// watchMachinesLoop watches for changes provided by the given
// machinesWatcher and starts machine goroutines to deal with them,
// using the provided newMachineContext function to create the
// appropriate context for each new machine tag.
func watchMachinesLoop(context updaterContext, machinesWatcher watcher.StringsWatcher) (err error) {
	p := &updater{
		context:     context,
		machines:    make(map[names.MachineTag]chan struct{}),
		machineDead: make(chan machine),
	}
	defer func() {
		// TODO(fwereade): is this a home-grown sync.WaitGroup or something?
		// strongly suspect these machine goroutines could be managed rather
		// less opaquely if we made them all workers.
		for len(p.machines) > 0 {
			delete(p.machines, (<-p.machineDead).Tag())
		}
	}()
	for {
		select {
		case <-p.context.dying():
			return p.context.errDying()
		case ids, ok := <-machinesWatcher.Changes():
			if !ok {
				return errors.New("machines watcher closed")
			}
			tags := make([]names.MachineTag, len(ids))
			for i := range ids {
				tags[i] = names.NewMachineTag(ids[i])
			}
			if err := p.startMachines(tags); err != nil {
				return err
			}
		case m := <-p.machineDead:
			delete(p.machines, m.Tag())
		}
	}
}

func NewProvisionerTask(
	controllerUUID string,
	machineTag names.MachineTag,
	harvestMode config.HarvestMode,
	machineGetter MachineGetter,
	toolsFinder ToolsFinder,
	machineWatcher watcher.StringsWatcher,
	retryWatcher watcher.NotifyWatcher,
	broker environs.InstanceBroker,
	auth authentication.AuthenticationProvider,
	imageStream string,
	retryStartInstanceStrategy RetryStrategy,
) (ProvisionerTask, error) {
	machineChanges := machineWatcher.Changes()
	workers := []worker.Worker{machineWatcher}
	var retryChanges watcher.NotifyChannel
	if retryWatcher != nil {
		retryChanges = retryWatcher.Changes()
		workers = append(workers, retryWatcher)
	}
	task := &provisionerTask{
		controllerUUID:             controllerUUID,
		machineTag:                 machineTag,
		machineGetter:              machineGetter,
		toolsFinder:                toolsFinder,
		machineChanges:             machineChanges,
		retryChanges:               retryChanges,
		broker:                     broker,
		auth:                       auth,
		harvestMode:                harvestMode,
		harvestModeChan:            make(chan config.HarvestMode, 1),
		machines:                   make(map[string]*apiprovisioner.Machine),
		imageStream:                imageStream,
		retryStartInstanceStrategy: retryStartInstanceStrategy,
	}
	err := catacomb.Invoke(catacomb.Plan{
		Site: &task.catacomb,
		Work: task.loop,
		Init: workers,
	})
	if err != nil {
		return nil, errors.Trace(err)
	}
	return task, nil
}

// watchLoop watches the machine for units added or removed.
func (md *machineData) watchLoop(unitw watcher.StringsWatcher) error {
	if err := md.catacomb.Add(unitw); err != nil {
		return errors.Trace(err)
	}
	for {
		select {
		case <-md.catacomb.Dying():
			return md.catacomb.ErrDying()
		case change, ok := <-unitw.Changes():
			if !ok {
				return errors.New("machine units watcher closed")
			}
			select {
			case md.fw.unitsChange <- &unitsChange{md, change}:
			case <-md.catacomb.Dying():
				return md.catacomb.ErrDying()
			}
		}
	}
}