Add a default fileno function to the BusABC class. (#877)

Modify Notifier to handle the exception raised if the bus doesn't override it.

can/bus.py

@@ -199,7 +199,7 @@ def send_periodic(
             Disable to instead manage tasks manually.
         :return:
             A started task instance. Note the task can be stopped (and depending on
-            the backend modified) by calling the :meth:`stop` method.
+            the backend modified) by calling the task's :meth:`stop` method.
 
         .. note::
 
@@ -430,3 +430,6 @@ def _detect_available_configs() -> List[can.typechecking.AutoDetectedConfig]:
                  for usage in the interface's bus constructor.
         """
         raise NotImplementedError()
+
+    def fileno(self) -> int:
+        raise NotImplementedError("fileno is not implemented using current CAN bus")

can/notifier.py

@@ -61,23 +61,26 @@ def add_bus(self, bus: BusABC):
         :param bus:
             CAN bus instance.
         """
-        if (
-            self._loop is not None
-            and hasattr(bus, "fileno")
-            and bus.fileno() >= 0  # type: ignore
-        ):
-            # Use file descriptor to watch for messages
-            reader = bus.fileno()  # type: ignore
+        reader: int = -1
+        try:
+            reader = bus.fileno()
+        except NotImplementedError:
+            # Bus doesn't support fileno, we fall back to thread based reader
+            pass
+
+        if self._loop is not None and reader >= 0:
+            # Use bus file descriptor to watch for messages
             self._loop.add_reader(reader, self._on_message_available, bus)
+            self._readers.append(reader)
         else:
-            reader = threading.Thread(
+            reader_thread = threading.Thread(
                 target=self._rx_thread,
                 args=(bus,),
                 name='can.notifier for bus "{}"'.format(bus.channel_info),
             )
-            reader.daemon = True
-            reader.start()
-        self._readers.append(reader)
+            reader_thread.daemon = True
+            reader_thread.start()
+            self._readers.append(reader_thread)
 
     def stop(self, timeout: float = 5):
         """Stop notifying Listeners when new :class:`~can.Message` objects arrive