def remote_startCommand(self, stepref, stepId, command, args):
        """
        This gets invoked by L{buildbot.process.step.RemoteCommand.start}, as
        part of various master-side BuildSteps, to start various commands
        that actually do the build. I return nothing. Eventually I will call
        .commandComplete() to notify the master-side RemoteCommand that I'm
        done.
        """
        stepId = decode(stepId)
        command = decode(command)
        args = decode(args)

        self.activity()

        if self.command:
            log.msg("leftover command, dropping it")
            self.stopCommand()

        try:
            factory = registry.getFactory(command)
        except KeyError:
            raise UnknownCommand(u"unrecognized WorkerCommand '{0}'".format(command))
        self.command = factory(self, stepId, args)

        log.msg(u" startCommand:{0} [id {1}]".format(command, stepId))
        self.remoteStep = stepref
        self.remoteStep.notifyOnDisconnect(self.lostRemoteStep)
        d = self.command.doStart()
        d.addCallback(lambda res: None)
        d.addBoth(self.commandComplete)
        return None