CES-2235 Delay publication of the Charon service to the bus

Tweak the initialization order of the Charon DBus service. We should
initially create the `dbus.service.Object` *without* claiming a
well-known name on the bus. If we immediately claim a name within the
`FileService.__init__()` initializer, the DBus service will (briefly)
become visible on the DBus bus in a half-initialized state. The symptom
of this is clients seeing empty or incomplete introspection data on
`nl.ultimaker.charon`.

If, instead, we first allow `FileService` to completely finish its
`__init__()`, and then separately claim the well-known name in
`FileService.publish()`, this publishes the service to the bus
"atomically".

Author: onnodb

Charon/Service/FileService.py

@@ -18,15 +18,31 @@
 #   Note: This class does not currently use type hinting since type hints,
 #   dbus-python decorators and Python 3.4 do not mix well.
 class FileService(dbus.service.Object):
+
     def __init__(self, dbus_bus: dbus.Bus) -> None:
+        self.__dbus_bus = dbus_bus
         super().__init__(
-            bus_name = dbus.service.BusName("nl.ultimaker.charon", dbus_bus),
-            object_path = "/nl/ultimaker/charon"
+            conn=self.__dbus_bus,
+            object_path="/nl/ultimaker/charon",
+            # Postpone claiming a well-known name until the class is fully initialized.
+            # If we do this right now, the DBus service will be published to the
+            # bus in an incomplete state, and clients connecting early on may
+            # encounter, for instance, emptry introspection data on this service.
+            bus_name=None,
         )
+        self.__bus_name = None
 
         log.debug("FileService initialized")
         self.__queue = RequestQueue.RequestQueue()
 
+    ## Publish the fully initialized DBus service to the bus
+    def publish(self) -> None:
+        # Store a reference to the BusName for as long as the DBus service is alive;
+        # otherwise it gets GC'd, and the well-known name gets released again.
+        # Taking ownership of this well-known name is also the trigger for other services
+        # to notice that we are alive & available.
+        self.__bus_name = dbus.service.BusName("nl.ultimaker.charon", self.__dbus_bus)
+
     ##  Start a request for data from a file.
     #
     #   This function will start a request for data from a certain file.

Charon/Service/main.py

@@ -31,5 +31,6 @@
     _bus = dbus.SystemBus(private=True, mainloop=dbus.mainloop.glib.DBusGMainLoop())
 
 _service = Charon.Service.FileService(_bus)
+_service.publish()
 
 _loop.run()