[Twisted-Python] What to do when a service fails to start, also, deferred and startService

[Terry Jones]

Following up on myself....  Here's an updated/working version of the
MultiService subclass I proposed the other day:

    class GuardedMultiService(service.MultiService):

        def __init__(self, guardService):
            service.MultiService.__init__(self)
            self.guardService = guardService
            self.startDeferred = defer.Deferred()
            self.stopDeferred = defer.Deferred()

        def startService(self):
            def startOthers(_):
                log.msg("Guard service startup succeeded. Starting other services.")
                service.MultiService.startService(self)
            def initFailed(f):
                log.msg("Guard service startup failed.")
                return f
            d = defer.maybeDeferred(self.guardService.startService)
            d.addCallbacks(startOthers, initFailed)
            d.chainDeferred(self.startDeferred)

        def stopService(self):
            d = service.MultiService.stopService(self)
            d.addCallback(lambda _: self.guardService.stopService())
            d.chainDeferred(self.stopDeferred)
            return self.stopDeferred


This is pretty much as I described earlier.

I've added self.startDeferred and self.stopDeferred deferreds.  These give
the programmer using this class (who, I imagine, is probably doing so in
the context of writing a Twistd plugin - i.e., writing the makeService
method in a class implementing IServiceMaker and IPlugin) the chance to add
callbacks to the chain of events that happen when the service is started
and stopped. Their use is optional.

It's easy to see how to make guardService default to None, which would make
it possible to merge the above into MultiService itself.

If there's any interest at all in this, I'm happy to open a ticket, run
tests, submit a patch, etc.

Terry