Rename some pgstats callbacks related to flush of entries

The two callbacks have_fixed_pending_cb and flush_fixed_cb have been
introduced in fc415ed to provide a way for fixed-numbered
statistics to control the flush of their data.  These are renamed to
respectively have_static_pending_cb and flush_static_cb.  The
restriction that these only apply to fixed-numbered stats is removed.

A follow-up patch will make use of them for backend statistics.  This
stats kind is variable-numbered, and patches are under discussion to
track WAL data for IO and backend stats which cannot use
PgStat_EntryRef->pending as pending data would be touched in critical
sections, where no memory allocation can happen.

Per discussion with Andres Freund.

Author: Bertrand Drouvot
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/66efowskppsns35v5u2m7k4sdnl7yoz5bo64tdjwq7r5lhplrz@y7dme5xwh2r5

Author: michaelpq

src/backend/utils/activity/pgstat.c

@@ -437,8 +437,8 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.shared_data_off = offsetof(PgStatShared_IO, stats),
 		.shared_data_len = sizeof(((PgStatShared_IO *) 0)->stats),
 
-		.flush_fixed_cb = pgstat_io_flush_cb,
-		.have_fixed_pending_cb = pgstat_io_have_pending_cb,
+		.flush_static_cb = pgstat_io_flush_cb,
+		.have_static_pending_cb = pgstat_io_have_pending_cb,
 		.init_shmem_cb = pgstat_io_init_shmem_cb,
 		.reset_all_cb = pgstat_io_reset_all_cb,
 		.snapshot_cb = pgstat_io_snapshot_cb,
@@ -455,8 +455,8 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.shared_data_off = offsetof(PgStatShared_SLRU, stats),
 		.shared_data_len = sizeof(((PgStatShared_SLRU *) 0)->stats),
 
-		.flush_fixed_cb = pgstat_slru_flush_cb,
-		.have_fixed_pending_cb = pgstat_slru_have_pending_cb,
+		.flush_static_cb = pgstat_slru_flush_cb,
+		.have_static_pending_cb = pgstat_slru_have_pending_cb,
 		.init_shmem_cb = pgstat_slru_init_shmem_cb,
 		.reset_all_cb = pgstat_slru_reset_all_cb,
 		.snapshot_cb = pgstat_slru_snapshot_cb,
@@ -474,8 +474,8 @@ static const PgStat_KindInfo pgstat_kind_builtin_infos[PGSTAT_KIND_BUILTIN_SIZE]
 		.shared_data_len = sizeof(((PgStatShared_Wal *) 0)->stats),
 
 		.init_backend_cb = pgstat_wal_init_backend_cb,
-		.flush_fixed_cb = pgstat_wal_flush_cb,
-		.have_fixed_pending_cb = pgstat_wal_have_pending_cb,
+		.flush_static_cb = pgstat_wal_flush_cb,
+		.have_static_pending_cb = pgstat_wal_have_pending_cb,
 		.init_shmem_cb = pgstat_wal_init_shmem_cb,
 		.reset_all_cb = pgstat_wal_reset_all_cb,
 		.snapshot_cb = pgstat_wal_snapshot_cb,
@@ -713,22 +713,17 @@ pgstat_report_stat(bool force)
 	{
 		bool		do_flush = false;
 
-		/* Check for pending fixed-numbered stats */
+		/* Check for pending stats */
 		for (PgStat_Kind 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->have_fixed_pending_cb == NULL);
-				continue;
-			}
-			if (!kind_info->have_fixed_pending_cb)
+			if (!kind_info->have_static_pending_cb)
 				continue;
 
-			if (kind_info->have_fixed_pending_cb())
+			if (kind_info->have_static_pending_cb())
 			{
 				do_flush = true;
 				break;
@@ -789,25 +784,20 @@ pgstat_report_stat(bool force)
 
 	partial_flush = false;
 
-	/* flush of variable-numbered stats */
+	/* flush of variable-numbered stats tracked in pending entries list */
 	partial_flush |= pgstat_flush_pending_entries(nowait);
 
-	/* flush of fixed-numbered stats */
+	/* flush of other stats kinds */
 	for (PgStat_Kind 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->flush_fixed_cb == NULL);
-			continue;
-		}
-		if (!kind_info->flush_fixed_cb)
+		if (!kind_info->flush_static_cb)
 			continue;
 
-		partial_flush |= kind_info->flush_fixed_cb(nowait);
+		partial_flush |= kind_info->flush_static_cb(nowait);
 	}
 
 	last_flush = now;

src/include/utils/pgstat_internal.h

@@ -156,8 +156,8 @@ typedef struct PgStat_EntryRef
 	 * Pending statistics data that will need to be flushed to shared memory
 	 * stats eventually. Each stats kind utilizing pending data defines what
 	 * format its pending data has and needs to provide a
-	 * PgStat_KindInfo->flush_pending_cb callback to merge pending into shared
-	 * stats.
+	 * PgStat_KindInfo->flush_pending_cb callback to merge pending entries
+	 * into the shared stats hash table.
 	 */
 	void	   *pending;
 	dlist_node	pending_node;	/* membership in pgStatPending list */
@@ -260,7 +260,8 @@ typedef struct PgStat_KindInfo
 
 	/*
 	 * For variable-numbered stats: flush pending stats. Required if pending
-	 * data is used.  See flush_fixed_cb for fixed-numbered stats.
+	 * data is used. See flush_static_cb when dealing with stats data that
+	 * that cannot use PgStat_EntryRef->pending.
 	 */
 	bool		(*flush_pending_cb) (PgStat_EntryRef *sr, bool nowait);
 
@@ -289,17 +290,23 @@ typedef struct PgStat_KindInfo
 	void		(*init_shmem_cb) (void *stats);
 
 	/*
-	 * For fixed-numbered statistics: Flush pending stats. Returns true if
-	 * some of the stats could not be flushed, due to lock contention for
-	 * example. Optional.
+	 * For fixed-numbered or variable-numbered statistics: Flush pending stats
+	 * entries, for stats kinds that do not use PgStat_EntryRef->pending.
+	 *
+	 * Returns true if some of the stats could not be flushed, due to lock
+	 * contention for example. Optional.
 	 */
-	bool		(*flush_fixed_cb) (bool nowait);
+	bool		(*flush_static_cb) (bool nowait);
 
 	/*
-	 * For fixed-numbered statistics: Check for pending stats in need of
-	 * flush. Returns true if there are any stats pending for flush. Optional.
+	 * For fixed-numbered or variable-numbered statistics: Check for pending
+	 * stats in need of flush with flush_static_cb, when these do not use
+	 * PgStat_EntryRef->pending.
+	 *
+	 * Returns true if there are any stats pending for flush, triggering
+	 * flush_static_cb. Optional.
 	 */
-	bool		(*have_fixed_pending_cb) (void);
+	bool		(*have_static_pending_cb) (void);
 
 	/*
 	 * For fixed-numbered statistics: Reset All.