Allow RX only in the FDM loop

flightgear_python/fg_if.py

@@ -13,13 +13,13 @@
 from .general_util import EventPipe, strip_end
 from .fg_util import FGConnectionError, FGCommunicationError, fix_fg_radian_parsing
 
-rx_callback_type = Callable[[Struct, EventPipe], Struct]
+rx_callback_type = Callable[[Struct, EventPipe], Optional[Struct]]
 """
 RX callback function type, signature should be:
 
 .. code-block:: python
 
-    def rx_cb(fdm_data: Construct.Struct, event_pipe: EventPipe):
+    def rx_cb(fdm_data: Construct.Struct, event_pipe: EventPipe) -> Optional[Construct.Struct]:
 """
 
 
@@ -93,20 +93,18 @@ def _fg_packet_roundtrip(self):
         s = fix_fg_radian_parsing(s)
 
         # Call user method
-        if self.event_pipe.is_set() and self.event_pipe.child_poll():
-            # only update when we have data to send
-            s = self.fg_rx_cb(s, self.event_pipe)
-        else:
-            print('Receiving FG updates but no data to send', flush=True)
-        sys.stdout.flush()
-        tx_msg = self.fg_net_struct.build(dict(**s))
+        s = self.fg_rx_cb(s, self.event_pipe)
+        sys.stdout.flush()  # flush so that `print()` works
+
         # Send data back to FG
-        if self.fg_tx_sock is not None:
+        if self.fg_tx_sock is not None and s is not None:
+            tx_msg = self.fg_net_struct.build(dict(**s))
             self.fg_tx_sock.sendto(tx_msg, self.fg_tx_addr)
-        else:
-            print(f'Warning: TX not connected, not sending updates to FG for RX {self.fg_rx_sock.getsockname()}')
 
     def _rx_process(self):
+        if self.fg_tx_sock is None:
+            print(f'Warning: TX not connected, not sending updates to FG for RX {self.fg_rx_sock.getsockname()}')
+
         self.fg_rx_sock.settimeout(self.rx_timeout_s)
         while True:
             self._fg_packet_roundtrip()

tests/test_fdm_connection.py

@@ -60,3 +60,22 @@ def rx_cb(fdm_data, event_pipe):
         run_idx, callback_version = fdm_c.event_pipe.parent_recv()
         assert run_idx == i
         assert callback_version == fdm_version
+
+
+@pytest.mark.parametrize('fdm_version', supported_fdm_versions)
+def test_fdm_only_rx(mocker, fdm_version):
+    def rx_cb(fdm_data, event_pipe):
+        callback_version = fdm_data['version']
+        event_pipe.child_send((callback_version,))
+
+    fdm_c = FDMConnection(fdm_version)
+
+    setup_fdm_mock(mocker, fdm_version, fdm_c.fg_net_struct.sizeof())
+
+    fdm_c.connect_rx('localhost', 55550, rx_cb)
+
+    for i in range(5):
+        # manually call the process instead of having the process spawn
+        fdm_c._fg_packet_roundtrip()
+        callback_version, = fdm_c.event_pipe.parent_recv()
+        assert callback_version == fdm_version