maybe send ack works needs a running trasport

This fixes a regression, a context switch was introduced after the check
for the state of the datagram server, this context switch exposed a race
that allowed a call to `transport.send` on a closed transport.

raiden/network/protocol.py

@@ -590,8 +590,17 @@ def _maybe_send_ack(self, receiver_address, messagedata):
         """ ACK must not go into the queue, otherwise nodes will deadlock
         waiting for the confirmation.
         """
+        host_port = self.get_host_port(receiver_address)
+
+        # ACKs are sent at the end of the receive method, after the message is
+        # sucessfully processed. It may be the case that the server is stopped
+        # after the message is received but before the ack is sent, under that
+        # circumstance the udp socket would be unavaiable and then an exception
+        # is raised.
+        #
+        # This check verifies the udp socket is still available before trying
+        # to send the ack. There must be *no context-switches after this test*.
         if self.transport.server.started:
-            host_port = self.get_host_port(receiver_address)
             self.transport.send(
                 self.raiden,
                 host_port,

raiden/network/transport.py

@@ -102,6 +102,9 @@ def send(self, sender, host_port, bytes_):
         if sleep_timeout:
             gevent.sleep(sleep_timeout)
 
+        if not hasattr(self.server, 'socket'):
+            raise RuntimeError('trying to send a message on a closed server')
+
         self.server.sendto(bytes_, host_port)
 
         # enable debugging using the DummyNetwork callbacks