Add support for symbol link inside docker tar file (#987)

* Add support for symbol link inside docker tar file

* docker tar file containing multiple images will have symbol link for
layer files to save space, this will read from the target file instead
of the symbol link file

* Add unit test for symbol link inside tar file

* Fix format

pkg/v1/tarball/image.go

@@ -23,6 +23,8 @@ import (
 	"io"
 	"io/ioutil"
 	"os"
+	"path"
+	"path/filepath"
 	"sync"
 
 	"github.com/google/go-containerregistry/internal/gzip"
@@ -216,6 +218,10 @@ func extractFileFromTar(opener Opener, filePath string) (io.ReadCloser, error) {
 			return nil, err
 		}
 		if hdr.Name == filePath {
+			if hdr.Typeflag == tar.TypeSymlink || hdr.Typeflag == tar.TypeLink {
+				currentDir := filepath.Dir(filePath)
+				return extractFileFromTar(opener, path.Join(currentDir, hdr.Linkname))
+			}
 			close = false
 			return tarFile{
 				Reader: tf,

pkg/v1/tarball/image_test.go

@@ -15,9 +15,11 @@
 package tarball
 
 import (
+	"io/ioutil"
 	"testing"
 
 	"github.com/google/go-containerregistry/pkg/name"
+	v1 "github.com/google/go-containerregistry/pkg/v1"
 	"github.com/google/go-containerregistry/pkg/v1/validate"
 )
 
@@ -95,3 +97,33 @@ func TestBundleMultiple(t *testing.T) {
 		})
 	}
 }
+
+func TestLayerLink(t *testing.T) {
+	tag, err := name.NewTag("bazel/v1/tarball:test_image_3", name.WeakValidation)
+	if err != nil {
+		t.Fatalf("Error creating tag: %v", err)
+	}
+	img, err := ImageFromPath("testdata/test_link.tar", &tag)
+	if err != nil {
+		t.Fatalf("Error loading image: %v", img)
+	}
+	hash := v1.Hash{
+		Algorithm: "sha256",
+		Hex:       "8897395fd26dc44ad0e2a834335b33198cb41ac4d98dfddf58eced3853fa7b17",
+	}
+	layer, err := img.LayerByDiffID(hash)
+	if err != nil {
+		t.Fatalf("Error getting layer by diff ID: %v, %v", hash, err)
+	}
+	rc, err := layer.Uncompressed()
+	if err != nil {
+		t.Fatal(err)
+	}
+	bs, err := ioutil.ReadAll(rc)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(bs) == 0 {
+		t.Errorf("layer.Uncompressed() returned a link file")
+	}
+}