Add the option to exclude certain layers from the tarball.

pkg/v1/tarball/write.go

@@ -26,9 +26,15 @@ import (
 	"github.com/google/go-containerregistry/pkg/v1"
 )
 
+// LayerFilter defines a function for filtering layers.
+//  True - indicates the layer should be kept,
+//  False - indicates the layer should be excluded.
+type LayerFilter func(v1.Layer) (bool, error)
+
 // WriteOptions are used to expose optional information to guide or
 // control the image write.
 type WriteOptions struct {
+	LayerFilter LayerFilter
 	// TODO(mattmoor): Whether to store things compressed?
 }
 
@@ -66,6 +72,16 @@ func Write(tag name.Tag, img v1.Image, wo *WriteOptions, w io.Writer) error {
 		return err
 	}
 
+	// Default filter function
+	filter := func(_ v1.Layer) (bool, error) {
+		return true, nil
+	}
+	if wo != nil {
+		if wo.LayerFilter != nil {
+			filter = wo.LayerFilter
+		}
+	}
+
 	// Write the layers.
 	layers, err := img.Layers()
 	if err != nil {
@@ -89,6 +105,15 @@ func Write(tag name.Tag, img v1.Image, wo *WriteOptions, w io.Writer) error {
 		// https://www.gnu.org/software/gzip/manual/html_node/Overview.html
 		layerFiles[i] = fmt.Sprintf("%s.tar.gz", hex)
 
+		// We filter late because the lenght of layerFiles must match the diff_ids
+		// in config file.  It is ok if the file doesn't exist when the daemon
+		// already has a given layer, since it won't try to read it.
+		if keep, err := filter(l); err != nil {
+			return err
+		} else if !keep {
+			continue
+		}
+
 		r, err := l.Compressed()
 		if err != nil {
 			return err

pkg/v1/tarball/write_test.go

@@ -15,8 +15,11 @@
 package tarball
 
 import (
+	"archive/tar"
+	"io"
 	"io/ioutil"
 	"os"
+	"strings"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -84,6 +87,72 @@ func TestWrite(t *testing.T) {
 	}
 }
 
+func TestFilteredWrite(t *testing.T) {
+	// Make a tempfile for tarball writes.
+	fp, err := ioutil.TempFile("", "")
+	if err != nil {
+		t.Fatalf("Error creating temp file.")
+	}
+	t.Log(fp.Name())
+	defer fp.Close()
+	defer os.Remove(fp.Name())
+
+	// Make a random image
+	randImage, err := random.Image(256, 8)
+	if err != nil {
+		t.Fatalf("Error creating random image.")
+	}
+	tag, err := name.NewTag("gcr.io/foo/bar:latest", name.StrictValidation)
+	if err != nil {
+		t.Fatalf("Error creating test tag.")
+	}
+
+	layers, err := randImage.Layers()
+	if err != nil {
+		t.Fatalf("Layers() = %v", err)
+	}
+	rld, err := layers[0].Digest()
+	if err != nil {
+		t.Fatalf("Digest() = %v", err)
+	}
+
+	wo := &WriteOptions{
+		LayerFilter: func(l v1.Layer) (bool, error) {
+			// Filter the first layer in the image.
+			if ld, err := l.Digest(); err != nil {
+				return false, err
+			} else {
+				return ld != rld, nil
+			}
+		},
+	}
+
+	if err := WriteToFile(fp.Name(), tag, randImage, wo); err != nil {
+		t.Fatalf("Unexpected error writing tarball: %v", err)
+	}
+
+	f, err := os.Open(fp.Name())
+	if err != nil {
+		t.Fatalf("os.Open() = %v", err)
+	}
+	defer f.Close()
+
+	tarReader := tar.NewReader(f)
+	for {
+		header, err := tarReader.Next()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			t.Fatalf("scanning tarfile: %v", err)
+		}
+
+		if strings.Contains(header.Name, rld.Hex) {
+			t.Errorf("Saw file %v in tarball, want %v elided.", header.Name, rld)
+		}
+	}
+}
+
 func assertImageLayersMatchManifestLayers(t *testing.T, i v1.Image) {
 	t.Helper()