Add a bit of debug logging to backend_read_statsfile().

This is in hopes of learning more about what causes "pgstat wait timeout"
warnings in the buildfarm.  This patch should probably be reverted once
we've learned what we can.  As coded, it will result in regression test
"failures" at half the delay that the existing code does, so I expect
to see a few more than before.

src/backend/postmaster/pgstat.c

@@ -3754,8 +3754,10 @@ pgstat_read_statsfile_timestamp(bool permanent, TimestampTz *ts)
 static void
 backend_read_statsfile(void)
 {
+	TimestampTz cur_ts;
 	TimestampTz min_ts;
 	int			count;
+	int			last_delay_errno = 0;
 
 	/* already read it? */
 	if (pgStatDBHash)
@@ -3776,12 +3778,11 @@ backend_read_statsfile(void)
 	 * PGSTAT_STAT_INTERVAL; and we don't want to lie to the collector about
 	 * what our cutoff time really is.
 	 */
+	cur_ts = GetCurrentTimestamp();
 	if (IsAutoVacuumWorkerProcess())
-		min_ts = TimestampTzPlusMilliseconds(GetCurrentTimestamp(),
-											 -PGSTAT_RETRY_DELAY);
+		min_ts = TimestampTzPlusMilliseconds(cur_ts, -PGSTAT_RETRY_DELAY);
 	else
-		min_ts = TimestampTzPlusMilliseconds(GetCurrentTimestamp(),
-											 -PGSTAT_STAT_INTERVAL);
+		min_ts = TimestampTzPlusMilliseconds(cur_ts, -PGSTAT_STAT_INTERVAL);
 
 	/*
 	 * Loop until fresh enough stats file is available or we ran out of time.
@@ -3798,9 +3799,29 @@ backend_read_statsfile(void)
 			file_ts >= min_ts)
 			break;
 
+		/* Make debugging printouts once we've waited unreasonably long */
+		if (count >= PGSTAT_POLL_LOOP_COUNT/2)
+		{
+			TimestampTz now_ts = GetCurrentTimestamp();
+
+#ifdef HAVE_INT64_TIMESTAMP
+			elog(WARNING, "pgstat waiting for " INT64_FORMAT " usec (%d loops), file timestamp " INT64_FORMAT " target timestamp " INT64_FORMAT " last errno %d",
+				 now_ts - cur_ts, count,
+				 file_ts, min_ts,
+				 last_delay_errno);
+#else
+			elog(WARNING, "pgstat waiting for %.6f sec (%d loops), file timestamp %.6f target timestamp %.6f last errno %d",
+				 now_ts - cur_ts, count,
+				 file_ts, min_ts,
+				 last_delay_errno);
+#endif
+		}
+
 		/* Not there or too old, so kick the collector and wait a bit */
+		errno = 0;
 		pgstat_send_inquiry(min_ts);
 		pg_usleep(PGSTAT_RETRY_DELAY * 1000L);
+		last_delay_errno = errno;
 	}
 
 	if (count >= PGSTAT_POLL_LOOP_COUNT)