package layer // import "github.com/docker/docker/layer" import ( "archive/tar" "bytes" "fmt" "io" ) // DigestSHA256EmptyTar is the canonical sha256 digest of empty tar file - // (1024 NULL bytes) const DigestSHA256EmptyTar = DiffID("sha256:5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef") type emptyLayer struct{} // EmptyLayer is a layer that corresponds to empty tar. var EmptyLayer = &emptyLayer{} func (el *emptyLayer) TarStream() (io.ReadCloser, error) { buf := new(bytes.Buffer) tarWriter := tar.NewWriter(buf) tarWriter.Close() return io.NopCloser(buf), nil } func (el *emptyLayer) TarStreamFrom(p ChainID) (io.ReadCloser, error) { if p == "" { return el.TarStream() } return nil, fmt.Errorf("can't get parent tar stream of an empty layer") } func (el *emptyLayer) ChainID() ChainID { return ChainID(DigestSHA256EmptyTar) } func (el *emptyLayer) DiffID() DiffID { return DigestSHA256EmptyTar } func (el *emptyLayer) Parent() Layer { return nil } func (el *emptyLayer) Size() (size int64, err error) { return 0, nil } func (el *emptyLayer) DiffSize() (size int64, err error) { return 0, nil } func (el *emptyLayer) Metadata() (map[string]string, error) { return make(map[string]string), nil } // IsEmpty returns true if the layer is an EmptyLayer func IsEmpty(diffID DiffID) bool { return diffID == DigestSHA256EmptyTar }