func (u *UndertakerAPI) environResourceWatcher() params.NotifyWatchResult {
	var nothing params.NotifyWatchResult
	machines, err := u.st.AllMachines()
	if err != nil {
		nothing.Error = common.ServerError(err)
		return nothing
	}
	services, err := u.st.AllServices()
	if err != nil {
		nothing.Error = common.ServerError(err)
		return nothing
	}
	var watchers []state.NotifyWatcher
	for _, machine := range machines {
		watchers = append(watchers, machine.Watch())
	}
	for _, service := range services {
		watchers = append(watchers, service.Watch())
	}

	watch := common.NewMultiNotifyWatcher(watchers...)

	if _, ok := <-watch.Changes(); ok {
		return params.NotifyWatchResult{
			NotifyWatcherId: u.resources.Register(watch),
		}
	}
	nothing.Error = common.ServerError(watcher.EnsureErr(watch))
	return nothing
}

// WatchBlockDevices watches for changes to the specified machines' block devices.
func (s *StorageProvisionerAPI) WatchBlockDevices(args params.Entities) (params.NotifyWatchResults, error) {
	canAccess, err := s.getBlockDevicesAuthFunc()
	if err != nil {
		return params.NotifyWatchResults{}, common.ServerError(common.ErrPerm)
	}
	results := params.NotifyWatchResults{
		Results: make([]params.NotifyWatchResult, len(args.Entities)),
	}
	one := func(arg params.Entity) (string, error) {
		machineTag, err := names.ParseMachineTag(arg.Tag)
		if err != nil {
			return "", err
		}
		if !canAccess(machineTag) {
			return "", common.ErrPerm
		}
		w := s.st.WatchBlockDevices(machineTag)
		if _, ok := <-w.Changes(); ok {
			return s.resources.Register(w), nil
		}
		return "", watcher.EnsureErr(w)
	}
	for i, arg := range args.Entities {
		var result params.NotifyWatchResult
		id, err := one(arg)
		if err != nil {
			result.Error = common.ServerError(err)
		} else {
			result.NotifyWatcherId = id
		}
		results.Results[i] = result
	}
	return results, nil
}

// WatchMachineErrorRetry returns a NotifyWatcher that notifies when
// the provisioner should retry provisioning machines with transient errors.
func (p *ProvisionerAPI) WatchMachineErrorRetry() (params.NotifyWatchResult, error) {
	result := params.NotifyWatchResult{}
	if !p.authorizer.AuthModelManager() {
		return result, common.ErrPerm
	}
	watch := newWatchMachineErrorRetry()
	// Consume any initial event and forward it to the result.
	if _, ok := <-watch.Changes(); ok {
		result.NotifyWatcherId = p.resources.Register(watch)
	} else {
		return result, watcher.EnsureErr(watch)
	}
	return result, nil
}

// WatchForModelConfigChanges returns a NotifyWatcher that observes
// changes to the environment configuration.
// Note that although the NotifyWatchResult contains an Error field,
// it's not used because we are only returning a single watcher,
// so we use the regular error return.
func (e *ModelWatcher) WatchForModelConfigChanges() (params.NotifyWatchResult, error) {
	result := params.NotifyWatchResult{}
	watch := e.st.WatchForModelConfigChanges()
	// Consume the initial event. Technically, API
	// calls to Watch 'transmit' the initial event
	// in the Watch response. But NotifyWatchers
	// have no state to transmit.
	if _, ok := <-watch.Changes(); ok {
		result.NotifyWatcherId = e.resources.Register(watch)
	} else {
		return result, watcher.EnsureErr(watch)
	}
	return result, nil
}

func (api *ProxyUpdaterAPI) oneWatch() params.NotifyWatchResult {
	var result params.NotifyWatchResult

	watch := common.NewMultiNotifyWatcher(
		api.backend.WatchForModelConfigChanges(),
		api.backend.WatchAPIHostPorts())

	if _, ok := <-watch.Changes(); ok {
		result = params.NotifyWatchResult{
			NotifyWatcherId: api.resources.Register(watch),
		}
	}
	result.Error = common.ServerError(watcher.EnsureErr(watch))
	return result
}

// WatchForRebootEvent starts a watcher to track if there is a new
// reboot request on the machines ID or any of its parents (in case we are a container).
func (r *RebootAPI) WatchForRebootEvent() (params.NotifyWatchResult, error) {
	err := common.ErrPerm
	var watch state.NotifyWatcher
	var result params.NotifyWatchResult

	if r.auth.AuthOwner(r.machine.Tag()) {
		watch = r.machine.WatchForRebootEvent()
		err = nil
		// Consume the initial event. Technically, API
		// calls to Watch 'transmit' the initial event
		// in the Watch response. But NotifyWatchers
		// have no state to transmit.
		if _, ok := <-watch.Changes(); ok {
			result.NotifyWatcherId = r.resources.Register(watch)
		} else {
			err = watcher.EnsureErr(watch)
		}
	}
	result.Error = common.ServerError(err)
	return result, nil
}