Fix some gaps in pg_stat_io with WAL receiver and WAL summarizer

The WAL receiver and WAL summarizer processes gain each one a call to
pgstat_report_wal(), to make sure that they report their WAL statistics
to pgstats, gathering data for pg_stat_io.

In the WAL receiver, the stats reports are timed with status updates sent
to the primary, that depend on wal_receiver_status_interval and
wal_receiver_timeout.  This is a conservative choice, but perhaps we
could be more aggressive with the frequency of the stats reports.  An
interesting historical fact is that the WAL receiver does writes and
syncs of WAL, but it has never reported its statistics to pgstats in
pg_stat_wal.

In the WAL summarizer, the stats reports are done each time the process
waits for WAL.

While on it, pg_stat_io is adjusted so as these two processes do not
report any rows when IOObject is not WAL, making the view easier to use
with less rows.

Two tests are added in TAP, checking statistics for the WAL summarizer
and the WAL receiver.  Status updates in the WAL receiver are currently
possible in the recovery test 001_stream_rep.pl.

Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/Z8UKZyVSHUUQJHNb@paquier.xyz

src/backend/postmaster/walsummarizer.c

@@ -33,6 +33,7 @@
 #include "common/blkreftable.h"
 #include "libpq/pqsignal.h"
 #include "miscadmin.h"
+#include "pgstat.h"
 #include "postmaster/auxprocess.h"
 #include "postmaster/interrupt.h"
 #include "postmaster/walsummarizer.h"
@@ -1636,6 +1637,9 @@ summarizer_wait_for_wal(void)
 			sleep_quanta -= pages_read_since_last_sleep;
 	}
 
+	/* Report pending statistics to the cumulative stats system. */
+	pgstat_report_wal(false);
+
 	/* OK, now sleep. */
 	(void) WaitLatch(MyLatch,
 					 WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,

src/backend/replication/walreceiver.c

@@ -583,6 +583,16 @@ WalReceiverMain(const void *startup_data, size_t startup_data_len)
 					 */
 					bool		requestReply = false;
 
+					/*
+					 * Report pending statistics to the cumulative stats
+					 * system.  This location is useful for the report as it
+					 * is not within a tight loop in the WAL receiver, to
+					 * avoid bloating pgstats with requests, while also making
+					 * sure that the reports happen each time a status update
+					 * is sent.
+					 */
+					pgstat_report_wal(false);
+
 					/*
 					 * Check if time since last receive from primary has
 					 * reached the configured limit.

src/backend/utils/activity/pgstat_io.c

@@ -435,12 +435,22 @@ pgstat_tracks_io_object(BackendType bktype, IOObject io_object,
 	 */
 	no_temp_rel = bktype == B_AUTOVAC_LAUNCHER || bktype == B_BG_WRITER ||
 		bktype == B_CHECKPOINTER || bktype == B_AUTOVAC_WORKER ||
-		bktype == B_STANDALONE_BACKEND || bktype == B_STARTUP;
+		bktype == B_STANDALONE_BACKEND || bktype == B_STARTUP ||
+		bktype == B_WAL_SUMMARIZER || bktype == B_WAL_WRITER ||
+		bktype == B_WAL_RECEIVER;
 
 	if (no_temp_rel && io_context == IOCONTEXT_NORMAL &&
 		io_object == IOOBJECT_TEMP_RELATION)
 		return false;
 
+	/*
+	 * Some BackendTypes only perform IO under IOOBJECT_WAL, hence exclude all
+	 * rows for all the other objects for these.
+	 */
+	if ((bktype == B_WAL_SUMMARIZER || bktype == B_WAL_RECEIVER ||
+		 bktype == B_WAL_WRITER) && io_object != IOOBJECT_WAL)
+		return false;
+
 	/*
 	 * Some BackendTypes do not currently perform any IO in certain
 	 * IOContexts, and, while it may not be inherently incorrect for them to

src/bin/pg_walsummary/t/002_blocks.pl

@@ -46,6 +46,13 @@ SELECT EXISTS (
 EOM
 ok($result, "WAL summarization caught up after insert");
 
+# The WAL summarizer should have generated some IO statistics.
+my $stats_reads = $node1->safe_psql(
+	'postgres',
+	qq{SELECT sum(reads) > 0 FROM pg_stat_io
+   WHERE backend_type = 'walsummarizer' AND object = 'wal'});
+is($stats_reads, 't', "WAL summarizer generates statistics for WAL reads");
+
 # Find the highest LSN that is summarized on disk.
 my $summarized_lsn = $node1->safe_psql('postgres', <<EOM);
 SELECT MAX(end_lsn) AS summarized_lsn FROM pg_available_wal_summaries()

src/test/recovery/t/001_stream_rep.pl

@@ -506,6 +506,13 @@ $node_standby_2->append_conf('postgresql.conf', "primary_slot_name = ''");
 $node_standby_2->enable_streaming($node_primary);
 $node_standby_2->reload;
 
+# The WAL receiver should have generated some IO statistics.
+my $stats_reads = $node_standby_1->safe_psql(
+	'postgres',
+	qq{SELECT sum(writes) > 0 FROM pg_stat_io
+   WHERE backend_type = 'walreceiver' AND object = 'wal'});
+is($stats_reads, 't', "WAL receiver generates statistics for WAL writes");
+
 # be sure do not streaming from cascade
 $node_standby_1->stop;