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

[Peter Westlake]

On Wed, Nov 27, 2013, at 19:20, exarkun at twistedmatrix.com wrote:
... 
> What about something like this instead?
> 
>   @implementer(IService)
>   class Runner(object):
>       ...
>       @classmethod
>       def loadFromWhatever(cls, name):
>           return deferLater(reactor, Runner, name)
> 
>       def __init__(self, name):
>           self.name = name
> 
>       def startService(self):
>           self.running = True
>           print 'realStartService', self.name, reactor.running
> 
>   def parent(service):
>       application.setServiceParent(service)

A small detail: should this be service.setServiceParent(application)?

Peter.
 
>   loading = Runner.initializeFromWhatever("foo")
>   loading.addCallback(parent)
>   loading.addCallback(lambda ignored: 
> Runner.initializeFromWhatever("bar"))
>   loading.addCallback(parent)
>   loading.addErrback(stopTheReactorOrWhatever)
> 
> The advantage I see of this approach is that a `Runner` never exists in
> the service hierarchy until it is fully initialized, started, and 
> running.
> 
> If a `Runner` is only partially ready and the process shuts down then 
> its `stopService` method isn't called because it's not part of the 
> service hierarchy.
> 
> I could definitely imagine a library to help with this kind of thing. 
> For example, perhaps you want the above encapsulated as:
> 
>   asynchronouslySetUpServices(application, [
>       lambda: Runner.initializeFromWhatever("foo"),
>       lambda: Runner.initialifrFromWhatever("bar")])
> 
> And maybe then you want to add in some logic so that if the application 
> gets shut down while some things are still being initialized then you 
> cancel their Deferred.  Then you have good cleanup support for the 
> uninitialized case - without complicating `stopService` (the cleanup 
> logic is isolated in the implementation of Deferred cancellation where 
> it belongs - eg, with this `deferLater`-based asynchronousness it's 
> alreay present since deferLater implements cancellation already).
> 
> Jean-Paul
> 
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