// applicationSetCharm sets the charm for the given for the application.
func (api *API) applicationSetCharm(
	appName string,
	application Application,
	url string,
	channel csparams.Channel,
	configSettingsStrings map[string]string,
	configSettingsYAML string,
	forceSeries,
	forceUnits bool,
	resourceIDs map[string]string,
	storageConstraints map[string]params.StorageConstraints,
) error {
	curl, err := charm.ParseURL(url)
	if err != nil {
		return errors.Trace(err)
	}
	sch, err := api.backend.Charm(curl)
	if err != nil {
		return errors.Trace(err)
	}
	var settings charm.Settings
	if configSettingsYAML != "" {
		settings, err = sch.Config().ParseSettingsYAML([]byte(configSettingsYAML), appName)
	} else if len(configSettingsStrings) > 0 {
		settings, err = parseSettingsCompatible(sch.Config(), configSettingsStrings)
	}
	if err != nil {
		return errors.Annotate(err, "parsing config settings")
	}
	var stateStorageConstraints map[string]state.StorageConstraints
	if len(storageConstraints) > 0 {
		stateStorageConstraints = make(map[string]state.StorageConstraints)
		for name, cons := range storageConstraints {
			stateCons := state.StorageConstraints{Pool: cons.Pool}
			if cons.Size != nil {
				stateCons.Size = *cons.Size
			}
			if cons.Count != nil {
				stateCons.Count = *cons.Count
			}
			stateStorageConstraints[name] = stateCons
		}
	}
	cfg := state.SetCharmConfig{
		Charm:              api.stateCharm(sch),
		Channel:            channel,
		ConfigSettings:     settings,
		ForceSeries:        forceSeries,
		ForceUnits:         forceUnits,
		ResourceIDs:        resourceIDs,
		StorageConstraints: stateStorageConstraints,
	}
	return application.SetCharm(cfg)
}

// AddToUnit validates and creates additional storage instances for units.
// This method handles bulk add operations and
// a failure on one individual storage instance does not block remaining
// instances from being processed.
// A "CHANGE" block can block this operation.
func (a *API) AddToUnit(args params.StoragesAddParams) (params.ErrorResults, error) {
	// Check if changes are allowed and the operation may proceed.
	blockChecker := common.NewBlockChecker(a.storage)
	if err := blockChecker.ChangeAllowed(); err != nil {
		return params.ErrorResults{}, errors.Trace(err)
	}

	if len(args.Storages) == 0 {
		return params.ErrorResults{}, nil
	}

	serverErr := func(err error) params.ErrorResult {
		if errors.IsNotFound(err) {
			err = common.ErrPerm
		}
		return params.ErrorResult{Error: common.ServerError(err)}
	}

	paramsToState := func(p params.StorageConstraints) state.StorageConstraints {
		s := state.StorageConstraints{Pool: p.Pool}
		if p.Size != nil {
			s.Size = *p.Size
		}
		if p.Count != nil {
			s.Count = *p.Count
		}
		return s
	}

	result := make([]params.ErrorResult, len(args.Storages))
	for i, one := range args.Storages {
		u, err := names.ParseUnitTag(one.UnitTag)
		if err != nil {
			result[i] = serverErr(
				errors.Annotatef(err, "parsing unit tag %v", one.UnitTag))
			continue
		}

		err = a.storage.AddStorageForUnit(u,
			one.StorageName,
			paramsToState(one.Constraints))
		if err != nil {
			result[i] = serverErr(
				errors.Annotatef(err, "adding storage %v for %v", one.StorageName, one.UnitTag))
		}
	}
	return params.ErrorResults{Results: result}, nil
}