Use pgstat_kind_infos to write fixed shared statistics

This is similar to 9004abf620, but this time for the write part of the
stats file.  The code is changed so as, rather than referring to
individual members of PgStat_Snapshot in an order based on their
PgStat_Kind value, a loop based on pgstat_kind_infos is used to retrieve
the contents to write from the snapshot structure, for a size of
PgStat_KindInfo's shared_data_len.

This requires the addition to PgStat_KindInfo of an offset to track the
location of each fixed-numbered stats in PgStat_Snapshot.  This change
is useful to make this area of the code more easily pluggable, and
reduces the knowledge of specific fixed-numbered kinds in pgstat.c.

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

src/backend/utils/activity/pgstat.c

@@ -349,6 +349,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.fixed_amount = true,
 
+		.snapshot_ctl_off = offsetof(PgStat_Snapshot, archiver),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, archiver),
 		.shared_data_off = offsetof(PgStatShared_Archiver, stats),
 		.shared_data_len = sizeof(((PgStatShared_Archiver *) 0)->stats),
@@ -362,6 +363,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.fixed_amount = true,
 
+		.snapshot_ctl_off = offsetof(PgStat_Snapshot, bgwriter),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, bgwriter),
 		.shared_data_off = offsetof(PgStatShared_BgWriter, stats),
 		.shared_data_len = sizeof(((PgStatShared_BgWriter *) 0)->stats),
@@ -375,6 +377,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.fixed_amount = true,
 
+		.snapshot_ctl_off = offsetof(PgStat_Snapshot, checkpointer),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, checkpointer),
 		.shared_data_off = offsetof(PgStatShared_Checkpointer, stats),
 		.shared_data_len = sizeof(((PgStatShared_Checkpointer *) 0)->stats),
@@ -388,6 +391,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.fixed_amount = true,
 
+		.snapshot_ctl_off = offsetof(PgStat_Snapshot, io),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, io),
 		.shared_data_off = offsetof(PgStatShared_IO, stats),
 		.shared_data_len = sizeof(((PgStatShared_IO *) 0)->stats),
@@ -401,6 +405,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.fixed_amount = true,
 
+		.snapshot_ctl_off = offsetof(PgStat_Snapshot, slru),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, slru),
 		.shared_data_off = offsetof(PgStatShared_SLRU, stats),
 		.shared_data_len = sizeof(((PgStatShared_SLRU *) 0)->stats),
@@ -414,6 +419,7 @@ static const PgStat_KindInfo pgstat_kind_infos[PGSTAT_NUM_KINDS] = {
 
 		.fixed_amount = true,
 
+		.snapshot_ctl_off = offsetof(PgStat_Snapshot, wal),
 		.shared_ctl_off = offsetof(PgStat_ShmemControl, wal),
 		.shared_data_off = offsetof(PgStatShared_Wal, stats),
 		.shared_data_len = sizeof(((PgStatShared_Wal *) 0)->stats),
@@ -1371,47 +1377,21 @@ pgstat_write_statsfile(void)
 	format_id = PGSTAT_FILE_FORMAT_ID;
 	write_chunk_s(fpout, &format_id);
 
-	/*
-	 * XXX: The following could now be generalized to just iterate over
-	 * pgstat_kind_infos instead of knowing about the different kinds of
-	 * stats.
-	 */
-
-	/*
-	 * Write archiver stats struct
-	 */
-	pgstat_build_snapshot_fixed(PGSTAT_KIND_ARCHIVER);
-	write_chunk_s(fpout, &pgStatLocal.snapshot.archiver);
-
-	/*
-	 * Write bgwriter stats struct
-	 */
-	pgstat_build_snapshot_fixed(PGSTAT_KIND_BGWRITER);
-	write_chunk_s(fpout, &pgStatLocal.snapshot.bgwriter);
-
-	/*
-	 * Write checkpointer stats struct
-	 */
-	pgstat_build_snapshot_fixed(PGSTAT_KIND_CHECKPOINTER);
-	write_chunk_s(fpout, &pgStatLocal.snapshot.checkpointer);
+	/* Write various stats structs for fixed number of objects */
+	for (int kind = PGSTAT_KIND_FIRST_VALID; kind <= PGSTAT_KIND_LAST; kind++)
+	{
+		char	   *ptr;
+		const PgStat_KindInfo *info = pgstat_get_kind_info(kind);
 
-	/*
-	 * Write IO stats struct
-	 */
-	pgstat_build_snapshot_fixed(PGSTAT_KIND_IO);
-	write_chunk_s(fpout, &pgStatLocal.snapshot.io);
+		if (!info->fixed_amount)
+			continue;
 
-	/*
-	 * Write SLRU stats struct
-	 */
-	pgstat_build_snapshot_fixed(PGSTAT_KIND_SLRU);
-	write_chunk_s(fpout, &pgStatLocal.snapshot.slru);
+		Assert(info->snapshot_ctl_off != 0);
 
-	/*
-	 * Write WAL stats struct
-	 */
-	pgstat_build_snapshot_fixed(PGSTAT_KIND_WAL);
-	write_chunk_s(fpout, &pgStatLocal.snapshot.wal);
+		pgstat_build_snapshot_fixed(kind);
+		ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
+		write_chunk(fpout, ptr, info->shared_data_len);
+	}
 
 	/*
 	 * Walk through the stats entries

src/include/utils/pgstat_internal.h

@@ -199,6 +199,12 @@ typedef struct PgStat_KindInfo
 	 */
 	uint32		shared_size;
 
+	/*
+	 * The offset of the statistics struct in the cached statistics snapshot
+	 * PgStat_Snapshot, for fixed-numbered statistics.
+	 */
+	uint32		snapshot_ctl_off;
+
 	/*
 	 * The offset of the statistics struct in the containing shared memory
 	 * control structure PgStat_ShmemControl, for fixed-numbered statistics.