Fix retransmission of packets.

Previously the padding bytes were being included twice in retransmitted
packets. This fixes that bug and introduces a test to ensure that
retransmitted packets are transmitted exactly.

rig/machine_control/scp_connection.py

@@ -281,8 +281,7 @@ def __init__(self, callback, packet, expected_args, extra_timeout):
                         raise TimeoutError(self.n_tries)
 
                     # Otherwise we retransmit it
-                    self.sock.send(b"\x00\x00" + outstanding.packet)
-                    # self.sock.send(outstanding.packet)
+                    self.sock.send(outstanding.packet)
                     outstanding.n_tries += 1
                     outstanding.time_sent = current_time
 

tests/machine_control/test_scp_connection.py

@@ -144,6 +144,21 @@ def test_single_packet_times_out(self, mock_conn):
         """Test correct operation for transmitting a single packet which is
         never acknowledged.
         """
+        # Create a callable for the socket send that asserts that we always
+        # send the same packet.
+        class Send(object):
+            def __init__(self):
+                self.last_packet = None
+
+            def __call__(self, packet):
+                if self.last_packet is None:
+                    self.last_packet = packet
+                else:
+                    assert self.last_packet == packet
+
+        mock_conn.sock.send.side_effect = Send()
+
+        # Create a generator of packets to send
         def packets():
             # Yield a single packet
             yield scpcall(3, 5, 0, 12)