+ /* this will drop the final reference to the deleted ports,
+ * which will in turn call their destructors, which will in
+ * turn call the backend to unregister them.
+ *
+ * There will no connect/disconnect or register/unregister
+ * callbacks from the backend until we get here, because
+ * they are driven by the Port destructor. The destructor
+ * will not execute until we drop the final reference,
+ * which all happens right .... here.
+ */
+ deleted_ports.clear ();
+