Add PgStat_KindInfo.init_shmem_cb

This new callback gives fixed-numbered stats the possibility to take
actions based on the area of shared memory allocated for them.

This removes from pgstat_shmem.c any knowledge specific to the types
of fixed-numbered stats, and the initializations happen in their own
files.  Like b68b29bc8f, this change is useful to make this area of
the code more pluggable, so as custom fixed-numbered stats can take
actions after their shared memory area is initialized.

Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/Zot5bxoPYdS7yaoy@paquier.xyz

src/backend/utils/activity/pgstat.c

@@ -354,6 +354,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.shared_data_off = offsetof(PgStatShared_Archiver, stats),
 		.shared_data_len = sizeof(((PgStatShared_Archiver *) 0)->stats),
 
+		.init_shmem_cb = pgstat_archiver_init_shmem_cb,
 		.reset_all_cb = pgstat_archiver_reset_all_cb,
 		.snapshot_cb = pgstat_archiver_snapshot_cb,
 	},
@@ -368,6 +369,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.shared_data_off = offsetof(PgStatShared_BgWriter, stats),
 		.shared_data_len = sizeof(((PgStatShared_BgWriter *) 0)->stats),
 
+		.init_shmem_cb = pgstat_bgwriter_init_shmem_cb,
 		.reset_all_cb = pgstat_bgwriter_reset_all_cb,
 		.snapshot_cb = pgstat_bgwriter_snapshot_cb,
 	},
@@ -382,6 +384,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.shared_data_off = offsetof(PgStatShared_Checkpointer, stats),
 		.shared_data_len = sizeof(((PgStatShared_Checkpointer *) 0)->stats),
 
+		.init_shmem_cb = pgstat_checkpointer_init_shmem_cb,
 		.reset_all_cb = pgstat_checkpointer_reset_all_cb,
 		.snapshot_cb = pgstat_checkpointer_snapshot_cb,
 	},
@@ -396,6 +399,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.shared_data_off = offsetof(PgStatShared_IO, stats),
 		.shared_data_len = sizeof(((PgStatShared_IO *) 0)->stats),
 
+		.init_shmem_cb = pgstat_io_init_shmem_cb,
 		.reset_all_cb = pgstat_io_reset_all_cb,
 		.snapshot_cb = pgstat_io_snapshot_cb,
 	},
@@ -410,6 +414,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.shared_data_off = offsetof(PgStatShared_SLRU, stats),
 		.shared_data_len = sizeof(((PgStatShared_SLRU *) 0)->stats),
 
+		.init_shmem_cb = pgstat_slru_init_shmem_cb,
 		.reset_all_cb = pgstat_slru_reset_all_cb,
 		.snapshot_cb = pgstat_slru_snapshot_cb,
 	},
@@ -424,6 +429,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 		.shared_data_off = offsetof(PgStatShared_Wal, stats),
 		.shared_data_len = sizeof(((PgStatShared_Wal *) 0)->stats),
 
+		.init_shmem_cb = pgstat_wal_init_shmem_cb,
 		.reset_all_cb = pgstat_wal_reset_all_cb,
 		.snapshot_cb = pgstat_wal_snapshot_cb,
 	},

src/backend/utils/activity/pgstat_archiver.c

@@ -62,6 +62,14 @@ pgstat_fetch_stat_archiver(void)
 	return &pgStatLocal.snapshot.archiver;
 }
 
+void
+pgstat_archiver_init_shmem_cb(void *stats)
+{
+	PgStatShared_Archiver *stats_shmem = (PgStatShared_Archiver *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_archiver_reset_all_cb(TimestampTz ts)
 {

src/backend/utils/activity/pgstat_bgwriter.c

@@ -75,6 +75,14 @@ pgstat_fetch_stat_bgwriter(void)
 	return &pgStatLocal.snapshot.bgwriter;
 }
 
+void
+pgstat_bgwriter_init_shmem_cb(void *stats)
+{
+	PgStatShared_BgWriter *stats_shmem = (PgStatShared_BgWriter *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_bgwriter_reset_all_cb(TimestampTz ts)
 {

src/backend/utils/activity/pgstat_checkpointer.c

@@ -84,6 +84,14 @@ pgstat_fetch_stat_checkpointer(void)
 	return &pgStatLocal.snapshot.checkpointer;
 }
 
+void
+pgstat_checkpointer_init_shmem_cb(void *stats)
+{
+	PgStatShared_Checkpointer *stats_shmem = (PgStatShared_Checkpointer *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_checkpointer_reset_all_cb(TimestampTz ts)
 {

src/backend/utils/activity/pgstat_io.c

@@ -251,6 +251,15 @@ pgstat_get_io_object_name(IOObject io_object)
 	pg_unreachable();
 }
 
+void
+pgstat_io_init_shmem_cb(void *stats)
+{
+	PgStatShared_IO *stat_shmem = (PgStatShared_IO *) stats;
+
+	for (int i = 0; i < BACKEND_NUM_TYPES; i++)
+		LWLockInitialize(&stat_shmem->locks[i], LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_io_reset_all_cb(TimestampTz ts)
 {

src/backend/utils/activity/pgstat_shmem.c

@@ -196,17 +196,18 @@ StatsShmemInit(void)
 
 		pg_atomic_init_u64(&ctl->gc_request_count, 1);
 
-
 		/* initialize fixed-numbered stats */
-		LWLockInitialize(&ctl->archiver.lock, LWTRANCHE_PGSTATS_DATA);
-		LWLockInitialize(&ctl->bgwriter.lock, LWTRANCHE_PGSTATS_DATA);
-		LWLockInitialize(&ctl->checkpointer.lock, LWTRANCHE_PGSTATS_DATA);
-		LWLockInitialize(&ctl->slru.lock, LWTRANCHE_PGSTATS_DATA);
-		LWLockInitialize(&ctl->wal.lock, LWTRANCHE_PGSTATS_DATA);
+		for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+		{
+			const PgStat_KindInfo *kind_info = pgstat_get_kind_info(kind);
+			char	   *ptr;
 
-		for (int i = 0; i < BACKEND_NUM_TYPES; i++)
-			LWLockInitialize(&ctl->io.locks[i],
-							 LWTRANCHE_PGSTATS_DATA);
+			if (!kind_info->fixed_amount)
+				continue;
+
+			ptr = ((char *) ctl) + kind_info->shared_ctl_off;
+			kind_info->init_shmem_cb(ptr);
+		}
 	}
 	else
 	{

src/backend/utils/activity/pgstat_slru.c

@@ -192,6 +192,14 @@ pgstat_slru_flush(bool nowait)
 	return false;
 }
 
+void
+pgstat_slru_init_shmem_cb(void *stats)
+{
+	PgStatShared_SLRU *stats_shmem = (PgStatShared_SLRU *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_slru_reset_all_cb(TimestampTz ts)
 {

src/backend/utils/activity/pgstat_wal.c

@@ -163,6 +163,14 @@ pgstat_have_pending_wal(void)
 		PendingWalStats.wal_sync != 0;
 }
 
+void
+pgstat_wal_init_shmem_cb(void *stats)
+{
+	PgStatShared_Wal *stats_shmem = (PgStatShared_Wal *) stats;
+
+	LWLockInitialize(&stats_shmem->lock, LWTRANCHE_PGSTATS_DATA);
+}
+
 void
 pgstat_wal_reset_all_cb(TimestampTz ts)
 {