From a04bb65f70dafdf462e0478ad19e6de56df89bfc Mon Sep 17 00:00:00 2001
From: Robert Haas <rhaas@postgresql.org>
Date: Fri, 17 Jul 2015 09:12:03 -0400
Subject: Add new function pg_notification_queue_usage.

This tells you what fraction of NOTIFY's queue is currently filled.

Brendan Jurd, reviewed by Merlin Moncure and Gurjeet Singh.  A few
further tweaks by me.
---
 src/backend/commands/async.c | 50 ++++++++++++++++++++++++++++++++++----------
 1 file changed, 39 insertions(+), 11 deletions(-)

(limited to 'src/backend/commands/async.c')

diff --git a/src/backend/commands/async.c b/src/backend/commands/async.c
index 2826b7e43c4..3b71174b826 100644
--- a/src/backend/commands/async.c
+++ b/src/backend/commands/async.c
@@ -371,6 +371,7 @@ static bool asyncQueueIsFull(void);
 static bool asyncQueueAdvance(volatile QueuePosition *position, int entryLength);
 static void asyncQueueNotificationToEntry(Notification *n, AsyncQueueEntry *qe);
 static ListCell *asyncQueueAddEntries(ListCell *nextNotify);
+static double asyncQueueUsage(void);
 static void asyncQueueFillWarning(void);
 static bool SignalBackends(void);
 static void asyncQueueReadAllNotifications(void);
@@ -1362,26 +1363,37 @@ asyncQueueAddEntries(ListCell *nextNotify)
 }
 
 /*
- * Check whether the queue is at least half full, and emit a warning if so.
- *
- * This is unlikely given the size of the queue, but possible.
- * The warnings show up at most once every QUEUE_FULL_WARN_INTERVAL.
+ * SQL function to return the fraction of the notification queue currently
+ * occupied.
+ */
+Datum
+pg_notification_queue_usage(PG_FUNCTION_ARGS)
+{
+	double		usage;
+
+	LWLockAcquire(AsyncQueueLock, LW_SHARED);
+	usage = asyncQueueUsage();
+	LWLockRelease(AsyncQueueLock);
+
+	PG_RETURN_FLOAT8(usage);
+}
+
+/*
+ * Return the fraction of the queue that is currently occupied.
  *
- * Caller must hold exclusive AsyncQueueLock.
+ * The caller must hold AysncQueueLock in (at least) shared mode.
  */
-static void
-asyncQueueFillWarning(void)
+static double
+asyncQueueUsage(void)
 {
 	int			headPage = QUEUE_POS_PAGE(QUEUE_HEAD);
 	int			tailPage = QUEUE_POS_PAGE(QUEUE_TAIL);
 	int			occupied;
-	double		fillDegree;
-	TimestampTz t;
 
 	occupied = headPage - tailPage;
 
 	if (occupied == 0)
-		return;					/* fast exit for common case */
+		return (double) 0;		/* fast exit for common case */
 
 	if (occupied < 0)
 	{
@@ -1389,8 +1401,24 @@ asyncQueueFillWarning(void)
 		occupied += QUEUE_MAX_PAGE + 1;
 	}
 
-	fillDegree = (double) occupied / (double) ((QUEUE_MAX_PAGE + 1) / 2);
+	return (double) occupied / (double) ((QUEUE_MAX_PAGE + 1) / 2);
+}
+
+/*
+ * Check whether the queue is at least half full, and emit a warning if so.
+ *
+ * This is unlikely given the size of the queue, but possible.
+ * The warnings show up at most once every QUEUE_FULL_WARN_INTERVAL.
+ *
+ * Caller must hold exclusive AsyncQueueLock.
+ */
+static void
+asyncQueueFillWarning(void)
+{
+	double		fillDegree;
+	TimestampTz t;
 
+	fillDegree = asyncQueueUsage();
 	if (fillDegree < 0.5)
 		return;
 
-- 
cgit v1.2.3