Improve error message when attempting to change consumer type (#6408)

Before this change, an attempt to change consumer type (by either
setting or removing deliver subject), resulted in `max waiting can not
be updated` error as `MaxWaiting` has a default value for pull
consumers. This ensures that changing consumer type is checked before
max waiting to return a more useful error message.

Signed-off-by: Piotr Piotrowski
[piotr@synadia.com](mailto:piotr@synadia.com)

server/consumer.go

@@ -2099,9 +2099,6 @@ func (acc *Account) checkNewConsumerConfig(cfg, ncfg *ConsumerConfig) error {
 	if cfg.FlowControl != ncfg.FlowControl {
 		return errors.New("flow control can not be updated")
 	}
-	if cfg.MaxWaiting != ncfg.MaxWaiting {
-		return errors.New("max waiting can not be updated")
-	}
 
 	// Deliver Subject is conditional on if its bound.
 	if cfg.DeliverSubject != ncfg.DeliverSubject {
@@ -2116,6 +2113,10 @@ func (acc *Account) checkNewConsumerConfig(cfg, ncfg *ConsumerConfig) error {
 		}
 	}
 
+	if cfg.MaxWaiting != ncfg.MaxWaiting {
+		return errors.New("max waiting can not be updated")
+	}
+
 	// Check if BackOff is defined, MaxDeliver is within range.
 	if lbo := len(ncfg.BackOff); lbo > 0 && ncfg.MaxDeliver != -1 && lbo > ncfg.MaxDeliver {
 		return NewJSConsumerMaxDeliverBackoffError()

server/jetstream_test.go

@@ -9279,6 +9279,47 @@ func TestJetStreamPullConsumerMaxWaiting(t *testing.T) {
 	}
 }
 
+func TestJetStreamChangeConsumerType(t *testing.T) {
+	s := RunBasicJetStreamServer(t)
+	defer s.Shutdown()
+
+	nc, js := jsClientConnect(t, s)
+	defer nc.Close()
+
+	_, err := js.AddStream(&nats.StreamConfig{Name: "TEST", Subjects: []string{"test.*"}})
+	require_NoError(t, err)
+
+	// create pull consumer
+	_, err = js.AddConsumer("TEST", &nats.ConsumerConfig{
+		Name:      "pull",
+		AckPolicy: nats.AckExplicitPolicy,
+	})
+	require_NoError(t, err)
+
+	// cannot update pull -> push
+	_, err = js.UpdateConsumer("TEST", &nats.ConsumerConfig{
+		Name:           "pull",
+		AckPolicy:      nats.AckExplicitPolicy,
+		DeliverSubject: "foo",
+	})
+	require_Contains(t, err.Error(), "can not update pull consumer to push based")
+
+	// create push consumer
+	_, err = js.AddConsumer("TEST", &nats.ConsumerConfig{
+		Name:           "push",
+		AckPolicy:      nats.AckExplicitPolicy,
+		DeliverSubject: "foo",
+	})
+	require_NoError(t, err)
+
+	// cannot change push -> pull
+	_, err = js.UpdateConsumer("TEST", &nats.ConsumerConfig{
+		Name:      "push",
+		AckPolicy: nats.AckExplicitPolicy,
+	})
+	require_Contains(t, err.Error(), "can not update push consumer to pull based")
+}
+
 ////////////////////////////////////////
 // Benchmark placeholders
 // TODO(dlc) - move