inline functions like
> pgstat_get_entry_len() to retrieve the parts of the custom data in the
> snapshot and shmem control structures for fixed-numbered stats.  That
> would limit what extensions need to know about
> pgStatLocal.shmem->custom_data[] and
> pgStatLocal.snapshot.custom_data[], which is easy to use incorrectly.
> They don't need to know about pgStatLocal at all, either.
> 
> Thinking over the weekend on this patch, splitting injection_stats.c
> into two separate files to act as two templates for the variable and
> fixed-numbered cases would be more friendly to developers, as well.


I've been toying a bit with these two ideas, and the result is
actually neater:
- The example for fixed-numbered stats is now in its own new file,
called injection_stats_fixed.c.
- Stats in the dshash are at the same location, injection_stats.c.
- pgstat_internal.h gains two inline routines called
pgstat_get_custom_shmem_data and pgstat_get_custom_snapshot_data that
hide completely the snapshot structure for extensions when it comes to
custom fixed-numbered stats, see the new injection_stats_fixed.c that
uses them.
--
Michael

From 54aa07f04489aca5dc2fed02121639d5fb453abd Mon Sep 17 00:00:00 2001
From: Michael Paquier <mich...@paquier.xyz>
Date: Mon, 8 Jul 2024 12:28:26 +0900
Subject: [PATCH v7 1/6] Switch PgStat_Kind from enum to uint32

A follow-up patch is planned to make this counter extensible, and
keeping a trace of the kind behind a type is useful in the internal
routines used by pgstats.  While on it, switch pgstat_is_kind_valid() to
use PgStat_Kind, to be more consistent with its callers.
---
 src/include/pgstat.h                | 35 ++++++++++++++---------------
 src/backend/utils/activity/pgstat.c |  6 ++---
 2 files changed, 20 insertions(+), 21 deletions(-)

diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 6b99bb8aad..cc97274708 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -32,26 +32,25 @@
 #define PG_STAT_TMP_DIR		"pg_stat_tmp"
 
 /* The types of statistics entries */
-typedef enum PgStat_Kind
-{
-	/* use 0 for INVALID, to catch zero-initialized data */
-	PGSTAT_KIND_INVALID = 0,
+#define PgStat_Kind uint32
 
-	/* stats for variable-numbered objects */
-	PGSTAT_KIND_DATABASE,		/* database-wide statistics */
-	PGSTAT_KIND_RELATION,		/* per-table statistics */
-	PGSTAT_KIND_FUNCTION,		/* per-function statistics */
-	PGSTAT_KIND_REPLSLOT,		/* per-slot statistics */
-	PGSTAT_KIND_SUBSCRIPTION,	/* per-subscription statistics */
+/* use 0 for INVALID, to catch zero-initialized data */
+#define PGSTAT_KIND_INVALID 0
 
-	/* stats for fixed-numbered objects */
-	PGSTAT_KIND_ARCHIVER,
-	PGSTAT_KIND_BGWRITER,
-	PGSTAT_KIND_CHECKPOINTER,
-	PGSTAT_KIND_IO,
-	PGSTAT_KIND_SLRU,
-	PGSTAT_KIND_WAL,
-} PgStat_Kind;
+/* stats for variable-numbered objects */
+#define PGSTAT_KIND_DATABASE	1	/* database-wide statistics */
+#define PGSTAT_KIND_RELATION	2	/* per-table statistics */
+#define PGSTAT_KIND_FUNCTION	3	/* per-function statistics */
+#define PGSTAT_KIND_REPLSLOT	4	/* per-slot statistics */
+#define PGSTAT_KIND_SUBSCRIPTION	5	/* per-subscription statistics */
+
+/* stats for fixed-numbered objects */
+#define PGSTAT_KIND_ARCHIVER	6
+#define PGSTAT_KIND_BGWRITER	7
+#define PGSTAT_KIND_CHECKPOINTER	8
+#define PGSTAT_KIND_IO	9
+#define PGSTAT_KIND_SLRU	10
+#define PGSTAT_KIND_WAL	11
 
 #define PGSTAT_KIND_FIRST_VALID PGSTAT_KIND_DATABASE
 #define PGSTAT_KIND_LAST PGSTAT_KIND_WAL
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index 2e22bf2707..e3058b32c0 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -182,7 +182,7 @@ static void pgstat_prep_snapshot(void);
 static void pgstat_build_snapshot(void);
 static void pgstat_build_snapshot_fixed(PgStat_Kind kind);
 
-static inline bool pgstat_is_kind_valid(int ikind);
+static inline bool pgstat_is_kind_valid(PgStat_Kind kind);
 
 
 /* ----------
@@ -1298,9 +1298,9 @@ pgstat_get_kind_from_str(char *kind_str)
 }
 
 static inline bool
-pgstat_is_kind_valid(int ikind)
+pgstat_is_kind_valid(PgStat_Kind kind)
 {
-	return ikind >= PGSTAT_KIND_FIRST_VALID && ikind <= PGSTAT_KIND_LAST;
+	return kind >= PGSTAT_KIND_FIRST_VALID && kind <= PGSTAT_KIND_LAST;
 }
 
 const PgStat_KindInfo *
-- 
2.45.2

From 07c4be73bb31c06b1c44b2457ab5425f2426cc8c Mon Sep 17 00:00:00 2001
From: Michael Paquier <mich...@paquier.xyz>
Date: Mon, 8 Jul 2024 14:03:17 +0900
Subject: [PATCH v7 2/6] Introduce pluggable APIs for Cumulative Statistics

This commit adds support in the backend for $subject, allowing
out-of-core extensions to add their own custom statistics kinds.  The
stats kinds are divided into two parts for efficiency:
- The built-in stats kinds, with designated initializers.
- The custom kinds, able to use a range of IDs (128 slots available as
of this patch), with information saved in TopMemoryContext.

Custom cumulative statistics can only be loaded with
shared_preload_libraries at startup, and must allocate a unique ID
shared across all the PostgreSQL extension ecosystem with the following
wiki page:
https://wiki.postgresql.org/wiki/CustomCumulativeStats

This is able to support fixed-numbered (like WAL, archiver, bgwriter)
and variable-numbered stats kinds.
---
 src/include/pgstat.h                      |  35 +++-
 src/include/utils/pgstat_internal.h       |  22 ++-
 src/backend/utils/activity/pgstat.c       | 231 +++++++++++++++++++---
 src/backend/utils/activity/pgstat_shmem.c |  31 ++-
 src/backend/utils/adt/pgstatfuncs.c       |   2 +-
 5 files changed, 287 insertions(+), 34 deletions(-)

diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index cc97274708..21c3705ce4 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -34,6 +34,10 @@
 /* The types of statistics entries */
 #define PgStat_Kind uint32
 
+/* Range of IDs allowed, for built-in and custom kinds */
+#define PGSTAT_KIND_MIN	1		/* Minimum ID allowed */
+#define PGSTAT_KIND_MAX	256		/* Maximum ID allowed */
+
 /* use 0 for INVALID, to catch zero-initialized data */
 #define PGSTAT_KIND_INVALID 0
 
@@ -52,9 +56,34 @@
 #define PGSTAT_KIND_SLRU	10
 #define PGSTAT_KIND_WAL	11
 
-#define PGSTAT_KIND_FIRST_VALID PGSTAT_KIND_DATABASE
-#define PGSTAT_KIND_LAST PGSTAT_KIND_WAL
-#define PGSTAT_NUM_KINDS (PGSTAT_KIND_LAST + 1)
+#define PGSTAT_KIND_MIN_BUILTIN PGSTAT_KIND_DATABASE
+#define PGSTAT_KIND_MAX_BUILTIN PGSTAT_KIND_WAL
+
+/* Custom stats kinds */
+
+/* Range of IDs allowed for custom stats kinds */
+#define PGSTAT_KIND_CUSTOM_MIN	128
+#define PGSTAT_KIND_CUSTOM_MAX	PGSTAT_KIND_MAX
+#define PGSTAT_KIND_CUSTOM_SIZE	(PGSTAT_KIND_CUSTOM_MAX - PGSTAT_KIND_CUSTOM_MIN + 1)
+
+/*
+ * PgStat_Kind to use for extensions that require an ID, but are still in
+ * development and have not reserved their own unique kind ID yet. See:
+ * https://wiki.postgresql.org/wiki/CustomCumulativeStats
+ */
+#define PGSTAT_KIND_EXPERIMENTAL	128
+
+static inline bool
+pgstat_is_kind_builtin(PgStat_Kind kind)
+{
+	return kind > PGSTAT_KIND_INVALID && kind <= PGSTAT_KIND_MAX_BUILTIN;
+}
+
+static inline bool
+pgstat_is_kind_custom(PgStat_Kind kind)
+{
+	return kind >= PGSTAT_KIND_CUSTOM_MIN && kind <= PGSTAT_KIND_CUSTOM_MAX;
+}
 
 /* Values for track_functions GUC variable --- order is significant! */
 typedef enum TrackFunctionsLevel
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 778f625ca1..39f63362a3 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -195,7 +195,8 @@ typedef struct PgStat_KindInfo
 
 	/*
 	 * The size of an entry in the shared stats hash table (pointed to by
-	 * PgStatShared_HashEntry->body).
+	 * PgStatShared_HashEntry->body).  For fixed-numbered statistics, this is
+	 * the size of an entry in PgStat_ShmemControl->custom_data.
 	 */
 	uint32		shared_size;
 
@@ -446,6 +447,13 @@ typedef struct PgStat_ShmemControl
 	PgStatShared_IO io;
 	PgStatShared_SLRU slru;
 	PgStatShared_Wal wal;
+
+	/*
+	 * Custom stats data with fixed-numbered objects, indexed by (PgStat_Kind
+	 * - PGSTAT_KIND_CUSTOM_MIN).
+	 */
+	void	   *custom_data[PGSTAT_KIND_CUSTOM_SIZE];
+
 } PgStat_ShmemControl;
 
 
@@ -459,7 +467,7 @@ typedef struct PgStat_Snapshot
 	/* time at which snapshot was taken */
 	TimestampTz snapshot_timestamp;
 
-	bool		fixed_valid[PGSTAT_NUM_KINDS];
+	bool		fixed_valid[PGSTAT_KIND_MAX_BUILTIN + 1];
 
 	PgStat_ArchiverStats archiver;
 
@@ -473,6 +481,14 @@ typedef struct PgStat_Snapshot
 
 	PgStat_WalStats wal;
 
+	/*
+	 * Data in snapshot for custom fixed-numbered statistics, indexed by
+	 * (PgStat_Kind - PGSTAT_KIND_CUSTOM_MIN).  Each entry is allocated in
+	 * TopMemoryContext, for a size of shared_data_len.
+	 */
+	bool		custom_valid[PGSTAT_KIND_CUSTOM_SIZE];
+	void	   *custom_data[PGSTAT_KIND_CUSTOM_SIZE];
+
 	/* to free snapshot in bulk */
 	MemoryContext context;
 	struct pgstat_snapshot_hash *stats;
@@ -516,6 +532,8 @@ static inline void *pgstat_get_entry_data(PgStat_Kind kind, PgStatShared_Common
  */
 
 extern const PgStat_KindInfo *pgstat_get_kind_info(PgStat_Kind kind);
+extern void pgstat_register_kind(PgStat_Kind kind,
+								 const PgStat_KindInfo *kind_info);
 
 #ifdef USE_ASSERT_CHECKING
 extern void pgstat_assert_is_up(void);
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index e3058b32c0..bb70b81e49 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -49,8 +49,16 @@
  * pgStatPending list. Pending statistics updates are flushed out by
  * pgstat_report_stat().
  *
+ * It is possible for external modules to define custom statistics kinds,
+ * that can use the same properties as any built-in stats kinds.  Each custom
+ * stats kind needs to assign a unique ID to ensure that it does not overlap
+ * with other extensions.  In order to reserve a unique stats kind ID, refer
+ * to https://wiki.postgresql.org/wiki/CustomCumulativeStats.
+ *
  * The behavior of different kinds of statistics is determined by the kind's
- * entry in pgstat_kind_infos, see PgStat_KindInfo for details.
+ * entry in pgstat_kind_builtin_infos for all the built-in statistics kinds
+ * defined, and pgstat_kind_custom_infos for custom kinds registered at
+ * startup by pgstat_register_kind().  See PgStat_KindInfo for details.
  *
  * The consistency of read accesses to statistics can be configured using the
  * stats_fetch_consistency GUC (see config.sgml and monitoring.sgml for the
@@ -174,6 +182,8 @@ typedef struct PgStat_SnapshotEntry
 static void pgstat_write_statsfile(void);
 static void pgstat_read_statsfile(void);
 
+static void pgstat_init_snapshot_fixed(void);
+
 static void pgstat_reset_after_failure(void);
 
 static bool pgstat_flush_pending_entries(bool nowait);
@@ -251,7 +261,7 @@ static bool pgstat_is_shutdown = false;
 
 
 /*
- * The different kinds of statistics.
+ * The different kinds of built-in statistics.
  *
  * If reasonably possible, handling specific to one kind of stats should go
  * through this abstraction, rather than making more of pgstat.c aware.
@@ -263,7 +273,7 @@ static bool pgstat_is_shutdown = false;
  * seem to be a great way of doing that, given the split across multiple
  * files.
  */
-static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
+static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_MAX_BUILTIN + 1] = {
 
 	/* stats kinds for variable-numbered objects */
 
@@ -436,6 +446,15 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 	},
 };
 
+/*
+ * Information about custom statistics kinds.
+ *
+ * These are saved in a different array than the built-in kinds to save
+ * in clarity with the initializations.
+ *
+ * Indexed by PGSTAT_KIND_CUSTOM_MIN, of size PGSTAT_KIND_CUSTOM_SIZE.
+ */
+static const PgStat_KindInfo **pgstat_kind_custom_infos = NULL;
 
 /* ------------------------------------------------------------
  * Functions managing the state of the stats system for all backends.
@@ -586,6 +605,8 @@ pgstat_initialize(void)
 
 	pgstat_init_wal();
 
+	pgstat_init_snapshot_fixed();
+
 	/* Set up a process-exit hook to clean up */
 	before_shmem_exit(pgstat_shutdown_hook, 0);
 
@@ -829,6 +850,8 @@ pgstat_clear_snapshot(void)
 
 	memset(&pgStatLocal.snapshot.fixed_valid, 0,
 		   sizeof(pgStatLocal.snapshot.fixed_valid));
+	memset(&pgStatLocal.snapshot.custom_valid, 0,
+		   sizeof(pgStatLocal.snapshot.custom_valid));
 	pgStatLocal.snapshot.stats = NULL;
 	pgStatLocal.snapshot.mode = PGSTAT_FETCH_CONSISTENCY_NONE;
 
@@ -992,7 +1015,29 @@ pgstat_snapshot_fixed(PgStat_Kind kind)
 	else
 		pgstat_build_snapshot_fixed(kind);
 
-	Assert(pgStatLocal.snapshot.fixed_valid[kind]);
+	if (pgstat_is_kind_builtin(kind))
+		Assert(pgStatLocal.snapshot.fixed_valid[kind]);
+	else if (pgstat_is_kind_custom(kind))
+		Assert(pgStatLocal.snapshot.custom_valid[kind - PGSTAT_KIND_CUSTOM_MIN]);
+}
+
+static void
+pgstat_init_snapshot_fixed(void)
+{
+	/*
+	 * Initialize fixed-numbered statistics data in snapshots, only for custom
+	 * stats kinds.
+	 */
+	for (int kind = PGSTAT_KIND_CUSTOM_MIN; kind <= PGSTAT_KIND_CUSTOM_MAX; kind++)
+	{
+		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+
+		if (!kind_info || !kind_info->fixed_amount)
+			continue;
+
+		pgStatLocal.snapshot.custom_data[kind - PGSTAT_KIND_CUSTOM_MIN] =
+			MemoryContextAlloc(TopMemoryContext, kind_info->shared_data_len);
+	}
 }
 
 static void
@@ -1088,10 +1133,12 @@ pgstat_build_snapshot(void)
 	/*
 	 * Build snapshot of all fixed-numbered stats.
 	 */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
+		if (!kind_info)
+			continue;
 		if (!kind_info->fixed_amount)
 		{
 			Assert(kind_info->snapshot_cb == NULL);
@@ -1108,6 +1155,20 @@ static void
 pgstat_build_snapshot_fixed(PgStat_Kind kind)
 {
 	const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+	int			idx;
+	bool	   *valid;
+
+	/* Position in fixed_valid or custom_valid */
+	if (pgstat_is_kind_builtin(kind))
+	{
+		idx = kind;
+		valid = pgStatLocal.snapshot.fixed_valid;
+	}
+	else
+	{
+		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+		valid = pgStatLocal.snapshot.custom_valid;
+	}
 
 	Assert(kind_info->fixed_amount);
 	Assert(kind_info->snapshot_cb != NULL);
@@ -1115,21 +1176,21 @@ pgstat_build_snapshot_fixed(PgStat_Kind kind)
 	if (pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_NONE)
 	{
 		/* rebuild every time */
-		pgStatLocal.snapshot.fixed_valid[kind] = false;
+		valid[idx] = false;
 	}
-	else if (pgStatLocal.snapshot.fixed_valid[kind])
+	else if (valid[idx])
 	{
 		/* in snapshot mode we shouldn't get called again */
 		Assert(pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_CACHE);
 		return;
 	}
 
-	Assert(!pgStatLocal.snapshot.fixed_valid[kind]);
+	Assert(!valid[idx]);
 
 	kind_info->snapshot_cb();
 
-	Assert(!pgStatLocal.snapshot.fixed_valid[kind]);
-	pgStatLocal.snapshot.fixed_valid[kind] = true;
+	Assert(!valid[idx]);
+	valid[idx] = true;
 }
 
 
@@ -1285,30 +1346,127 @@ pgstat_flush_pending_entries(bool nowait)
 PgStat_Kind
 pgstat_get_kind_from_str(char *kind_str)
 {
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN_BUILTIN; kind <= PGSTAT_KIND_MAX_BUILTIN; kind++)
 	{
-		if (pg_strcasecmp(kind_str, pgstat_kind_infos[kind].name) == 0)
+		if (pg_strcasecmp(kind_str, pgstat_kind_builtin_infos[kind].name) == 0)
 			return kind;
 	}
 
+	/* Check the custom set of cumulative stats */
+	if (pgstat_kind_custom_infos)
+	{
+		for (int kind = 0; kind < PGSTAT_KIND_CUSTOM_SIZE; kind++)
+		{
+			if (pgstat_kind_custom_infos[kind] &&
+				pg_strcasecmp(kind_str, pgstat_kind_custom_infos[kind]->name) == 0)
+				return kind + PGSTAT_KIND_CUSTOM_MIN;
+		}
+	}
+
 	ereport(ERROR,
 			(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 			 errmsg("invalid statistics kind: \"%s\"", kind_str)));
-	return PGSTAT_KIND_DATABASE;	/* avoid compiler warnings */
+	return PGSTAT_KIND_INVALID; /* avoid compiler warnings */
 }
 
 static inline bool
 pgstat_is_kind_valid(PgStat_Kind kind)
 {
-	return kind >= PGSTAT_KIND_FIRST_VALID && kind <= PGSTAT_KIND_LAST;
+	return pgstat_is_kind_builtin(kind) || pgstat_is_kind_custom(kind);
 }
 
 const PgStat_KindInfo *
 pgstat_get_kind_info(PgStat_Kind kind)
 {
-	Assert(pgstat_is_kind_valid(kind));
+	if (pgstat_is_kind_builtin(kind))
+		return &pgstat_kind_builtin_infos[kind];
 
-	return &pgstat_kind_infos[kind];
+	if (pgstat_is_kind_custom(kind))
+	{
+		uint32		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+		if (pgstat_kind_custom_infos == NULL ||
+			pgstat_kind_custom_infos[idx] == NULL)
+			return NULL;
+		return pgstat_kind_custom_infos[idx];
+	}
+
+	return NULL;
+}
+
+/*
+ * Register a new stats kind.
+ *
+ * PgStat_Kinds must be globally unique across all extensions. Refer
+ * to https://wiki.postgresql.org/wiki/CustomCumulativeStats to reserve a
+ * unique ID for your extension, to avoid conflicts with other extension
+ * developers. During development, use PGSTAT_KIND_EXPERIMENTAL to avoid
+ * needlessly reserving a new ID.
+ */
+void
+pgstat_register_kind(PgStat_Kind kind, const PgStat_KindInfo *kind_info)
+{
+	uint32		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+	if (kind_info->name == NULL || strlen(kind_info->name) == 0)
+		ereport(ERROR,
+				(errmsg("custom cumulative statistics name is invalid"),
+				 errhint("Provide a non-empty name for the custom cumulative statistics.")));
+
+	if (!pgstat_is_kind_custom(kind))
+		ereport(ERROR, (errmsg("custom cumulative statistics ID %u is out of range", kind),
+						errhint("Provide a custom cumulative statistics ID between %u and %u.",
+								PGSTAT_KIND_CUSTOM_MIN, PGSTAT_KIND_CUSTOM_MAX)));
+
+	if (!process_shared_preload_libraries_in_progress)
+		ereport(ERROR,
+				(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+				 errdetail("Custom cumulative statistics must be registered while initializing modules in \"shared_preload_libraries\".")));
+
+	/*
+	 * Check some data for fixed-numbered stats.
+	 */
+	if (kind_info->fixed_amount)
+	{
+		if (kind_info->shared_size == 0)
+			ereport(ERROR,
+					(errmsg("custom cumulative statistics property is invalid"),
+					 errhint("Custom cumulative statistics require a shared memory size for fixed-numbered objects.")));
+	}
+
+	/*
+	 * If pgstat_kind_custom_infos is not available yet, allocate it.
+	 */
+	if (pgstat_kind_custom_infos == NULL)
+	{
+		pgstat_kind_custom_infos = (const PgStat_KindInfo **)
+			MemoryContextAllocZero(TopMemoryContext,
+								   sizeof(PgStat_KindInfo *) * PGSTAT_KIND_CUSTOM_SIZE);
+	}
+
+	if (pgstat_kind_custom_infos[idx] != NULL &&
+		pgstat_kind_custom_infos[idx]->name != NULL)
+		ereport(ERROR,
+				(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+				 errdetail("Custom cumulative statistics \"%s\" already registered with the same ID.",
+						   pgstat_kind_custom_infos[idx]->name)));
+
+	/* check for existing custom stats with the same name */
+	for (int existing_kind = 0; existing_kind < PGSTAT_KIND_CUSTOM_SIZE; existing_kind++)
+	{
+		if (pgstat_kind_custom_infos[existing_kind] == NULL)
+			continue;
+		if (!pg_strcasecmp(pgstat_kind_custom_infos[existing_kind]->name, kind_info->name))
+			ereport(ERROR,
+					(errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+					 errdetail("Existing cumulative statistics with ID %u has the same name.", existing_kind + PGSTAT_KIND_CUSTOM_MIN)));
+	}
+
+	/* Register it */
+	pgstat_kind_custom_infos[idx] = kind_info;
+	ereport(LOG,
+			(errmsg("registered custom cumulative statistics \"%s\" with ID %u",
+					kind_info->name, kind)));
 }
 
 /*
@@ -1388,18 +1546,22 @@ pgstat_write_statsfile(void)
 	write_chunk_s(fpout, &format_id);
 
 	/* Write various stats structs for fixed number of objects */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		char	   *ptr;
 		const PgStat_KindInfo *info = pgstat_get_kind_info(kind);
 
-		if (!info->fixed_amount)
+		if (!info || !info->fixed_amount)
 			continue;
 
-		Assert(info->snapshot_ctl_off != 0);
+		if (pgstat_is_kind_builtin(kind))
+			Assert(info->snapshot_ctl_off != 0);
 
 		pgstat_build_snapshot_fixed(kind);
-		ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
+		if (pgstat_is_kind_builtin(kind))
+			ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
+		else
+			ptr = pgStatLocal.snapshot.custom_data[kind - PGSTAT_KIND_CUSTOM_MIN];
 
 		fputc(PGSTAT_FILE_ENTRY_FIXED, fpout);
 		write_chunk_s(fpout, &kind);
@@ -1422,6 +1584,17 @@ pgstat_write_statsfile(void)
 		if (ps->dropped)
 			continue;
 
+		/*
+		 * This discards data related to custom stats kinds that are unknown
+		 * to this process.
+		 */
+		if (!pgstat_is_kind_valid(ps->key.kind))
+		{
+			elog(WARNING, "found unknown stats entry %u/%u/%u",
+				 ps->key.kind, ps->key.dboid, ps->key.objoid);
+			continue;
+		}
+
 		shstats = (PgStatShared_Common *) dsa_get_address(pgStatLocal.dsa, ps->body);
 
 		kind_info = pgstat_get_kind_info(ps->key.kind);
@@ -1570,8 +1743,16 @@ pgstat_read_statsfile(void)
 						goto error;
 
 					/* Load back stats into shared memory */
-					ptr = ((char *) shmem) + info->shared_ctl_off +
-						info->shared_data_off;
+					if (pgstat_is_kind_builtin(kind))
+						ptr = ((char *) shmem) + info->shared_ctl_off +
+							info->shared_data_off;
+					else
+					{
+						int		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+						ptr = ((char *) shmem->custom_data[idx]) +
+							info->shared_data_off;
+					}
 
 					if (!read_chunk(fpin, ptr, info->shared_data_len))
 						goto error;
@@ -1638,7 +1819,7 @@ pgstat_read_statsfile(void)
 					if (found)
 					{
 						dshash_release_lock(pgStatLocal.shared_hash, p);
-						elog(WARNING, "found duplicate stats entry %d/%u/%u",
+						elog(WARNING, "found duplicate stats entry %u/%u/%u",
 							 key.kind, key.dboid, key.objoid);
 						goto error;
 					}
@@ -1696,11 +1877,11 @@ pgstat_reset_after_failure(void)
 	TimestampTz ts = GetCurrentTimestamp();
 
 	/* reset fixed-numbered stats */
-	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 	{
 		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 
-		if (!kind_info->fixed_amount)
+		if (!kind_info || !kind_info->fixed_amount)
 			continue;
 
 		kind_info->reset_all_cb(ts);
diff --git a/src/backend/utils/activity/pgstat_shmem.c b/src/backend/utils/activity/pgstat_shmem.c
index 1c2b69d563..ca3d3eecfb 100644
--- a/src/backend/utils/activity/pgstat_shmem.c
+++ b/src/backend/utils/activity/pgstat_shmem.c
@@ -131,6 +131,21 @@ StatsShmemSize(void)
 	sz = MAXALIGN(sizeof(PgStat_ShmemControl));
 	sz = add_size(sz, pgstat_dsa_init_size());
 
+	/* Add shared memory for all the custom fixed-numbered statistics */
+	for (int kind = PGSTAT_KIND_CUSTOM_MIN; kind <= PGSTAT_KIND_CUSTOM_MAX; kind++)
+	{
+		const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+
+		if (!kind_info)
+			continue;
+		if (!kind_info->fixed_amount)
+			continue;
+
+		Assert(kind_info->shared_size != 0);
+
+		sz += MAXALIGN(kind_info->shared_size);
+	}
+
 	return sz;
 }
 
@@ -197,15 +212,25 @@ StatsShmemInit(void)
 		pg_atomic_init_u64(&ctl->gc_request_count, 1);
 
 		/* initialize fixed-numbered stats */
-		for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+		for (int kind = PGSTAT_KIND_MIN; kind <= PGSTAT_KIND_MAX; kind++)
 		{
 			const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
 			char	   *ptr;
 
-			if (!kind_info->fixed_amount)
+			if (!kind_info || !kind_info->fixed_amount)
 				continue;
 
-			ptr = ((char *) ctl) + kind_info->shared_ctl_off;
+			if (pgstat_is_kind_builtin(kind))
+				ptr = ((char *) ctl) + kind_info->shared_ctl_off;
+			else
+			{
+				int			idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+				Assert(kind_info->shared_size != 0);
+				ctl->custom_data[idx] = ShmemAlloc(kind_info->shared_size);
+				ptr = ctl->custom_data[idx];
+			}
+
 			kind_info->init_shmem_cb(ptr);
 		}
 	}
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 3876339ee1..3221137123 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1696,7 +1696,7 @@ pg_stat_reset(PG_FUNCTION_ARGS)
  * Reset some shared cluster-wide counters
  *
  * When adding a new reset target, ideally the name should match that in
- * pgstat_kind_infos, if relevant.
+ * pgstat_kind_builtin_infos, if relevant.
  */
 Datum
 pg_stat_reset_shared(PG_FUNCTION_ARGS)
-- 
2.45.2

From e475faaa8943356379923433f80125efbe68b9d0 Mon Sep 17 00:00:00 2001
From: Michael Paquier <mich...@paquier.xyz>
Date: Thu, 18 Jul 2024 14:10:35 +0900
Subject: [PATCH v7 3/6] Add helper routines to retrieve custom data for
 fixed-numbered stats

This is useful for extensions to get snapshot and shmem data for custom
statistics, so as these don't need to know about the current snapshots.
---
 src/include/utils/pgstat_internal.h | 32 +++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 39f63362a3..6bbd49388c 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -525,6 +525,8 @@ static inline int pgstat_cmp_hash_key(const void *a, const void *b, size_t size,
 static inline uint32 pgstat_hash_hash_key(const void *d, size_t size, void *arg);
 static inline size_t pgstat_get_entry_len(PgStat_Kind kind);
 static inline void *pgstat_get_entry_data(PgStat_Kind kind, PgStatShared_Common *entry);
+static inline void *pgstat_get_custom_shmem_data(PgStat_Kind kind);
+static inline void *pgstat_get_custom_snapshot_data(PgStat_Kind kind);
 
 
 /*
@@ -842,4 +844,34 @@ pgstat_get_entry_data(PgStat_Kind kind, PgStatShared_Common *entry)
 	return ((char *) (entry)) + off;
 }
 
+/*
+ * Returns a pointer to the shared memory area of custom stats for
+ * fixed-numbered statistics.
+ */
+static inline void *
+pgstat_get_custom_shmem_data(PgStat_Kind kind)
+{
+	int		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+	Assert(pgstat_is_kind_custom(kind));
+	Assert(pgstat_get_kind_info(kind)->fixed_amount);
+
+	return pgStatLocal.shmem->custom_data[idx];
+}
+
+/*
+ * Returns a pointer to the portion of custom data for fixed-numbered
+ * statistics in the current snapshot.
+ */
+static inline void *
+pgstat_get_custom_snapshot_data(PgStat_Kind kind)
+{
+	int		idx = kind - PGSTAT_KIND_CUSTOM_MIN;
+
+	Assert(pgstat_is_kind_custom(kind));
+	Assert(pgstat_get_kind_info(kind)->fixed_amount);
+
+	return pgStatLocal.snapshot.custom_data[idx];
+}
+
 #endif							/* PGSTAT_INTERNAL_H */
-- 
2.45.2

From 75e499b54c7c6ab79dadffe6eacb943df0a55038 Mon Sep 17 00:00:00 2001
From: Michael Paquier <mich...@paquier.xyz>
Date: Thu, 18 Jul 2024 14:14:58 +0900
Subject: [PATCH v7 4/6] doc: Add section for Custom Cumulative Statistics APIs

This provides a short description of what can be done, with a pointer to
the template in the tree.
---
 doc/src/sgml/xfunc.sgml | 62 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)

diff --git a/doc/src/sgml/xfunc.sgml b/doc/src/sgml/xfunc.sgml
index 7e92e89846..839c78c102 100644
--- a/doc/src/sgml/xfunc.sgml
+++ b/doc/src/sgml/xfunc.sgml
@@ -3703,6 +3703,68 @@ extern bool InjectionPointDetach(const char *name);
     </para>
    </sect2>
 
+   <sect2 id="xfunc-addin-custom-cumulative-statistics">
+    <title>Custom Cumulative Statistics</title>
+
+    <para>
+     Is is possible for add-ins written in C-language to use custom types
+     of cumulative statistics registered in the
+     <link linkend="monitoring-stats-setup">Cumulative Statistics System</link>.
+    </para>
+
+    <para>
+     First, define a <literal>PgStat_KindInfo</literal> that includes all
+     the information related to the custom type registered. For example:
+<programlisting>
+static const PgStat_KindInfo custom_stats = {
+    .name = "custom_stats",
+    .fixed_amount = false,
+    .shared_size = sizeof(PgStatShared_Custom),
+    .shared_data_off = offsetof(PgStatShared_Custom, stats),
+    .shared_data_len = sizeof(((PgStatShared_Custom *) 0)->stats),
+    .pending_size = sizeof(PgStat_StatCustomEntry),
+}
+</programlisting>
+
+     Then, each backend that needs to use this custom type needs to register
+     it with <literal>pgstat_register_kind</literal> and a unique ID used to
+     store the entries related to this type of statistics:
+<programlisting>
+extern PgStat_Kind pgstat_add_kind(PgStat_Kind kind,
+                                   const PgStat_KindInfo *kind_info);
+</programlisting>
+     While developing a new extension, use
+     <literal>PGSTAT_KIND_EXPERIMENTAL</literal> for
+     <parameter>kind</parameter>. When you are ready to release the extension
+     to users, reserve a kind ID at the
+     <ulink url="https://wiki.postgresql.org/wiki/CustomCumulativeStats";>
+     Custom Cumulative Statistics</ulink> page.
+    </para>
+
+    <para>
+     The details of the API for <literal>PgStat_KindInfo</literal> can
+     be found in <filename>src/include/utils/pgstat_internal.h</filename>.
+    </para>
+
+    <para>
+     The type of statistics registered is associated with a name and a unique
+     ID shared across the server in shared memory. Each backend using a
+     custom type of statistics maintains a local cache storing the information
+     of each custom <literal>PgStat_KindInfo</literal>.
+    </para>
+
+    <para>
+     Place the extension module implementing the custom cumulative statistics
+     type in <xref linkend="guc-shared-preload-libraries"/> so that it will
+     be loaded early during <productname>PostgreSQL</productname> startup.
+    </para>
+
+    <para>
+     An example describing how to register and use custom statistics can be
+     found in <filename>src/test/modules/injection_points</filename>.
+    </para>
+   </sect2>
+
    <sect2 id="extend-cpp">
     <title>Using C++ for Extensibility</title>
 
-- 
2.45.2

From 63ea23477e13322587bf100ddc85ffecfbe1a627 Mon Sep 17 00:00:00 2001
From: Michael Paquier <mich...@paquier.xyz>
Date: Thu, 18 Jul 2024 14:22:54 +0900
Subject: [PATCH v7 5/6] injection_points: Add statistics for custom points

This acts as a template of what can be achieved with the pluggable
cumulative stats APIs, while being useful on its own for injection
points.

Currently, the only data gathered is the number of times an injection
point is called.  This can be extended as required.  All the routines
related to the stats are located in their own file, for clarity.

A TAP test is included to provide coverage for these new APIs, showing
the persistency of the data across restarts.
---
 src/test/modules/injection_points/Makefile    |  11 +-
 .../injection_points--1.0.sql                 |  10 +
 .../injection_points/injection_points.c       |  27 +++
 .../injection_points/injection_stats.c        | 197 ++++++++++++++++++
 .../injection_points/injection_stats.h        |  23 ++
 src/test/modules/injection_points/meson.build |   9 +
 .../modules/injection_points/t/001_stats.pl   |  48 +++++
 src/tools/pgindent/typedefs.list              |   2 +
 8 files changed, 325 insertions(+), 2 deletions(-)
 create mode 100644 src/test/modules/injection_points/injection_stats.c
 create mode 100644 src/test/modules/injection_points/injection_stats.h
 create mode 100644 src/test/modules/injection_points/t/001_stats.pl

diff --git a/src/test/modules/injection_points/Makefile b/src/test/modules/injection_points/Makefile
index 2ffd2f77ed..7b9cd12a2a 100644
--- a/src/test/modules/injection_points/Makefile
+++ b/src/test/modules/injection_points/Makefile
@@ -1,7 +1,10 @@
 # src/test/modules/injection_points/Makefile
 
-MODULES = injection_points
-
+MODULE_big = injection_points
+OBJS = \
+	$(WIN32RES) \
+	injection_points.o \
+	injection_stats.o
 EXTENSION = injection_points
 DATA = injection_points--1.0.sql
 PGFILEDESC = "injection_points - facility for injection points"
@@ -11,9 +14,13 @@ REGRESS_OPTS = --dlpath=$(top_builddir)/src/test/regress
 
 ISOLATION = inplace
 
+TAP_TESTS = 1
+
 # The injection points are cluster-wide, so disable installcheck
 NO_INSTALLCHECK = 1
 
+export enable_injection_points enable_injection_points
+
 ifdef USE_PGXS
 PG_CONFIG = pg_config
 PGXS := $(shell $(PG_CONFIG) --pgxs)
diff --git a/src/test/modules/injection_points/injection_points--1.0.sql b/src/test/modules/injection_points/injection_points--1.0.sql
index 0f280419a5..b98d571889 100644
--- a/src/test/modules/injection_points/injection_points--1.0.sql
+++ b/src/test/modules/injection_points/injection_points--1.0.sql
@@ -74,3 +74,13 @@ CREATE FUNCTION injection_points_detach(IN point_name TEXT)
 RETURNS void
 AS 'MODULE_PATHNAME', 'injection_points_detach'
 LANGUAGE C STRICT PARALLEL UNSAFE;
+
+--
+-- injection_points_stats_numcalls()
+--
+-- Reports statistics, if any, related to the given injection point.
+--
+CREATE FUNCTION injection_points_stats_numcalls(IN point_name TEXT)
+RETURNS bigint
+AS 'MODULE_PATHNAME', 'injection_points_stats_numcalls'
+LANGUAGE C STRICT;
diff --git a/src/test/modules/injection_points/injection_points.c b/src/test/modules/injection_points/injection_points.c
index 15f9d0233c..acec4e95b0 100644
--- a/src/test/modules/injection_points/injection_points.c
+++ b/src/test/modules/injection_points/injection_points.c
@@ -18,6 +18,7 @@
 #include "postgres.h"
 
 #include "fmgr.h"
+#include "injection_stats.h"
 #include "miscadmin.h"
 #include "nodes/pg_list.h"
 #include "nodes/value.h"
@@ -170,6 +171,9 @@ injection_points_cleanup(int code, Datum arg)
 		char	   *name = strVal(lfirst(lc));
 
 		(void) InjectionPointDetach(name);
+
+		/* Remove stats entry */
+		pgstat_drop_inj(name);
 	}
 }
 
@@ -182,6 +186,8 @@ injection_error(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	elog(ERROR, "error triggered for injection point %s", name);
 }
 
@@ -193,6 +199,8 @@ injection_notice(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	elog(NOTICE, "notice triggered for injection point %s", name);
 }
 
@@ -211,6 +219,8 @@ injection_wait(const char *name, const void *private_data)
 	if (!injection_point_allowed(condition))
 		return;
 
+	pgstat_report_inj(name);
+
 	/*
 	 * Use the injection point name for this custom wait event.  Note that
 	 * this custom wait event name is not released, but we don't care much for
@@ -299,6 +309,10 @@ injection_points_attach(PG_FUNCTION_ARGS)
 		inj_list_local = lappend(inj_list_local, makeString(pstrdup(name)));
 		MemoryContextSwitchTo(oldctx);
 	}
+
+	/* Add entry for stats */
+	pgstat_create_inj(name);
+
 	PG_RETURN_VOID();
 }
 
@@ -431,5 +445,18 @@ injection_points_detach(PG_FUNCTION_ARGS)
 		MemoryContextSwitchTo(oldctx);
 	}
 
+	/* Remove stats entry */
+	pgstat_drop_inj(name);
+
 	PG_RETURN_VOID();
 }
+
+
+void
+_PG_init(void)
+{
+	if (!process_shared_preload_libraries_in_progress)
+		return;
+
+	pgstat_register_inj();
+}
diff --git a/src/test/modules/injection_points/injection_stats.c b/src/test/modules/injection_points/injection_stats.c
new file mode 100644
index 0000000000..78042074ff
--- /dev/null
+++ b/src/test/modules/injection_points/injection_stats.c
@@ -0,0 +1,197 @@
+/*--------------------------------------------------------------------------
+ *
+ * injection_stats.c
+ *		Code for statistics of injection points.
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/injection_points/injection_stats.c
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "fmgr.h"
+
+#include "common/hashfn.h"
+#include "injection_stats.h"
+#include "pgstat.h"
+#include "utils/builtins.h"
+#include "utils/pgstat_internal.h"
+
+/* Structures for statistics of injection points */
+typedef struct PgStat_StatInjEntry
+{
+	PgStat_Counter numcalls;	/* number of times point has been run */
+} PgStat_StatInjEntry;
+
+typedef struct PgStatShared_InjectionPoint
+{
+	PgStatShared_Common header;
+	PgStat_StatInjEntry stats;
+} PgStatShared_InjectionPoint;
+
+static bool injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait);
+
+static const PgStat_KindInfo injection_stats = {
+	.name = "injection_points",
+	.fixed_amount = false,		/* Bounded by the number of points */
+
+	/* Injection points are system-wide */
+	.accessed_across_databases = true,
+
+	.shared_size = sizeof(PgStatShared_InjectionPoint),
+	.shared_data_off = offsetof(PgStatShared_InjectionPoint, stats),
+	.shared_data_len = sizeof(((PgStatShared_InjectionPoint *) 0)->stats),
+	.pending_size = sizeof(PgStat_StatInjEntry),
+	.flush_pending_cb = injection_stats_flush_cb,
+};
+
+/*
+ * Compute stats entry idx from point name with a 4-byte hash.
+ */
+#define PGSTAT_INJ_IDX(name) hash_bytes((const unsigned char *) name, strlen(name))
+
+/*
+ * Kind ID reserved for statistics of injection points.
+ */
+#define PGSTAT_KIND_INJECTION	129
+
+/* Track if stats are loaded */
+static bool inj_stats_loaded = false;
+
+/*
+ * Callback for stats handling
+ */
+static bool
+injection_stats_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
+{
+	PgStat_StatInjEntry *localent;
+	PgStatShared_InjectionPoint *shfuncent;
+
+	localent = (PgStat_StatInjEntry *) entry_ref->pending;
+	shfuncent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+
+	if (!pgstat_lock_entry(entry_ref, nowait))
+		return false;
+
+	shfuncent->stats.numcalls += localent->numcalls;
+	return true;
+}
+
+/*
+ * Support function for the SQL-callable pgstat* functions.  Returns
+ * a pointer to the injection point statistics struct.
+ */
+static PgStat_StatInjEntry *
+pgstat_fetch_stat_injentry(const char *name)
+{
+	PgStat_StatInjEntry *entry = NULL;
+
+	if (!inj_stats_loaded)
+		return NULL;
+
+	/* Compile the lookup key as a hash of the point name */
+	entry = (PgStat_StatInjEntry *) pgstat_fetch_entry(PGSTAT_KIND_INJECTION,
+													   InvalidOid,
+													   PGSTAT_INJ_IDX(name));
+	return entry;
+}
+
+/*
+ * Workhorse to do the registration work, called in _PG_init().
+ */
+void
+pgstat_register_inj(void)
+{
+	pgstat_register_kind(PGSTAT_KIND_INJECTION, &injection_stats);
+
+	/* mark stats as loaded */
+	inj_stats_loaded = true;
+}
+
+/*
+ * Report injection point creation.
+ */
+void
+pgstat_create_inj(const char *name)
+{
+	PgStat_EntryRef *entry_ref;
+	PgStatShared_InjectionPoint *shstatent;
+
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_INJECTION, InvalidOid,
+											PGSTAT_INJ_IDX(name), false);
+	shstatent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+
+	/* initialize shared memory data */
+	memset(&shstatent->stats, 0, sizeof(shstatent->stats));
+	pgstat_unlock_entry(entry_ref);
+}
+
+/*
+ * Report injection point drop.
+ */
+void
+pgstat_drop_inj(const char *name)
+{
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	if (!pgstat_drop_entry(PGSTAT_KIND_INJECTION, InvalidOid,
+						   PGSTAT_INJ_IDX(name)))
+		pgstat_request_entry_refs_gc();
+}
+
+/*
+ * Report statistics for injection point.
+ *
+ * This is simple because the set of stats to report currently is simple:
+ * track the number of times a point has been run.
+ */
+void
+pgstat_report_inj(const char *name)
+{
+	PgStat_EntryRef *entry_ref;
+	PgStatShared_InjectionPoint *shstatent;
+	PgStat_StatInjEntry *statent;
+
+	/* leave if disabled */
+	if (!inj_stats_loaded)
+		return;
+
+	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_INJECTION, InvalidOid,
+											PGSTAT_INJ_IDX(name), false);
+
+	shstatent = (PgStatShared_InjectionPoint *) entry_ref->shared_stats;
+	statent = &shstatent->stats;
+
+	/* Update the injection point statistics */
+	statent->numcalls++;
+
+	pgstat_unlock_entry(entry_ref);
+}
+
+/*
+ * SQL function returning the number of times an injection point
+ * has been called.
+ */
+PG_FUNCTION_INFO_V1(injection_points_stats_numcalls);
+Datum
+injection_points_stats_numcalls(PG_FUNCTION_ARGS)
+{
+	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
+	PgStat_StatInjEntry *entry = pgstat_fetch_stat_injentry(name);
+
+	if (entry == NULL)
+		PG_RETURN_NULL();
+
+	PG_RETURN_INT64(entry->numcalls);
+}
diff --git a/src/test/modules/injection_points/injection_stats.h b/src/test/modules/injection_points/injection_stats.h
new file mode 100644
index 0000000000..3e99705483
--- /dev/null
+++ b/src/test/modules/injection_points/injection_stats.h
@@ -0,0 +1,23 @@
+/*--------------------------------------------------------------------------
+ *
+ * injection_stats.h
+ *		Definitions for statistics of injection points.
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/injection_points/injection_stats.h
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#ifndef INJECTION_STATS
+#define INJECTION_STATS
+
+extern void pgstat_register_inj(void);
+extern void pgstat_create_inj(const char *name);
+extern void pgstat_drop_inj(const char *name);
+extern void pgstat_report_inj(const char *name);
+
+#endif
diff --git a/src/test/modules/injection_points/meson.build b/src/test/modules/injection_points/meson.build
index 3c23c14d81..a52fe5121e 100644
--- a/src/test/modules/injection_points/meson.build
+++ b/src/test/modules/injection_points/meson.build
@@ -6,6 +6,7 @@ endif
 
 injection_points_sources = files(
   'injection_points.c',
+  'injection_stats.c',
 )
 
 if host_system == 'windows'
@@ -42,4 +43,12 @@ tests += {
       'inplace',
     ],
   },
+  'tap': {
+    'env': {
+      'enable_injection_points': get_option('injection_points') ? 'yes' : 'no',
+    },
+    'tests': [
+      't/001_stats.pl',
+    ],
+  },
 }
diff --git a/src/test/modules/injection_points/t/001_stats.pl b/src/test/modules/injection_points/t/001_stats.pl
new file mode 100644
index 0000000000..7d5a96e522
--- /dev/null
+++ b/src/test/modules/injection_points/t/001_stats.pl
@@ -0,0 +1,48 @@
+
+# Copyright (c) 2024, PostgreSQL Global Development Group
+
+use strict;
+use warnings FATAL => 'all';
+use locale;
+
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+# Test persistency of statistics generated for injection points.
+if ($ENV{enable_injection_points} ne 'yes')
+{
+	plan skip_all => 'Injection points not supported by this build';
+}
+
+# Node initialization
+my $node = PostgreSQL::Test::Cluster->new('master');
+$node->init;
+$node->append_conf('postgresql.conf',
+	"shared_preload_libraries = 'injection_points'");
+$node->start;
+$node->safe_psql('postgres', 'CREATE EXTENSION injection_points;');
+
+# This should count for two calls.
+$node->safe_psql('postgres',
+	"SELECT injection_points_attach('stats-notice', 'notice');");
+$node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
+$node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
+my $numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '2', 'number of stats calls');
+
+# Restart the node cleanly, stats should still be around.
+$node->restart;
+$numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '2', 'number of stats after clean restart');
+
+# On crash the stats are gone.
+$node->stop('immediate');
+$node->start;
+$numcalls = $node->safe_psql('postgres',
+	"SELECT injection_points_stats_numcalls('stats-notice');");
+is($numcalls, '', 'number of stats after clean restart');
+
+done_testing();
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index b4d7f9217c..c0625b8830 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2119,6 +2119,7 @@ PgStatShared_Common
 PgStatShared_Database
 PgStatShared_Function
 PgStatShared_HashEntry
+PgStatShared_InjectionPoint
 PgStatShared_IO
 PgStatShared_Relation
 PgStatShared_ReplSlot
@@ -2150,6 +2151,7 @@ PgStat_Snapshot
 PgStat_SnapshotEntry
 PgStat_StatDBEntry
 PgStat_StatFuncEntry
+PgStat_StatInjEntry
 PgStat_StatReplSlotEntry
 PgStat_StatSubEntry
 PgStat_StatTabEntry
-- 
2.45.2

From cd82e799d2bd2d05d217b2eaec646e0298677302 Mon Sep 17 00:00:00 2001
From: Michael Paquier <mich...@paquier.xyz>
Date: Thu, 18 Jul 2024 14:44:33 +0900
Subject: [PATCH v7 6/6] injection_points: Add example for fixed-numbered
 statistics

This acts as a template to show what can be achieved with fixed-numbered
stats (like WAL, bgwriter, etc.) for pluggable cumulative statistics.
---
 src/test/modules/injection_points/Makefile    |   3 +-
 .../injection_points--1.0.sql                 |  11 +
 .../injection_points/injection_points.c       |   4 +
 .../injection_points/injection_stats.h        |   7 +
 .../injection_points/injection_stats_fixed.c  | 192 ++++++++++++++++++
 src/test/modules/injection_points/meson.build |   1 +
 .../modules/injection_points/t/001_stats.pl   |  11 +-
 7 files changed, 227 insertions(+), 2 deletions(-)
 create mode 100644 src/test/modules/injection_points/injection_stats_fixed.c

diff --git a/src/test/modules/injection_points/Makefile b/src/test/modules/injection_points/Makefile
index 7b9cd12a2a..ed28cd13a8 100644
--- a/src/test/modules/injection_points/Makefile
+++ b/src/test/modules/injection_points/Makefile
@@ -4,7 +4,8 @@ MODULE_big = injection_points
 OBJS = \
 	$(WIN32RES) \
 	injection_points.o \
-	injection_stats.o
+	injection_stats.o \
+	injection_stats_fixed.o
 EXTENSION = injection_points
 DATA = injection_points--1.0.sql
 PGFILEDESC = "injection_points - facility for injection points"
diff --git a/src/test/modules/injection_points/injection_points--1.0.sql b/src/test/modules/injection_points/injection_points--1.0.sql
index b98d571889..1b2a4938a9 100644
--- a/src/test/modules/injection_points/injection_points--1.0.sql
+++ b/src/test/modules/injection_points/injection_points--1.0.sql
@@ -84,3 +84,14 @@ CREATE FUNCTION injection_points_stats_numcalls(IN point_name TEXT)
 RETURNS bigint
 AS 'MODULE_PATHNAME', 'injection_points_stats_numcalls'
 LANGUAGE C STRICT;
+
+--
+-- injection_points_stats_fixed()
+--
+-- Reports fixed-numbered statistics for injection points.
+CREATE FUNCTION injection_points_stats_fixed(OUT numattach int8,
+   OUT numdetach int8,
+   OUT numrun int8)
+RETURNS record
+AS 'MODULE_PATHNAME', 'injection_points_stats_fixed'
+LANGUAGE C STRICT;
diff --git a/src/test/modules/injection_points/injection_points.c b/src/test/modules/injection_points/injection_points.c
index acec4e95b0..dc02be1bbf 100644
--- a/src/test/modules/injection_points/injection_points.c
+++ b/src/test/modules/injection_points/injection_points.c
@@ -297,6 +297,7 @@ injection_points_attach(PG_FUNCTION_ARGS)
 		condition.pid = MyProcPid;
 	}
 
+	pgstat_report_inj_fixed(1, 0, 0);
 	InjectionPointAttach(name, "injection_points", function, &condition,
 						 sizeof(InjectionPointCondition));
 
@@ -342,6 +343,7 @@ injection_points_run(PG_FUNCTION_ARGS)
 {
 	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
 
+	pgstat_report_inj_fixed(0, 0, 1);
 	INJECTION_POINT(name);
 
 	PG_RETURN_VOID();
@@ -432,6 +434,7 @@ injection_points_detach(PG_FUNCTION_ARGS)
 {
 	char	   *name = text_to_cstring(PG_GETARG_TEXT_PP(0));
 
+	pgstat_report_inj_fixed(0, 1, 0);
 	if (!InjectionPointDetach(name))
 		elog(ERROR, "could not detach injection point \"%s\"", name);
 
@@ -459,4 +462,5 @@ _PG_init(void)
 		return;
 
 	pgstat_register_inj();
+	pgstat_register_inj_fixed();
 }
diff --git a/src/test/modules/injection_points/injection_stats.h b/src/test/modules/injection_points/injection_stats.h
index 3e99705483..d519f29f83 100644
--- a/src/test/modules/injection_points/injection_stats.h
+++ b/src/test/modules/injection_points/injection_stats.h
@@ -15,9 +15,16 @@
 #ifndef INJECTION_STATS
 #define INJECTION_STATS
 
+/* injection_stats.c */
 extern void pgstat_register_inj(void);
 extern void pgstat_create_inj(const char *name);
 extern void pgstat_drop_inj(const char *name);
 extern void pgstat_report_inj(const char *name);
 
+/* injection_stats_fixed.c */
+extern void pgstat_register_inj_fixed(void);
+extern void pgstat_report_inj_fixed(uint32 numattach,
+									uint32 numdetach,
+									uint32 numrun);
+
 #endif
diff --git a/src/test/modules/injection_points/injection_stats_fixed.c b/src/test/modules/injection_points/injection_stats_fixed.c
new file mode 100644
index 0000000000..75639328a8
--- /dev/null
+++ b/src/test/modules/injection_points/injection_stats_fixed.c
@@ -0,0 +1,192 @@
+/*--------------------------------------------------------------------------
+ *
+ * injection_stats_fixed.c
+ *		Code for fixed-numbered statistics of injection points.
+ *
+ * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/injection_points/injection_stats_fixed.c
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "fmgr.h"
+
+#include "common/hashfn.h"
+#include "funcapi.h"
+#include "injection_stats.h"
+#include "pgstat.h"
+#include "utils/builtins.h"
+#include "utils/pgstat_internal.h"
+
+/* Structures for statistics of injection points, fixed-size */
+typedef struct PgStat_StatInjFixedEntry
+{
+	PgStat_Counter numattach;	/* number of points attached */
+	PgStat_Counter numdetach;	/* number of points detached */
+	PgStat_Counter numrun;		/* number of points run */
+	TimestampTz stat_reset_timestamp;
+} PgStat_StatInjFixedEntry;
+
+typedef struct PgStatShared_InjectionPointFixed
+{
+	LWLock	lock;	/* protects all the counters */
+	uint32	changecount;
+	PgStat_StatInjFixedEntry stats;
+	PgStat_StatInjFixedEntry reset_offset;
+} PgStatShared_InjectionPointFixed;
+
+/* Callbacks for fixed-numbered stats */
+static void injection_stats_fixed_init_shmem_cb(void *stats);
+static void injection_stats_fixed_reset_all_cb(TimestampTz ts);
+static void injection_stats_fixed_snapshot_cb(void);
+
+static const PgStat_KindInfo injection_stats_fixed = {
+	.name = "injection_points_fixed",
+	.fixed_amount = true,
+
+	.shared_size = sizeof(PgStat_StatInjFixedEntry),
+	.shared_data_off = offsetof(PgStatShared_InjectionPointFixed, stats),
+	.shared_data_len = sizeof(((PgStatShared_InjectionPointFixed *) 0)->stats),
+
+	.init_shmem_cb = injection_stats_fixed_init_shmem_cb,
+	.reset_all_cb = injection_stats_fixed_reset_all_cb,
+	.snapshot_cb = injection_stats_fixed_snapshot_cb,
+};
+
+/*
+ * Kind ID reserved for statistics of injection points.
+ */
+#define PGSTAT_KIND_INJECTION_FIXED	130
+
+/* Track if fixed-numbered stats are loaded */
+static bool inj_fixed_loaded = false;
+
+static void
+injection_stats_fixed_init_shmem_cb(void *stats)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		(PgStatShared_InjectionPointFixed *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
+static void
+injection_stats_fixed_reset_all_cb(TimestampTz ts)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		pgstat_get_custom_shmem_data(PGSTAT_KIND_INJECTION_FIXED);
+
+	LWLockAcquire(&stats_shmem->lock, LW_EXCLUSIVE);
+	pgstat_copy_changecounted_stats(&stats_shmem->reset_offset,
+									&stats_shmem->stats,
+									sizeof(stats_shmem->stats),
+									&stats_shmem->changecount);
+	stats_shmem->stats.stat_reset_timestamp = ts;
+	LWLockRelease(&stats_shmem->lock);
+}
+
+static void
+injection_stats_fixed_snapshot_cb(void)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem =
+		pgstat_get_custom_shmem_data(PGSTAT_KIND_INJECTION_FIXED);
+	PgStat_StatInjFixedEntry *stat_snap =
+		pgstat_get_custom_snapshot_data(PGSTAT_KIND_INJECTION_FIXED);
+	PgStat_StatInjFixedEntry *reset_offset = &stats_shmem->reset_offset;
+	PgStat_StatInjFixedEntry reset;
+
+	pgstat_copy_changecounted_stats(stat_snap,
+									&stats_shmem->stats,
+									sizeof(stats_shmem->stats),
+									&stats_shmem->changecount);
+
+	LWLockAcquire(&stats_shmem->lock, LW_SHARED);
+	memcpy(&reset, reset_offset, sizeof(stats_shmem->stats));
+	LWLockRelease(&stats_shmem->lock);
+
+	/* compensate by reset offsets */
+#define FIXED_COMP(fld) stat_snap->fld -= reset.fld;
+	FIXED_COMP(numattach);
+	FIXED_COMP(numdetach);
+	FIXED_COMP(numrun);
+#undef FIXED_COMP
+}
+
+/*
+ * Workhorse to do the registration work, called in _PG_init().
+ */
+void
+pgstat_register_inj_fixed(void)
+{
+	pgstat_register_kind(PGSTAT_KIND_INJECTION_FIXED, &injection_stats_fixed);
+
+	/* mark stats as loaded */
+	inj_fixed_loaded = true;
+}
+
+/*
+ * Report fixed number of statistics for an injection point.
+ */
+void
+pgstat_report_inj_fixed(uint32 numattach,
+						uint32 numdetach,
+						uint32 numrun)
+{
+	PgStatShared_InjectionPointFixed *stats_shmem;
+
+	/* leave if disabled */
+	if (!inj_fixed_loaded)
+		return;
+
+	stats_shmem = pgstat_get_custom_shmem_data(PGSTAT_KIND_INJECTION_FIXED);
+
+	pgstat_begin_changecount_write(&stats_shmem->changecount);
+	stats_shmem->stats.numattach += numattach;
+	stats_shmem->stats.numdetach += numdetach;
+	stats_shmem->stats.numrun += numrun;
+	pgstat_end_changecount_write(&stats_shmem->changecount);
+}
+
+/*
+ * SQL function returning fixed-numbered statistics for injection points.
+ */
+PG_FUNCTION_INFO_V1(injection_points_stats_fixed);
+Datum
+injection_points_stats_fixed(PG_FUNCTION_ARGS)
+{
+	TupleDesc	tupdesc;
+	Datum		values[3] = {0};
+	bool		nulls[3] = {0};
+	PgStat_StatInjFixedEntry *stats;
+
+	if (!inj_fixed_loaded)
+		PG_RETURN_NULL();
+
+	pgstat_snapshot_fixed(PGSTAT_KIND_INJECTION_FIXED);
+	stats = pgstat_get_custom_snapshot_data(PGSTAT_KIND_INJECTION_FIXED);
+
+	/* Initialise attributes information in the tuple descriptor */
+	tupdesc = CreateTemplateTupleDesc(3);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "numattach",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "numdetach",
+					   INT8OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 3, "numrun",
+					   INT8OID, -1, 0);
+	BlessTupleDesc(tupdesc);
+
+	values[0] = Int64GetDatum(stats->numattach);
+	values[1] = Int64GetDatum(stats->numdetach);
+	values[2] = Int64GetDatum(stats->numrun);
+	nulls[0] = false;
+	nulls[1] = false;
+	nulls[2] = false;
+
+	/* Returns the record as Datum */
+	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
+}
diff --git a/src/test/modules/injection_points/meson.build b/src/test/modules/injection_points/meson.build
index a52fe5121e..c9e357f644 100644
--- a/src/test/modules/injection_points/meson.build
+++ b/src/test/modules/injection_points/meson.build
@@ -7,6 +7,7 @@ endif
 injection_points_sources = files(
   'injection_points.c',
   'injection_stats.c',
+  'injection_stats_fixed.c',
 )
 
 if host_system == 'windows'
diff --git a/src/test/modules/injection_points/t/001_stats.pl b/src/test/modules/injection_points/t/001_stats.pl
index 7d5a96e522..e3c69b94ca 100644
--- a/src/test/modules/injection_points/t/001_stats.pl
+++ b/src/test/modules/injection_points/t/001_stats.pl
@@ -31,18 +31,27 @@ $node->safe_psql('postgres', "SELECT injection_points_run('stats-notice');");
 my $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
 is($numcalls, '2', 'number of stats calls');
+my $fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '1|0|2', 'number of fixed stats');
 
 # Restart the node cleanly, stats should still be around.
 $node->restart;
 $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
 is($numcalls, '2', 'number of stats after clean restart');
+$fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '1|0|2', 'number of fixed stats after clean restart');
 
 # On crash the stats are gone.
 $node->stop('immediate');
 $node->start;
 $numcalls = $node->safe_psql('postgres',
 	"SELECT injection_points_stats_numcalls('stats-notice');");
-is($numcalls, '', 'number of stats after clean restart');
+is($numcalls, '', 'number of stats after crash');
+$fixedstats = $node->safe_psql('postgres',
+	"SELECT * FROM injection_points_stats_fixed();");
+is($fixedstats, '0|0|0', 'number of fixed stats after crash');
 
 done_testing();
-- 
2.45.2

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