Should Twisted timeouts be cancelled when operation finishes successfully?

Question

What is a recommended practice for handling a timeout that was set for a Twisted operation that finished successfully before the timeout expired?

Should the timeout be left running and depend on the Deferred.cancel() method having no effect on a finished Deferred?

Or is it better to explicitly cancel the timeout when the operation finishes?

The second option seems cleaner, but requires more complex and error-prone code (it is easy to leave a running timer, especially for operations that require several steps).

Answer 1

Maybe the best thing would be to add a timeout-cancelling callback to the deferred in the same place where you set the timeout.

from twisted.internet import reactor, task

def foo():
    return 'done'

def somethingPossiblyCanceled():
    return task.deferLater(reactor, 1, foo)

def addTimeout(d, duration):
    timeout = reactor.callLater(duration, d.cancel)
    def cancelTimeout(result):
        if timeout.active():
            timeout.cancel()
            print('(timeout canceled)')
        return result
    d.addBoth(cancelTimeout)

def main():
    d = somethingPossiblyCanceled()

    addTimeout(d, 2)

    def finished(result):
        print(result)
        reactor.stop()
    def canceled(failure):
        print('timed out: {0}'.format(failure))
        reactor.stop()
    d.addCallbacks(finished, canceled)

reactor.callWhenRunning(main)
reactor.run()