Remove debug logging for pgstat wait timeout.

This reverts commit 79b2ee20, which proved
to not be very informative; it looks like the "pgstat wait timeout"
warnings in the buildfarm are just a symptom of running on heavily loaded
machines, and there isn't any weird mechanism causing them to appear.

To try to reduce the frequency of buildfarm failures from this effect,
increase PGSTAT_MAX_WAIT_TIME from 5 seconds to 10.

Also, arrange to not send a fresh inquiry message every single time through
the loop, as that seems more likely to cause problems (by swamping the
collector) than fix them.  We'll now send an inquiry the first time through
the delay loop, and every 640 msec thereafter.

src/backend/postmaster/pgstat.c

@@ -80,12 +80,15 @@
 #define PGSTAT_STAT_INTERVAL	500		/* Minimum time between stats file
 										 * updates; in milliseconds. */
 
-#define PGSTAT_RETRY_DELAY		10		/* How long to wait between statistics
-										 * update requests; in milliseconds. */
+#define PGSTAT_RETRY_DELAY		10		/* How long to wait between checks for
+										 * a new file; in milliseconds. */
 
-#define PGSTAT_MAX_WAIT_TIME	5000	/* Maximum time to wait for a stats
+#define PGSTAT_MAX_WAIT_TIME	10000	/* Maximum time to wait for a stats
 										 * file update; in milliseconds. */
 
+#define PGSTAT_INQ_INTERVAL		640		/* How often to ping the collector for
+										 * a new file; in milliseconds. */
+
 #define PGSTAT_RESTART_INTERVAL 60		/* How often to attempt to restart a
 										 * failed statistics collector; in
 										 * seconds. */
@@ -94,6 +97,7 @@
 										 * death; in seconds. */
 
 #define PGSTAT_POLL_LOOP_COUNT	(PGSTAT_MAX_WAIT_TIME / PGSTAT_RETRY_DELAY)
+#define PGSTAT_INQ_LOOP_COUNT	(PGSTAT_INQ_INTERVAL / PGSTAT_RETRY_DELAY)
 
 
 /* ----------
@@ -3758,7 +3762,6 @@ backend_read_statsfile(void)
 	TimestampTz cur_ts;
 	TimestampTz min_ts;
 	int			count;
-	int			last_delay_errno = 0;
 
 	/* already read it? */
 	if (pgStatDBHash)
@@ -3788,7 +3791,7 @@ backend_read_statsfile(void)
 	/*
 	 * Loop until fresh enough stats file is available or we ran out of time.
 	 * The stats inquiry message is sent repeatedly in case collector drops
-	 * it.
+	 * it; but not every single time, as that just swamps the collector.
 	 */
 	for (count = 0; count < PGSTAT_POLL_LOOP_COUNT; count++)
 	{
@@ -3800,29 +3803,11 @@ 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;
+		if ((count % PGSTAT_INQ_LOOP_COUNT) == 0)
 			pgstat_send_inquiry(min_ts);
+
 		pg_usleep(PGSTAT_RETRY_DELAY * 1000L);
-		last_delay_errno = errno;
 	}
 
 	if (count >= PGSTAT_POLL_LOOP_COUNT)