// Storage is required to implement Filesystem.
func (f *filesystem) Storage() (names.StorageTag, error) {
	if f.doc.StorageId == "" {
		msg := fmt.Sprintf("filesystem %q is not assigned to any storage instance", f.Tag().Id())
		return names.StorageTag{}, errors.NewNotAssigned(nil, msg)
	}
	return names.NewStorageTag(f.doc.StorageId), nil
}

// StorageInstance is required to implement Volume.
func (v *volume) StorageInstance() (names.StorageTag, error) {
	if v.doc.StorageId == "" {
		msg := fmt.Sprintf("volume %q is not assigned to any storage instance", v.Tag().Id())
		return names.StorageTag{}, errors.NewNotAssigned(nil, msg)
	}
	return names.NewStorageTag(v.doc.StorageId), nil
}

// RestoreError makes a best effort at converting the given error
// back into an error originally converted by ServerError(). If the
// error could not be converted then false is returned.
func RestoreError(err error) (error, bool) {
	err = errors.Cause(err)

	if apiErr, ok := err.(*params.Error); !ok {
		return err, false
	} else if apiErr == nil {
		return nil, true
	}
	if params.ErrCode(err) == "" {
		return err, false
	}
	msg := err.Error()

	if singleton, ok := singletonError(err); ok {
		return singleton, true
	}

	// TODO(ericsnow) Support the other error types handled by ServerError().
	switch {
	case params.IsCodeUnauthorized(err):
		return errors.NewUnauthorized(nil, msg), true
	case params.IsCodeNotFound(err):
		// TODO(ericsnow) UnknownModelError should be handled here too.
		// ...by parsing msg?
		return errors.NewNotFound(nil, msg), true
	case params.IsCodeAlreadyExists(err):
		return errors.NewAlreadyExists(nil, msg), true
	case params.IsCodeNotAssigned(err):
		return errors.NewNotAssigned(nil, msg), true
	case params.IsCodeHasAssignedUnits(err):
		// TODO(ericsnow) Handle state.HasAssignedUnitsError here.
		// ...by parsing msg?
		return err, false
	case params.IsCodeNoAddressSet(err):
		// TODO(ericsnow) Handle isNoAddressSetError here.
		// ...by parsing msg?
		return err, false
	case params.IsCodeNotProvisioned(err):
		return errors.NewNotProvisioned(nil, msg), true
	case params.IsCodeUpgradeInProgress(err):
		// TODO(ericsnow) Handle state.UpgradeInProgressError here.
		// ...by parsing msg?
		return err, false
	case params.IsCodeMachineHasAttachedStorage(err):
		// TODO(ericsnow) Handle state.HasAttachmentsError here.
		// ...by parsing msg?
		return err, false
	case params.IsCodeNotSupported(err):
		return errors.NewNotSupported(nil, msg), true
	case params.IsBadRequest(err):
		return errors.NewBadRequest(nil, msg), true
	case params.IsMethodNotAllowed(err):
		return errors.NewMethodNotAllowed(nil, msg), true
	case params.ErrCode(err) == params.CodeDischargeRequired:
		// TODO(ericsnow) Handle DischargeRequiredError here.
		return err, false
	default:
		return err, false
	}
}

func (m *mockFilesystem) Storage() (names.StorageTag, error) {
	if m.storage != nil {
		return *m.storage, nil
	}
	return names.StorageTag{}, errors.NewNotAssigned(nil, "error from mock")
}

func (m *mockVolume) StorageInstance() (names.StorageTag, error) {
	if m.storage != nil {
		return *m.storage, nil
	}
	return names.StorageTag{}, errors.NewNotAssigned(nil, "error from mock")
}