twisted - get OS-chosen listen port

I am writing a twisted P2P client using the application framework. The listen port for incoming connections will be on a random (OS-determined) port. However, I need a way to determine what that port is after creating it:

import twisted... etc.

application = service.Application('vmesh')
peerservice = MyPeerService()
servicecollection = service.IServiceCollection(application)
factory = MyPeerFactory(peerservice)
server = internet.TCPServer(0, factory) # listen on random port
listen_port = server.getHost().port # ??? doesn't work...
server.setServiceParent(servicecollection)

I can't find anything in the docs about querying the port created by internet.TCPServer() or by reactor.listenTCP() which it forwards to. I can't simply wait for a connection to occur since the client has to announce its port in order for those connections to ever happen.

Solution

listenTCP returns an IListeningPort, which has a getHost() method that gives back an object with a port. For example:

>>> from twisted.internet import reactor
>>> from twisted.internet.protocol import Factory
>>> port = reactor.listenTCP(0, Factory())
>>> port.getHost().port
55791

However, TCPServer doesn't call listenTCP until it is started with privilegedStartService. Plus, the IListeningPort isn't actually exposed via a public API. So, you will need to write your own Service. Luckily, it's quite easy to do this; TCPServer doesn't do very much. You just need to write one that reports its port somewhere as soon as it starts listening. Here's an example:

from twisted.internet import reactor
from twisted.application.service import Service

class PortReporter(Service, object):
    def __init__(self, factory, reportPort):
        self.factory = factory
        self.reportPort = reportPort

    def privilegedStartService(self):
        self.listeningPort = reactor.listenTCP(0, self.factory)
        self.reportPort(self.listeningPort.getHost().port)
        return super(PortReporter, self).privilegedStartService()

    def stopService(self):
        self.listeningPort.stopListening()
        return super(PortReporter, self).stopService()

You can then use this in a tac file, like so:

from twisted.internet.protocol import Factory
from twisted.application.service import Application
application = Application("test")
def showPortNumber(n):
    print("The port number is: %d" % (n,))
PortReporter(Factory(), showPortNumber).setServiceParent(application)