[CHANGED] MQTT no longer requires server name to be set (#4679)

It was required because it was required by JetStream, but JetStream now
auto-generates a default name.

server/mqtt.go

@@ -598,9 +598,9 @@ func validateMQTTOptions(o *Options) error {
 	if mo.Port == 0 {
 		return nil
 	}
-	// We have to force the server name to be explicitly set. There are conditions
-	// where we need a unique, repeatable name.
-	if o.ServerName == _EMPTY_ {
+	// We have to force the server name to be explicitly set and be unique when
+	// in cluster mode.
+	if o.ServerName == _EMPTY_ && (o.Cluster.Port != 0 || o.Gateway.Port != 0) {
 		return errMQTTServerNameMustBeSet
 	}
 	// If there is a NoAuthUser, we need to have Users defined and

server/mqtt_test.go

@@ -375,6 +375,9 @@ func testMQTTDefaultTLSOptions(t *testing.T, verify bool) *Options {
 
 func TestMQTTServerNameRequired(t *testing.T) {
 	conf := createConfFile(t, []byte(`
+		cluster {
+			port: -1
+		}
 		mqtt {
 			port: -1
 		}
@@ -386,6 +389,19 @@ func TestMQTTServerNameRequired(t *testing.T) {
 	if _, err := NewServer(o); err == nil || err.Error() != errMQTTServerNameMustBeSet.Error() {
 		t.Fatalf("Expected error about requiring server name to be set, got %v", err)
 	}
+
+	conf = createConfFile(t, []byte(`
+		mqtt {
+			port: -1
+		}
+	`))
+	o, err = ProcessConfigFile(conf)
+	if err != nil {
+		t.Fatalf("Error processing config file: %v", err)
+	}
+	if _, err := NewServer(o); err == nil || err.Error() != errMQTTStandaloneNeedsJetStream.Error() {
+		t.Fatalf(`Expected errMQTTStandaloneNeedsJetStream ("next in line"), got %v`, err)
+	}
 }
 
 func TestMQTTStandaloneRequiresJetStream(t *testing.T) {