Support symlink layer in image import.

Signed-off-by: Lantao Liu <[email protected]>

pkg/containerd/importer/importer.go

@@ -60,6 +60,22 @@ func isLayerTar(name string) bool {
 	return slashes == 2 && strings.HasSuffix(name, "/layer.tar")
 }
 
+// followSymlinkLayer returns actual layer name of the symlink layer.
+// It returns "deadbeeddeadbeef/layer.tar" if the name is like
+// "../deadbeeddeadbeef/layer.tar", and returns error if the name
+// is not in "../xxxx/layer.tar" format.
+func followSymlinkLayer(name string) (string, error) {
+	parts := strings.Split(name, "/")
+	if len(parts) != 3 || parts[0] != ".." {
+		return "", errors.New("invalid symlink layer")
+	}
+	name = strings.TrimPrefix(name, "../")
+	if !isLayerTar(name) {
+		return "", errors.New("invalid layer tar")
+	}
+	return name, nil
+}
+
 // isDotJSON returns true if name is like "deadbeefdeadbeef.json"
 func isDotJSON(name string) bool {
 	slashes := len(strings.Split(name, "/"))
@@ -97,9 +113,10 @@ func Import(ctx context.Context, client *containerd.Client, reader io.Reader) (_
 
 	tr := tar.NewReader(reader)
 	var (
-		mfsts   []manifestDotJSON
-		layers  = make(map[string]ocispec.Descriptor) // key: filename (deadbeeddeadbeef/layer.tar)
-		configs = make(map[string]imageConfig)        // key: filename (deadbeeddeadbeef.json)
+		mfsts         []manifestDotJSON
+		symlinkLayers = make(map[string]string)             // key: filename (deadbeeddeadbeef/layer.tar), value: linkname (targetlayerid/layer.tar)
+		layers        = make(map[string]ocispec.Descriptor) // key: filename (deadbeeddeadbeef/layer.tar)
+		configs       = make(map[string]imageConfig)        // key: filename (deadbeeddeadbeef.json)
 	)
 	for {
 		hdr, err := tr.Next()
@@ -109,6 +126,14 @@ func Import(ctx context.Context, client *containerd.Client, reader io.Reader) (_
 		if err != nil {
 			return nil, errors.Wrap(err, "get next file")
 		}
+		if hdr.Typeflag == tar.TypeSymlink && isLayerTar(hdr.Name) {
+			linkname, err := followSymlinkLayer(hdr.Linkname)
+			if err != nil {
+				return nil, errors.Wrapf(err, "follow symlink layer from %q to %q", hdr.Name, hdr.Linkname)
+			}
+			symlinkLayers[hdr.Name] = linkname
+			continue
+		}
 		if hdr.Typeflag != tar.TypeReg && hdr.Typeflag != tar.TypeRegA {
 			continue
 		}
@@ -136,6 +161,13 @@ func Import(ctx context.Context, client *containerd.Client, reader io.Reader) (_
 			continue
 		}
 	}
+	for name, linkname := range symlinkLayers {
+		desc, ok := layers[linkname]
+		if !ok {
+			return nil, errors.Errorf("no target for symlink layer from %q to %q", name, linkname)
+		}
+		layers[name] = desc
+	}
 	var refs []string
 	defer func() {
 		if retErr == nil {