Fix panic in legacy tarball writer when config has no history (#569)

* Add package comments to fix lint errors

* Fix year

* Fix panic in legacy tarball writer when config has no history

* Address review comments

pkg/legacy/tarball/write.go

@@ -214,6 +214,13 @@ func MultiWrite(refToImage map[name.Reference]v1.Image, w io.Writer) error {
 		if err != nil {
 			return err
 		}
+		history := cfg.History
+		// Create a blank config history if the config didn't have a history.
+		if len(history) == 0 && len(layers) != 0 {
+			history = make([]v1.History, len(layers))
+		} else if len(layers) != len(history) {
+			return errors.Errorf("image config had layer history which did not match the number of layers, got len(history)=%d, len(layers)=%d, want len(history)=len(layers)", len(history), len(layers))
+		}
 		layerFiles := make([]string, len(layers))
 		var prev *v1Layer
 		for i, l := range layers {
@@ -222,9 +229,9 @@ func MultiWrite(refToImage map[name.Reference]v1.Image, w io.Writer) error {
 			}
 			var cur *v1Layer
 			if i < (len(layers) - 1) {
-				cur, err = newV1Layer(l, prev, cfg.History[i])
+				cur, err = newV1Layer(l, prev, history[i])
 			} else {
-				cur, err = newTopV1Layer(l, prev, cfg.History[i], cfg, cfgBlob)
+				cur, err = newTopV1Layer(l, prev, history[i], cfg, cfgBlob)
 			}
 			if err != nil {
 				return err

pkg/legacy/tarball/write_test.go

@@ -17,6 +17,7 @@ package tarball
 import (
 	"io/ioutil"
 	"os"
+	"strings"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -320,6 +321,77 @@ func TestWriteForeignLayers(t *testing.T) {
 	}
 }
 
+func TestMultiWriteNoHistory(t *testing.T) {
+	// Make a random image
+	img, err := random.Image(256, 8)
+	if err != nil {
+		t.Fatalf("Error creating random image: %v", err)
+	}
+	cfg, err := img.ConfigFile()
+	if err != nil {
+		t.Fatalf("Error getting image config: %v", err)
+	}
+	// Blank out the layer history.
+	cfg.History = nil
+	tag, err := name.NewTag("gcr.io/foo/bar:latest", name.StrictValidation)
+	if err != nil {
+		t.Fatalf("Error creating test tag: %v", err)
+	}
+	// Make a tempfile for tarball writes.
+	fp, err := ioutil.TempFile("", "")
+	if err != nil {
+		t.Fatalf("Error creating temp file: %v", err)
+	}
+	t.Log(fp.Name())
+	defer fp.Close()
+	defer os.Remove(fp.Name())
+	if err := Write(tag, img, fp); err != nil {
+		t.Fatalf("Unexpected error writing tarball: %v", err)
+	}
+	tarImage, err := tarball.ImageFromPath(fp.Name(), &tag)
+	if err != nil {
+		t.Fatalf("Unexpected error reading tarball: %v", err)
+	}
+	if err := validate.Image(tarImage); err != nil {
+		t.Fatalf("validate.Image(): %v", err)
+	}
+}
+
+func TestMultiWriteMismatchedHistory(t *testing.T) {
+	// Make a random image
+	img, err := random.Image(256, 8)
+	if err != nil {
+		t.Fatalf("Error creating random image: %v", err)
+	}
+	cfg, err := img.ConfigFile()
+	if err != nil {
+		t.Fatalf("Error getting image config: %v", err)
+	}
+	// Set the history such that number of history entries != layers. This
+	// should trigger an error during the image write.
+	cfg.History = make([]v1.History, 1)
+	tag, err := name.NewTag("gcr.io/foo/bar:latest", name.StrictValidation)
+	if err != nil {
+		t.Fatalf("Error creating test tag: %v", err)
+	}
+	// Make a tempfile for tarball writes.
+	fp, err := ioutil.TempFile("", "")
+	if err != nil {
+		t.Fatalf("Error creating temp file: %v", err)
+	}
+	t.Log(fp.Name())
+	defer fp.Close()
+	defer os.Remove(fp.Name())
+	err = Write(tag, img, fp)
+	if err == nil {
+		t.Fatal("Unexpected success writing tarball, got nil, want error.")
+	}
+	want := "image config had layer history which did not match the number of layers"
+	if !strings.Contains(err.Error(), want) {
+		t.Errorf("Got unexpected error when writing image with mismatched history & layer, got %v, want substring %q", err, want)
+	}
+}
+
 func assertLayersAreIdentical(t *testing.T, a, b v1.Image) {
 	t.Helper()