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.

doc/src/sgml/func.sgml

@@ -14805,6 +14805,12 @@ SELECT * FROM pg_ls_dir('.') WITH ORDINALITY AS t(ls,n);
        <entry>channel names that the session is currently listening on</entry>
       </row>
 
+      <row>
+       <entry><literal><function>pg_notification_queue_usage()</function></literal></entry>
+       <entry><type>double</type></entry>
+       <entry>fraction of the asynchronous notification queue currently occupied (0-1)</entry>
+      </row>
+
       <row>
        <entry><literal><function>pg_my_temp_schema()</function></literal></entry>
        <entry><type>oid</type></entry>
@@ -14945,10 +14951,19 @@ SET search_path TO <replaceable>schema</> <optional>, <replaceable>schema</>, ..
     <primary>pg_listening_channels</primary>
    </indexterm>
 
+   <indexterm>
+    <primary>pg_notification_queue_usage</primary>
+   </indexterm>
+
    <para>
     <function>pg_listening_channels</function> returns a set of names of
-    channels that the current session is listening to.  See <xref
-    linkend="sql-listen"> for more information.
+    asynchronous notification channels that the current session is listening
+    to.  <function>pg_notification_queue_usage</function> returns the
+    fraction of the total available space for notifications currently
+    occupied by notifications that are waiting to be processed, as a
+    <type>double</type> in the range 0-1.
+    See <xref linkend="sql-listen"> and <xref linkend="sql-notify">
+    for more information.
    </para>
 
    <indexterm>

doc/src/sgml/ref/notify.sgml

@@ -165,6 +165,11 @@ NOTIFY <replaceable class="PARAMETER">channel</replaceable> [ , <replaceable cla
    cleanup. In this case you should make sure that this session ends its
    current transaction so that cleanup can proceed.
   </para>
+  <para>
+   The function <function>pg_notification_queue_usage</function> returns the
+   fraction of the queue that is currently occupied by pending notifications.
+   See <xref linkend="functions-info"> for more information.
+  </para>
   <para>
    A transaction that has executed <command>NOTIFY</command> cannot be
    prepared for two-phase commit.

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;
 

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201507021
+#define CATALOG_VERSION_NO	201507171
 
 #endif

src/include/catalog/pg_proc.h

@@ -4046,10 +4046,14 @@ DATA(insert OID = 2856 (  pg_timezone_names		PGNSP PGUID 12 1 1000 0 0 f f f f t
 DESCR("get the available time zone names");
 DATA(insert OID = 2730 (  pg_get_triggerdef		PGNSP PGUID 12 1 0 0 0 f f f f t f s 2 0 25 "26 16" _null_ _null_ _null_ _null_ _null_ pg_get_triggerdef_ext _null_ _null_ _null_ ));
 DESCR("trigger description with pretty-print option");
+
+/* asynchronous notifications */
 DATA(insert OID = 3035 (  pg_listening_channels PGNSP PGUID 12 1 10 0 0 f f f f t t s 0 0 25 "" _null_ _null_ _null_ _null_ _null_ pg_listening_channels _null_ _null_ _null_ ));
 DESCR("get the channels that the current backend listens to");
 DATA(insert OID = 3036 (  pg_notify				PGNSP PGUID 12 1 0 0 0 f f f f f f v 2 0 2278 "25 25" _null_ _null_ _null_ _null_ _null_ pg_notify _null_ _null_ _null_ ));
 DESCR("send a notification event");
+DATA(insert OID = 3296 (  pg_notification_queue_usage	PGNSP PGUID 12 1 0 0 0 f f f f t f v 0 0 701 "" _null_ _null_ _null_ _null_ _null_ pg_notification_queue_usage _null_ _null_ _null_ ));
+DESCR("get the fraction of the asynchronous notification queue currently in use");
 
 /* non-persistent series generator */
 DATA(insert OID = 1066 (  generate_series PGNSP PGUID 12 1 1000 0 0 f f f f t t i 3 0 23 "23 23 23" _null_ _null_ _null_ _null_ _null_ generate_series_step_int4 _null_ _null_ _null_ ));

src/include/commands/async.h

@@ -37,6 +37,7 @@ extern void Async_UnlistenAll(void);
 /* notify-related SQL functions */
 extern Datum pg_listening_channels(PG_FUNCTION_ARGS);
 extern Datum pg_notify(PG_FUNCTION_ARGS);
+extern Datum pg_notification_queue_usage(PG_FUNCTION_ARGS);
 
 /* perform (or cancel) outbound notify processing at transaction commit */
 extern void PreCommit_Notify(void);

src/test/isolation/expected/async-notify.out

+Parsed test spec with 2 sessions
+
+starting permutation: listen begin check notify check
+step listen: LISTEN a;
+step begin: BEGIN;
+step check: SELECT pg_notification_queue_usage() > 0 AS nonzero;
+nonzero        
+
+f              
+step notify: SELECT count(pg_notify('a', s::text)) FROM generate_series(1, 1000) s;
+count          
+
+1000           
+step check: SELECT pg_notification_queue_usage() > 0 AS nonzero;
+nonzero        
+
+t              

src/test/isolation/specs/async-notify.spec

+# Verify that pg_notification_queue_usage correctly reports a non-zero result,
+# after submitting notifications while another connection is listening for
+# those notifications and waiting inside an active transaction.
+
+session "listener"
+step "listen"	{ LISTEN a; }
+step "begin"	{ BEGIN; }
+teardown		{ ROLLBACK; }
+
+session "notifier"
+step "check"	{ SELECT pg_notification_queue_usage() > 0 AS nonzero; }
+step "notify"	{ SELECT count(pg_notify('a', s::text)) FROM generate_series(1, 1000) s; }
+
+permutation "listen" "begin" "check" "notify" "check"

src/test/regress/expected/async.out

@@ -32,3 +32,11 @@ NOTIFY notify_async2;
 LISTEN notify_async2;
 UNLISTEN notify_async2;
 UNLISTEN *;
+-- Should return zero while there are no pending notifications.
+-- src/test/isolation/specs/async-notify.spec tests for actual usage.
+SELECT pg_notification_queue_usage();
+ pg_notification_queue_usage 
+-----------------------------
+                           0
+(1 row)
+

src/test/regress/sql/async.sql

@@ -17,3 +17,7 @@ NOTIFY notify_async2;
 LISTEN notify_async2;
 UNLISTEN notify_async2;
 UNLISTEN *;
+
+-- Should return zero while there are no pending notifications.
+-- src/test/isolation/specs/async-notify.spec tests for actual usage.
+SELECT pg_notification_queue_usage();