Question

I have just started to learn twisted and wrote a small tcp server/client using the Tcp4endpoint-class. Everything works fine, except for one thing.

In order to detect the event that an unavailble port is given to the server as listen port, I have added an errback to the endpoint-deferer. This errback is triggered, however, I am unable to exit the application from the errback. Reactor.stop causes another failure saying that reactor is not running, while for example sys.exit triggers another error. The output from the two latter is only seen when I do ctrl+c and gc hits.

My question is, is there any way to make the application exit (cleanly) after a listenFailure has occured?

Answer 1

A minimal example would help make your question more clear. However, based on many years of Twisted experience, I have an educated guess. I think you wrote a program something like this:

from twisted.internet import endpoints, reactor, protocol

factory = protocol.Factory()
factory.protocol = protocol.Protocol
endpoint = endpoints.TCP4ServerEndpoint(reactor, 8000)
d = endpoint.listen(factory)
def listenFailed(reason):
    reactor.stop()
d.addErrback(listenFailed)

reactor.run()

You are on the right track. Unfortunately you have an ordering issue. The reason reactor.stop fails with ReactorNotRunning is that the listen Deferred fails ”before” you call reactor.run. That is, it has already failed by the time you did d.addErrback(listenFailed), so listenFailed was called right away.

There are a number of solutions to this. One is to write a .tac file and use services:

from twisted.internet import endpoints, reactor, protocol
from twisted.application.internet import StreamServerEndpointService
from twisted.application.service import Application

application = Application("Some Kind Of Server")

factory = protocol.Factory()
factory.protocol = protocol.Protocol
endpoint = endpoints.TCP4ServerEndpoint(reactor, 8000)

service = StreamServerEndpointService(endpoint, factory)
service.setServiceParent(application)

This is run using twistd, like twistd -y thisfile.tac

Another option is to use the low-level feature that services are based on, reactor.callWhenRunning:

from twisted.internet import endpoints, reactor, protocol

factory = protocol.Factory()
factory.protocol = protocol.Protocol
endpoint = endpoints.TCP4ServerEndpoint(reactor, 8000)

def listen():
    d = endpoint.listen(factory)
    def listenFailed(reason):
        reactor.stop()
    d.addErrback(listenFailed)

reactor.callWhenRunning(listen)
reactor.run()