Fix subscription stress test in notifier (#269)

While I was trying to fix another problem in the notifier's subscription
stress test, I ended up introducing another one by using a single wait
group for both the notification sending goroutine and subscriptions
loops which caused the test intermittency described in #268. The test
was ending too soon, and channels would only receive 1-2 messages, and
sometimes zero (which would fail the test).

    notifier_test.go:462: Channel  0 contains   2 message(s)
    notifier_test.go:462: Channel  1 contains   2 message(s)
    notifier_test.go:462: Channel  2 contains   2 message(s)
    notifier_test.go:462: Channel  3 contains   2 message(s)
    notifier_test.go:462: Channel  4 contains   2 message(s)
    notifier_test.go:462: Channel  5 contains   2 message(s)
    notifier_test.go:462: Channel  6 contains   2 message(s)
    notifier_test.go:462: Channel  7 contains   0 message(s)

Here, fix the problem by putting in different symbols for the
notification send and subscription loops. The proper order of operations
to finish the test are:

1. Wait for all subscription groups to finish.
2. Tell the notification send loop to shut down.
3. Wait for the notification send loop to return (so as not to introduce
   any goroutine leaks like had been present before).

Fixes #268.

internal/notifier/notifier_test.go

@@ -410,13 +410,13 @@ func TestNotifier(t *testing.T) {
 			notifyChans[i] = make(chan TopicAndPayload, 1000)
 		}
 
-		var wg sync.WaitGroup
-
 		// Start a goroutine to send messages constantly.
-		shutdown := make(chan struct{})
-		wg.Add(1)
+		var (
+			sendNotificationsDone     = make(chan struct{})
+			sendNotificationsShutdown = make(chan struct{})
+		)
 		go func() {
-			defer wg.Done()
+			defer close(sendNotificationsDone)
 
 			ticker := time.NewTicker(10 * time.Millisecond)
 			defer ticker.Stop()
@@ -427,14 +427,15 @@ func TestNotifier(t *testing.T) {
 				select {
 				case <-ctx.Done():
 					return
-				case <-shutdown:
+				case <-sendNotificationsShutdown:
 					return
 				case <-ticker.C:
 					// loop again
 				}
 			}
 		}()
 
+		var wg sync.WaitGroup
 		wg.Add(len(notifyChans))
 		for i := range notifyChans {
 			notifyChan := notifyChans[i]
@@ -454,9 +455,9 @@ func TestNotifier(t *testing.T) {
 			}()
 		}
 
-		close(shutdown)
-		wg.Wait()
-		notifier.Stop()
+		wg.Wait()                        // wait for subscribe loops to finish all their work
+		close(sendNotificationsShutdown) // stop sending notifications
+		<-sendNotificationsDone          // wait for notifications goroutine to finish
 
 		for i := range notifyChans {
 			t.Logf("Channel %2d contains %3d message(s)", i, len(notifyChans[i]))