Use pgstat_kind_infos to write fixed shared statistics

This is similar to 9004abf6206e, 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

1 files changed, 18 insertions, 38 deletions

diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index c37c11b2ecb..65b937a85f4 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/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