Return an error for calls into unknown interfaces or unknown objects

Ryan pointed out on IRC that we didn't do anything here. Looking at
the code, it's painfully obvious that we should be returning an error
here since a comment already says that we've exhausted all possible
options.

Signed-off-by: David Zeuthen <davidz@redhat.com>

gio/gdbusconnection.c

@@ -5088,6 +5088,7 @@ static void
 distribute_method_call (GDBusConnection *connection,
                         GDBusMessage    *message)
 {
+  GDBusMessage *reply;
   ExportedObject *eo;
   ExportedSubtree *es;
   const gchar *object_path;
@@ -5154,7 +5155,14 @@ distribute_method_call (GDBusConnection *connection,
   if (handle_generic_unlocked (connection, message))
     goto out;
 
-  /* if we end up here, the message has not been not handled */
+  /* if we end up here, the message has not been not handled - so return an error saying this */
+  reply = g_dbus_message_new_method_error (message,
+                                           "org.freedesktop.DBus.Error.UnknownMethod",
+                                           _("No such interface `%s' on object at path %s"),
+                                           interface_name,
+                                           object_path);
+  g_dbus_connection_send_message_unlocked (connection, reply, NULL, NULL);
+  g_object_unref (reply);
 
  out:
   g_free (subtree_path);

gio/tests/gdbus-export.c

@@ -824,6 +824,18 @@ test_dispatch_thread_func (gpointer user_data)
   g_error_free (error);
   g_assert (value == NULL);
 
+  error = NULL;
+  value = g_dbus_proxy_call_sync (foo_proxy,
+                                  "org.example.FooXYZ.NonExistant",
+                                  NULL,
+                                  G_DBUS_CALL_FLAGS_NONE,
+                                  -1,
+                                  NULL,
+                                  &error);
+  g_assert_error (error, G_DBUS_ERROR, G_DBUS_ERROR_UNKNOWN_METHOD);
+  g_error_free (error);
+  g_assert (value == NULL);
+
   /* user properties */
   error = NULL;
   value = g_dbus_proxy_call_sync (foo_proxy,