[Twisted-Python] Could Service.startService return a Deferred?

[Peter Westlake]

On Wed, Nov 27, 2013, at 17:13, exarkun at twistedmatrix.com wrote:
> On 02:58 pm, peter.westlake at pobox.com wrote:
> ... So would it be possible for 
> >t.a.s.Service.startService
> >to be allowed to return a Deferred? Then the next service would only be
> >started up when the Deferred fired.
> 
> Probably not.
> 
> There is some discussion on 
> <https://twistedmatrix.com/trac/ticket/5941>.

That's helpful, thanks. I hadn't realized that startService was called
before the reactor started.

Instead, I've passed in a Deferred to each service that needs to wait.
How does this look?

# -*- mode: python -*-
from twisted.application import service
from twisted.internet import defer, reactor
from twisted.internet.task import deferLater

application = service.Application('Dependencies')

def report(error):
    print 'ERROR', error
    reactor.stop()

class Runner(service.Service):
    def __init__(self, baton):
        self.baton = baton

    def startService(self):
        print 'startService', self.name, reactor.running
        self.baton.addCallback(lambda ignore:deferLater(reactor, 1.0,
        self.realStartService))
        self.baton.addErrback(report)

    def realStartService(self):
        print 'realStartService', self.name, reactor.running

baton = defer.succeed('pass me to each service')

foo = Runner(baton)
foo.setName('foo')
foo.setServiceParent(application)

bar = Runner(baton)
bar.setName('bar')
bar.setServiceParent(application)

Peter.