Twisted's unit testing framework Trial is quite strict on tests that don't clean up after themselves. In particular, any test that doesn't properly close all of its connections is fails with ugly errors.

If you are already glazing over with apathy towards unit tests, unglaze now. Disconnecting is a little more involved than you might expect, and there's every chance you aren't doing it correctly.

Thanks to moshez and teratorn for reviewing this document. Any errors are still my fault.

How to Disconnect in Twisted, Really

You are writing a test for some network code. You have already written many unit tests for the protocol and the protocol factories, and you assume that the lower level socket calls all work. You have deep confidence in your code, but you need more.

You yearn to write an end-to-end test. To set up the server, have it really listen, have the client really connect. You want data to zoom back and forth over the wire like luxury sportscars on the Autobahn.

Fine.

You start with a server and a client.

You want the client to connect to the server. You want things to happen. When things have happened, you want to assert stuff.

After the assertions, you want to disconnect the client and you want the server to go away.

Good.

I'm not going to help you set things up, I'm not going to help you make things happen. I am going to help you make all your connections just go away.

Let me lay down the law:

  1. The server must stop listening.
  2. The client connection must disconnect.
  3. The server connection must disconnect.

You have to make each of these things happen. That's easy. Here's the tricky part: you have to wait for each of these things to happen.

To make the server stop listening:
    port = reactor.listenTCP(portNumber, factory)
    ...
    port.stopListening()
You can effect the last two with one stroke, using either of the following commands:
    protocol.transport.loseConnection()

OR

    connector = reactor.connectTCP(host, port, factory)
    ...
    connector.disconnect()

Now, just running these commands is not enough. Twisted is asynchronous: you have to wait for it to call you.

The way to wait for something in a Trial test is to return a Deferred from the test method.

stopListening is easy. It returns a Deferred if it is going to take a while. The others are harder. There is only one way to know when the connection has truly been lost: override connectionLost on a Protocol instance. Pass a Deferred to your Protocol and have connectionLost call callback on that Deferred. You will need to do this for both the client and the server protocols. Once you have your three Deferreds, return them in a DeferredList.

Simple.

UPDATE: exarkun tells me that there are two ways to know when the connection has truly been lost on the client side. ClientFactory.clientConnectionLost is the other one.

Summary

In order to clean up a test that connects a client to a server, you need to wait on three Deferreds: one for the listening port, one for the server protocol and one for the client protocol.

Below is an example demonstrating how to disconnect. As you can see, it is quite verbose. Trial does not provide any shortcuts, even though it should.

from twisted.internet import defer, protocol
from twisted.trial import unittest

class ServerProtocol(protocol.Protocol):
    def connectionLost(self, *a):
        self.factory.onConnectionLost.callback(self)

class ClientProtocol(protocol.Protocol):
    def connectionMade(self):
        self.factory.onConnectionMade.callback(self)

    def connectionLost(self, *a):
        self.factory.onConnectionLost.callback(self)


class TestDisconnect(unittest.TestCase):
    def setUp(self):
        self.serverDisconnected = defer.Deferred()
        self.serverPort = self._listenServer(self.serverDisconnected)
        connected = defer.Deferred()
        self.clientDisconnected = defer.Deferred()
        self.clientConnection = self._connectClient(connected,
                                                    self.clientDisconnected)
        return connected

    def _listenServer(self, d):
        from twisted.internet import reactor
        f = protocol.Factory()
        f.onConnectionLost = d
        f.protocol = ServerProtocol
        return reactor.listenTCP(0, f)

    def _connectClient(self, d1, d2):
        from twisted.internet import reactor
        factory = protocol.ClientFactory()
        factory.protocol = ClientProtocol
        factory.onConnectionMade = d1
        factory.onConnectionLost = d2
        return reactor.connectTCP('localhost',
                                  self.serverPort.getHost().port,
                                  factory)

    def tearDown(self):
        d = defer.maybeDeferred(self.serverPort.stopListening)
        self.clientConnection.disconnect()
        return defer.gatherResults([d,
                                    self.clientDisconnected,
                                    self.serverDisconnected])

    def test_disconnect(self):
        pass

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How to Disconnect in Twisted, Really by Jonathan Lange is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.