Add an ImageSource and ImageDestination implementation for local directories

This is consistent with the (skopeo layers) storage layout; otherwise it
is expected to be used primarily as an a debugging aid when working on
more complex image transfers (e.g. directly from OpenShift to a running
Docker daemon), allowing them to be split to two simpler problems
between one complex storage mechanism and a simple directory.

Not used yet, users will be added in future commits.

directory.go

@@ -0,0 +1,102 @@
+package main
+
+import (
+	"fmt"
+	"io"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/projectatomic/skopeo/types"
+)
+
+// manifestPath returns a path for the manifest within a directory using our conventions.
+func manifestPath(dir string) string {
+	return filepath.Join(dir, "manifest.json")
+}
+
+// manifestPath returns a path for a layer tarball within a directory using our conventions.
+func layerPath(dir string, digest string) string {
+	// FIXME: Should we keep the digest identification?
+	return filepath.Join(dir, strings.TrimPrefix(digest, "sha256:")+".tar")
+}
+
+// manifestPath returns a path for a signature within a directory using our conventions.
+func signaturePath(dir string, index int) string {
+	return filepath.Join(dir, fmt.Sprintf("signature-%d", index+1))
+}
+
+type dirImageDestination struct {
+	dir string
+}
+
+// NewDirImageDestination returns an ImageDestination for writing to an existing directory.
+func NewDirImageDestination(dir string) types.ImageDestination {
+	return &dirImageDestination{dir}
+}
+
+func (d *dirImageDestination) PutManifest(manifest []byte) error {
+	return ioutil.WriteFile(manifestPath(d.dir), manifest, 0644)
+}
+
+func (d *dirImageDestination) PutLayer(digest string, stream io.Reader) error {
+	layerFile, err := os.Create(layerPath(d.dir, digest))
+	if err != nil {
+		return err
+	}
+	defer layerFile.Close()
+	if _, err := io.Copy(layerFile, stream); err != nil {
+		return err
+	}
+	if err := layerFile.Sync(); err != nil {
+		return err
+	}
+	return nil
+}
+
+func (d *dirImageDestination) PutSignatures(signatures [][]byte) error {
+	for i, sig := range signatures {
+		if err := ioutil.WriteFile(signaturePath(d.dir, i), sig, 0644); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+type dirImageSource struct {
+	dir string
+}
+
+// NewDirImageSource returns an ImageSource reading from an existing directory.
+func NewDirImageSource(dir string) types.ImageSource {
+	return &dirImageSource{dir}
+}
+
+func (s *dirImageSource) GetManifest() ([]byte, string, error) {
+	manifest, err := ioutil.ReadFile(manifestPath(s.dir))
+	if err != nil {
+		return nil, "", err
+	}
+
+	return manifest, "", nil // FIXME? unverifiedCanonicalDigest value - really primarily used by dockerImage
+}
+
+func (s *dirImageSource) GetLayer(digest string) (io.ReadCloser, error) {
+	return os.Open(layerPath(s.dir, digest))
+}
+
+func (s *dirImageSource) GetSignatures() ([][]byte, error) {
+	signatures := [][]byte{}
+	for i := 0; ; i++ {
+		signature, err := ioutil.ReadFile(signaturePath(s.dir, i))
+		if err != nil {
+			if os.IsNotExist(err) {
+				break
+			}
+			return nil, err
+		}
+		signatures = append(signatures, signature)
+	}
+	return signatures, nil
+}