Report stats when replaying XLOG_RUNNING_XACTS

Previously stats in the startup process would only get reported during
shutdown of the startup process. It has been that way for a long time, but
became a lot more noticeable with the new pg_stat_io view, which separates out
IO done by different backend types...

While replaying after every XLOG_RUNNING_XACTS isn't the prettiest approach,
it has the advantage of being quite easy. Given that we're well past feature
freeze...

It's not a problem that we don't report stats more frequently with
wal_level=minimal, in that case stats can't be read before the stats process
has shut down.

Besides the above, this commit also changes pgstat_report_stat() to acquire
the timestamp with GetCurrentTimestamp() instead of
GetCurrentTransactionStopTimestamp().

Thanks to Melih Mutlu, Kyotaro Horiguchi for prototypes of other approaches to
solving this issue.

Reported-by: Fujii Masao <masao.fujii@oss.nttdata.com>
Discussion: https://postgr.es/m/5315aedc-fbca-1556-c5de-dc2e00b23a14@oss.nttdata.com

src/backend/storage/ipc/standby.c

@@ -1193,6 +1193,15 @@ standby_redo(XLogReaderState *record)
 		running.xids = xlrec->xids;
 
 		ProcArrayApplyRecoveryInfo(&running);
+
+		/*
+		 * The startup process currently has no convenient way to schedule
+		 * stats to be reported. XLOG_RUNNING_XACTS records issued at a
+		 * regular cadence, making this a convenient location to report
+		 * stats. While these records aren't generated with wal_level=minimal,
+		 * stats also cannot be accessed during WAL replay.
+		 */
+		pgstat_report_stat(true);
 	}
 	else if (info == XLOG_INVALIDATIONS)
 	{

src/backend/utils/activity/pgstat.c

@@ -615,10 +615,21 @@ pgstat_report_stat(bool force)
 	 */
 	Assert(!pgStatLocal.shmem->is_shutdown);
 
-	now = GetCurrentTransactionStopTimestamp();
-
-	if (!force)
+	if (force)
 	{
+		/*
+		 * Stats reports are forced either when it's been too long since stats
+		 * have been reported or in processes that force stats reporting to
+		 * happen at specific points (including shutdown). In the former case
+		 * the transaction stop time might be quite old, in the latter it
+		 * would never get cleared.
+		 */
+		now = GetCurrentTimestamp();
+	}
+	else
+	{
+		now = GetCurrentTransactionStopTimestamp();
+
 		if (pending_since > 0 &&
 			TimestampDifferenceExceeds(pending_since, now, PGSTAT_MAX_INTERVAL))
 		{