Add callbacks to control flush of fixed-numbered stats

This commit adds two callbacks in pgstats to have a better control of
the flush timing of pgstat_report_stat(), whose operation depends on the
three PGSTAT_*_INTERVAL variables:
- have_fixed_pending_cb(), to check if a stats kind has any pending
data waiting for a flush.  This is used as a fast path if there are no
pending statistics to flush, and this check is done for fixed-numbered
statistics only if there are no variable-numbered statistics to flush.
A flush will need to happen if at least one callback reports any pending
data.
- flush_fixed_cb(), to do the actual flush.

These callbacks are currently used by the SLRU, WAL and IO statistics,
generalizing the concept for all stats kinds (builtin and custom).

The SLRU and IO stats relied each on one global variable to determine
whether a flush should happen; these are now local to pgstat_slru.c and
pgstat_io.c, cleaning up a bit how the pending flush states are tracked
in pgstat.c.

pgstat_flush_io() and pgstat_flush_wal() are still required, but we do
not need to check their return result anymore.

Reviewed-by: Bertrand Drouvot, Kyotaro Horiguchi
Discussion: https://postgr.es/m/ZtaVO0N-aTwiAk3w@paquier.xyz

1 files changed, 20 insertions, 2 deletions

diff --git a/src/backend/utils/activity/pgstat_io.c b/src/backend/utils/activity/pgstat_io.c
index 8af55989eed..cc2ffc78aa9 100644
--- a/src/backend/utils/activity/pgstat_io.c
+++ b/src/backend/utils/activity/pgstat_io.c
@@ -29,7 +29,7 @@ typedef struct PgStat_PendingIO
 
 
 static PgStat_PendingIO PendingIOStats;
-bool		have_iostats = false;
+static bool have_iostats = false;
 
 
 /*
@@ -162,6 +162,24 @@ pgstat_fetch_stat_io(void)
 }
 
 /*
+ * Check if there any IO stats waiting for flush.
+ */
+bool
+pgstat_io_have_pending_cb(void)
+{
+	return have_iostats;
+}
+
+/*
+ * Simpler wrapper of pgstat_io_flush_cb()
+ */
+void
+pgstat_flush_io(bool nowait)
+{
+	(void) pgstat_io_flush_cb(nowait);
+}
+
+/*
  * Flush out locally pending IO statistics
  *
  * If no stats have been recorded, this function returns false.
@@ -170,7 +188,7 @@ pgstat_fetch_stat_io(void)
  * acquired. Otherwise, return false.
  */
 bool
-pgstat_flush_io(bool nowait)
+pgstat_io_flush_cb(bool nowait)
 {
 	LWLock	   *bktype_lock;
 	PgStat_BktypeIO *bktype_shstats;