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.

4 files changed, 43 insertions, 0 deletions

diff --git a/src/test/isolation/expected/async-notify.out b/src/test/isolation/expected/async-notify.out
new file mode 100644
index 00000000000..92d281a7d1f
--- /dev/null
+++ b/src/test/isolation/expected/async-notify.out
@@ -0,0 +1,17 @@
+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              
diff --git a/src/test/isolation/specs/async-notify.spec b/src/test/isolation/specs/async-notify.spec
new file mode 100644
index 00000000000..7f451b18a15
--- /dev/null
+++ b/src/test/isolation/specs/async-notify.spec
@@ -0,0 +1,14 @@
+# 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"
diff --git a/src/test/regress/expected/async.out b/src/test/regress/expected/async.out
index ae0d5df3b78..19cbe38e636 100644
--- a/src/test/regress/expected/async.out
+++ b/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)
+
diff --git a/src/test/regress/sql/async.sql b/src/test/regress/sql/async.sql
index af3a904e0f5..40f6e015387 100644
--- a/src/test/regress/sql/async.sql
+++ b/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();