func storageAttachmentInfo(st storageAccess, a state.StorageAttachment) (_ names.MachineTag, location string, _ error) {
	machineTag, err := st.UnitAssignedMachine(a.Unit())
	if errors.IsNotAssigned(err) {
		return names.MachineTag{}, "", nil
	} else if err != nil {
		return names.MachineTag{}, "", errors.Trace(err)
	}
	info, err := storagecommon.StorageAttachmentInfo(st, a, machineTag)
	if errors.IsNotProvisioned(err) {
		return machineTag, "", nil
	} else if err != nil {
		return names.MachineTag{}, "", errors.Trace(err)
	}
	return machineTag, info.Location, nil
}

// StorageAttachmentInfo returns the StorageAttachmentInfo for the specified
// StorageAttachment by gathering information from related entities (volumes,
// filesystems).
func StorageAttachmentInfo(
	st StorageInterface,
	att state.StorageAttachment,
	machineTag names.MachineTag,
) (*storage.StorageAttachmentInfo, error) {
	storageInstance, err := st.StorageInstance(att.StorageInstance())
	if err != nil {
		return nil, errors.Annotate(err, "getting storage instance")
	}
	switch storageInstance.Kind() {
	case state.StorageKindBlock:
		return volumeStorageAttachmentInfo(st, storageInstance, machineTag)
	case state.StorageKindFilesystem:
		return filesystemStorageAttachmentInfo(st, storageInstance, machineTag)
	}
	return nil, errors.Errorf("invalid storage kind %v", storageInstance.Kind())
}

func (api *API) createParamsStorageAttachment(si params.StorageDetails, sa state.StorageAttachment) (params.StorageDetails, error) {
	result := params.StorageDetails{Status: "pending"}
	result.StorageTag = sa.StorageInstance().String()
	if result.StorageTag != si.StorageTag {
		panic("attachment does not belong to storage instance")
	}
	result.UnitTag = sa.Unit().String()
	result.OwnerTag = si.OwnerTag
	result.Kind = si.Kind
	result.Persistent = si.Persistent
	// TODO(axw) set status according to whether storage has been provisioned.

	// This is only for provisioned attachments
	machineTag, err := api.storage.UnitAssignedMachine(sa.Unit())
	if err != nil {
		return params.StorageDetails{}, errors.Annotate(err, "getting unit for storage attachment")
	}
	info, err := common.StorageAttachmentInfo(api.storage, sa, machineTag)
	if err != nil {
		if errors.IsNotProvisioned(err) {
			// If Info returns an error, then the storage has not yet been provisioned.
			return result, nil
		}
		return params.StorageDetails{}, errors.Annotate(err, "getting storage attachment info")
	}
	result.Location = info.Location
	if result.Location != "" {
		result.Status = "attached"
	}
	return result, nil
}

func (s *StorageAPI) fromStateStorageAttachment(stateStorageAttachment state.StorageAttachment) (params.StorageAttachment, error) {
	machineTag, err := s.st.UnitAssignedMachine(stateStorageAttachment.Unit())
	if err != nil {
		return params.StorageAttachment{}, err
	}
	info, err := common.StorageAttachmentInfo(s.st, stateStorageAttachment, machineTag)
	if err != nil {
		return params.StorageAttachment{}, err
	}
	stateStorageInstance, err := s.st.StorageInstance(stateStorageAttachment.StorageInstance())
	if err != nil {
		return params.StorageAttachment{}, err
	}
	return params.StorageAttachment{
		stateStorageAttachment.StorageInstance().String(),
		stateStorageInstance.Owner().String(),
		stateStorageAttachment.Unit().String(),
		params.StorageKind(stateStorageInstance.Kind()),
		info.Location,
		params.Life(stateStorageAttachment.Life().String()),
	}, nil
}