fix(pubsub/pstest): prevent panic on undelievered messages

If a running pstest Server has a subscription without a dead letter
topic configured, and there's a pending message in the subscription that
is not received after the retention duration (10 minutes), the deliver
loop for the subscription will trigger a panic.

This commit prevents the panic by checking not trying to deliver to the
dead letter topic unless one is configured.

pubsub/pstest/fake.go

@@ -1121,7 +1121,7 @@ func (s *subscription) tryDeliverMessage(m *message, start int, now time.Time) (
 	return 0, false
 }
 
-var retentionDuration = 10 * time.Minute
+const retentionDuration = 10 * time.Minute
 
 // Must be called with the lock held.
 func (s *subscription) maintainMessages(now time.Time) {
@@ -1133,7 +1133,9 @@ func (s *subscription) maintainMessages(now time.Time) {
 		pubTime := m.proto.Message.PublishTime.AsTime()
 		// Remove messages that have been undelivered for a long time.
 		if !m.outstanding() && now.Sub(pubTime) > retentionDuration {
-			s.publishToDeadLetter(m)
+			if s.proto.DeadLetterPolicy != nil {
+				s.publishToDeadLetter(m)
+			}
 			delete(s.msgs, id)
 		}
 	}

pubsub/pstest/fake_test.go

@@ -1599,3 +1599,31 @@ func TestSubscriptionMessageOrdering(t *testing.T) {
 		ids = ids[len(pull.ReceivedMessages):]
 	}
 }
+
+func TestSubscriptionRetention(t *testing.T) {
+	ctx := context.Background()
+
+	s := NewServer()
+	defer s.Close()
+
+	start := time.Now()
+	s.SetTimeNowFunc(func() time.Time { return start })
+
+	const topicName = "projects/p/topics/t"
+	top, err := s.GServer.CreateTopic(ctx, &pb.Topic{Name: topicName})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := s.GServer.CreateSubscription(ctx, &pb.Subscription{
+		Name:                  "projects/p/subscriptions/s",
+		Topic:                 top.Name,
+		AckDeadlineSeconds:    30,
+		EnableMessageOrdering: true,
+	}); err != nil {
+		t.Fatal(err)
+	}
+	s.Publish(topicName, []byte("payload"), nil)
+
+	s.SetTimeNowFunc(func() time.Time { return start.Add(retentionDuration + 1) })
+	time.Sleep(1 * time.Second)
+}