Question

I’m running the twisted.words MSN protocol example from the Twisted documentation located here: http://twistedmatrix.com/projects/words/documentation/examples/msn_example.py

However when I run this sample script, I get that UnhandledError exception:

2012-05-12 15:39:51-0300 [-] Log opened.
2012-05-12 15:39:51-0300 [-] Starting factory <twisted.internet.protocol.ClientFactory instance at  0x8c6c18c>
2012-05-12 15:39:52-0300 [Dispatch,client] Starting factory <twisted.words.protocols.msn.NotificationFactory instance at 0x8c6c2cc>
2012-05-12 15:39:52-0300 [Dispatch,client] Stopping factory <twisted.internet.protocol.ClientFactory instance at 0x8c6c18c>
2012-05-12 15:39:53-0300 [Notification,client] Starting factory <twisted.internet.protocol.ClientFactory instance at 0x8c710ec>
2012-05-12 15:39:53-0300 [Notification,client] Unhandled Error
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/twisted/python/log.py", line 84, in callWithLogger
    return callWithContext({"system": lp}, func, *args, **kw)
  File "/usr/lib/python2.7/dist-packages/twisted/python/log.py", line 69, in callWithContext
    return context.call({ILogContext: newCtx}, func, *args, **kw)
  File "/usr/lib/python2.7/dist-packages/twisted/python/context.py", line 118, in callWithContext
    return self.currentContext().callWithContext(ctx, func, *args, **kw)
  File "/usr/lib/python2.7/dist-packages/twisted/python/context.py", line 81, in callWithContext
    return func(*args,**kw)
--- <exception caught here> ---
  File "/usr/lib/python2.7/dist-packages/twisted/internet/selectreactor.py", line 146, in _doReadOrWrite
    why = getattr(selectable, method)()
  File "/usr/lib/python2.7/dist-packages/twisted/internet/tcp.py", line 460, in doRead
    rval = self.protocol.dataReceived(data)
  File "/usr/lib/python2.7/dist-packages/twisted/protocols/basic.py", line 564, in dataReceived
    why = self.lineReceived(line)
  File "/usr/lib/python2.7/dist-packages/twisted/words/protocols/msn.py", line 670, in lineReceived
    handler(params.split())
  File "/usr/lib/python2.7/dist-packages/twisted/words/protocols/msn.py", line 848, in handle_USR
    authData=params[3])
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 108, in execute
    return succeed(result)
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 71, in succeed
    d.callback(result)
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 360, in callback
    assert not isinstance(result, Deferred)
exceptions.AssertionError: 

2012-05-12 15:39:53-0300 [Notification,client] Stopping factory <twisted.words.protocols.msn.NotificationFactory instance at 0x8c6c2cc>
2012-05-12 15:39:55-0300 [PassportNexus,client] Starting factory <twisted.internet.protocol.ClientFactory instance at 0x8d014cc>
2012-05-12 15:39:55-0300 [PassportNexus,client] Stopping factory <twisted.internet.protocol.ClientFactory instance at 0x8c710ec>
2012-05-12 15:39:56-0300 [PassportLogin,client] Stopping factory <twisted.internet.protocol.ClientFactory instance at 0x8d014cc>

• Python 2.7.2+
• Twisted 11.0.0
• pyOpenSSL 0.12

Answer 1

I’ve never used the t.w.protocols.msn protocol before, but it looks to me like there’s a bug in Twisted’s msn.py on line 846. Here’s the function in question:

def handle_USR(self, params):
    if len(params) != 4 and len(params) != 6:
        raise MSNProtocolError, "Invalid Number of Parameters for USR"

    mechanism = params[1]
    if mechanism == "OK":
        self.loggedIn(params[2], unquote(params[3]), int(params[4]))
    elif params[2].upper() == "S":
        # we need to obtain auth from a passport server
        f = self.factory
        d = execute(                                                   # <------- here
            _login, f.userHandle, f.password, f.passportServer,
            authData=params[3])
        d.addCallback(self._passportLogin)
        d.addErrback(self._passportError)

defer.execute is called with msn._login and its parameters, but msn._login is supposed to return a Deferred. It’s not appropriate to use defer.execute with a function that might return a Deferred, since it will be immediately wrapped in another Deferred via defer.succeed, and Deferreds can not hold other Deferreds as results.

The execute call there should be using defer.maybeDeferred instead. I would expect that if you add maybeDeferred to the imports from twisted.internet.defer at the top of the file and change this execute call to a maybeDeferred call, it will start working (or at least start failing due to a different bug 🙂 ).