func (p *ProvisionerAPI) watchOneMachineContainers(arg params.WatchContainer) (params.StringsWatchResult, error) {
	nothing := params.StringsWatchResult{}
	canAccess, err := p.getAuthFunc()
	if err != nil {
		return nothing, common.ErrPerm
	}
	tag, err := names.ParseMachineTag(arg.MachineTag)
	if err != nil {
		return nothing, common.ErrPerm
	}
	if !canAccess(tag) {
		return nothing, common.ErrPerm
	}
	machine, err := p.st.Machine(tag.Id())
	if err != nil {
		return nothing, err
	}
	var watch state.StringsWatcher
	if arg.ContainerType != "" {
		watch = machine.WatchContainers(instance.ContainerType(arg.ContainerType))
	} else {
		watch = machine.WatchAllContainers()
	}
	// Consume the initial event and forward it to the result.
	if changes, ok := <-watch.Changes(); ok {
		return params.StringsWatchResult{
			StringsWatcherId: p.resources.Register(watch),
			Changes:          changes,
		}, nil
	}
	return nothing, watcher.EnsureErr(watch)
}

func assertChange(c *gc.C, w state.StringsWatcher) {
	select {
	case <-w.Changes():
		return
	case <-time.After(coretesting.LongWait):
		c.Fatalf("no changes on watcher")
	}
}

func (s *StorageProvisionerAPI) watchAttachments(
	args params.Entities,
	watchEnvironAttachments func() state.StringsWatcher,
	watchMachineAttachments func(names.MachineTag) state.StringsWatcher,
	parseAttachmentIds func([]string) ([]params.MachineStorageId, error),
) (params.MachineStorageIdsWatchResults, error) {
	canAccess, err := s.getScopeAuthFunc()
	if err != nil {
		return params.MachineStorageIdsWatchResults{}, common.ServerError(common.ErrPerm)
	}
	results := params.MachineStorageIdsWatchResults{
		Results: make([]params.MachineStorageIdsWatchResult, len(args.Entities)),
	}
	one := func(arg params.Entity) (string, []params.MachineStorageId, error) {
		tag, err := names.ParseTag(arg.Tag)
		if err != nil || !canAccess(tag) {
			return "", nil, common.ErrPerm
		}
		var w state.StringsWatcher
		if tag, ok := tag.(names.MachineTag); ok {
			w = watchMachineAttachments(tag)
		} else {
			w = watchEnvironAttachments()
		}
		if stringChanges, ok := <-w.Changes(); ok {
			changes, err := parseAttachmentIds(stringChanges)
			if err != nil {
				w.Stop()
				return "", nil, err
			}
			return s.resources.Register(w), changes, nil
		}
		return "", nil, watcher.EnsureErr(w)
	}
	for i, arg := range args.Entities {
		var result params.MachineStorageIdsWatchResult
		id, changes, err := one(arg)
		if err != nil {
			result.Error = common.ServerError(err)
		} else {
			result.MachineStorageIdsWatcherId = id
			result.Changes = changes
		}
		results.Results[i] = result
	}
	return results, nil
}