storage/storage_transport.go
func (s *storageTransport) ParseReference(reference string) (types.ImageReference, error) {
	var store storage.Store
	// Check if there's a store location prefix.  If there is, then it
	// needs to match a store that was previously initialized using
	// storage.GetStore(), or be enough to let the storage library fill out
	// the rest using knowledge that it has from elsewhere.
	if reference[0] == '[' {
		closeIndex := strings.IndexRune(reference, ']')
		if closeIndex < 1 {
			return nil, ErrInvalidReference
		}
		storeSpec := reference[1:closeIndex]
		reference = reference[closeIndex+1:]
		// Peel off a "driver@" from the start.
		driverInfo := ""
		driverSplit := strings.SplitN(storeSpec, "@", 2)
		if len(driverSplit) != 2 {
			if storeSpec == "" {
				return nil, ErrInvalidReference
			}
		} else {
			driverInfo = driverSplit[0]
			if driverInfo == "" {
				return nil, ErrInvalidReference
			}
			storeSpec = driverSplit[1]
			if storeSpec == "" {
				return nil, ErrInvalidReference
			}
		}
		// Peel off a ":options" from the end.
		var options []string
		optionsSplit := strings.SplitN(storeSpec, ":", 2)
		if len(optionsSplit) == 2 {
			options = strings.Split(optionsSplit[1], ",")
			storeSpec = optionsSplit[0]
		}
		// Peel off a "+runroot" from the new end.
		runRootInfo := ""
		runRootSplit := strings.SplitN(storeSpec, "+", 2)
		if len(runRootSplit) == 2 {
			runRootInfo = runRootSplit[1]
			storeSpec = runRootSplit[0]
		}
		// The rest is our graph root.
		rootInfo := storeSpec
		// Check that any paths are absolute paths.
		if rootInfo != "" && !filepath.IsAbs(rootInfo) {
			return nil, ErrPathNotAbsolute
		}
		if runRootInfo != "" && !filepath.IsAbs(runRootInfo) {
			return nil, ErrPathNotAbsolute
		}
		store2, err := storage.GetStore(storage.StoreOptions{
			GraphDriverName:    driverInfo,
			GraphRoot:          rootInfo,
			RunRoot:            runRootInfo,
			GraphDriverOptions: options,
			UIDMap:             s.defaultUIDMap,
			GIDMap:             s.defaultGIDMap,
		})
		if err != nil {
			return nil, err
		}
		store = store2
	} else {
		// We didn't have a store spec, so use the default.
		store2, err := s.GetStore()
		if err != nil {
			return nil, err
		}
		store = store2
	}
	return s.ParseStoreReference(store, reference)
}
func (s *storageTransport) ParseReference(reference string) (types.ImageReference, error) {
	var store storage.Store
	// Check if there's a store location prefix.  If there is, then it
	// needs to match a store that was previously initialized using
	// storage.GetStore(), or be enough to let the storage library fill out
	// the rest using knowledge that it has from elsewhere.
	if reference[0] == '[' {
		closeIndex := strings.IndexRune(reference, ']')
		if closeIndex < 1 {
			return nil, ErrInvalidReference
		}
		storeSpec := reference[1:closeIndex]
		reference = reference[closeIndex+1:]
		// Peel off a "driver@" from the start.
		driverInfo := ""
		driverSplit := strings.SplitN(storeSpec, "@", 2)
		if len(driverSplit) != 2 {
			if storeSpec == "" {
				return nil, ErrInvalidReference
			}
		} else {
			driverInfo = driverSplit[0]
			if driverInfo == "" {
				return nil, ErrInvalidReference
			}
			storeSpec = driverSplit[1]
			if storeSpec == "" {
				return nil, ErrInvalidReference
			}
		}
		// Peel off a ":options" from the end.
		var options []string
		optionsSplit := strings.SplitN(storeSpec, ":", 2)
		if len(optionsSplit) == 2 {
			options = strings.Split(optionsSplit[1], ",")
			storeSpec = optionsSplit[0]
		}
		// Peel off a "+runroot" from the new end.
		runRootInfo := ""
		runRootSplit := strings.SplitN(storeSpec, "+", 2)
		if len(runRootSplit) == 2 {
			runRootInfo = runRootSplit[1]
			storeSpec = runRootSplit[0]
		}
		// The rest is our graph root.
		rootInfo := storeSpec
		// Check that any paths are absolute paths.
		if rootInfo != "" && !filepath.IsAbs(rootInfo) {
			return nil, ErrPathNotAbsolute
		}
		if runRootInfo != "" && !filepath.IsAbs(runRootInfo) {
			return nil, ErrPathNotAbsolute
		}
		store2, err := storage.GetStore(storage.StoreOptions{
			GraphDriverName:    driverInfo,
			GraphRoot:          rootInfo,
			RunRoot:            runRootInfo,
			GraphDriverOptions: options,
			UIDMap:             s.defaultUIDMap,
			GIDMap:             s.defaultGIDMap,
		})
		if err != nil {
			return nil, err
		}
		store = store2
	} else {
		// We didn't have a store spec, so use the default.
		store2, err := s.GetStore()
		if err != nil {
			return nil, err
		}
		store = store2
	}
	return s.ParseStoreReference(store, reference)
}
// ParseReference takes a name and a tag or digest and/or ID
// ("_name_"/"@_id_"/"_name_:_tag_"/"_name_:_tag_@_id_"/"_name_@_digest_"/"_name_@_digest_@_id_"/"_name_:_tag_@_digest_"/"_name_:_tag_@_digest_@_id_"),
// possibly prefixed with a store specifier in the form "[_graphroot_]" or
// "[_driver_@_graphroot_]" or "[_driver_@_graphroot_+_runroot_]" or
// "[_driver_@_graphroot_:_options_]" or "[_driver_@_graphroot_+_runroot_:_options_]",
// tries to figure out which it is, and returns it in a reference object.
// If _id_ is the ID of an image that's present in local storage, it can be truncated, and
// even be specified as if it were a _name_, value.