Bug: oras.Copy() may skip tagging if manifest blob already exists in the destination

copy.go

@@ -342,5 +342,23 @@ func prepareCopy(ctx context.Context, dst Target, dstRef string, proxy *cas.Prox
 		}
 	}
 
+	onCopySkipped := opts.OnCopySkipped
+	opts.OnCopySkipped = func(ctx context.Context, desc ocispec.Descriptor) error {
+		if onCopySkipped != nil {
+			if err := onCopySkipped(ctx, desc); err != nil {
+				return err
+			}
+		}
+		if !descriptor.EqualOCI(desc, root) {
+			return nil
+		}
+		// copy reference for skipped root
+		if refPusher, ok := dst.(registry.ReferencePusher); ok {
+			return copyCachedNodeWithReference(ctx, proxy, refPusher, desc, dstRef)
+		} else {
+			return dst.Tag(ctx, root, dstRef)
+		}
+	}
+
 	return nil
 }

copy_test.go

@@ -135,6 +135,114 @@ func TestCopy_FullCopy(t *testing.T) {
 	}
 }
 
+func TestCopy_ExistedRoot(t *testing.T) {
+	src := memory.New()
+	dst := memory.New()
+
+	// generate test content
+	var blobs [][]byte
+	var descs []ocispec.Descriptor
+	appendBlob := func(mediaType string, blob []byte) {
+		blobs = append(blobs, blob)
+		descs = append(descs, ocispec.Descriptor{
+			MediaType: mediaType,
+			Digest:    digest.FromBytes(blob),
+			Size:      int64(len(blob)),
+		})
+	}
+	generateManifest := func(config ocispec.Descriptor, layers ...ocispec.Descriptor) {
+		manifest := ocispec.Manifest{
+			Config: config,
+			Layers: layers,
+		}
+		manifestJSON, err := json.Marshal(manifest)
+		if err != nil {
+			t.Fatal(err)
+		}
+		appendBlob(ocispec.MediaTypeImageManifest, manifestJSON)
+	}
+
+	appendBlob(ocispec.MediaTypeImageConfig, []byte("config")) // Blob 0
+	appendBlob(ocispec.MediaTypeImageLayer, []byte("foo"))     // Blob 1
+	appendBlob(ocispec.MediaTypeImageLayer, []byte("bar"))     // Blob 2
+	generateManifest(descs[0], descs[1:3]...)                  // Blob 3
+
+	ctx := context.Background()
+	for i := range blobs {
+		err := src.Push(ctx, descs[i], bytes.NewReader(blobs[i]))
+		if err != nil {
+			t.Fatalf("failed to push test content to src: %d: %v", i, err)
+		}
+	}
+
+	root := descs[3]
+	ref := "foobar"
+	newTag := "newtag"
+	err := src.Tag(ctx, root, ref)
+	if err != nil {
+		t.Fatal("fail to tag root node", err)
+	}
+
+	var skippedCount int64
+	copyOpts := oras.CopyOptions{
+		CopyGraphOptions: oras.CopyGraphOptions{
+			OnCopySkipped: func(ctx context.Context, desc ocispec.Descriptor) error {
+				atomic.AddInt64(&skippedCount, 1)
+				return nil
+			},
+		},
+	}
+
+	// copy with src tag
+	gotDesc, err := oras.Copy(ctx, src, ref, dst, "", copyOpts)
+	if err != nil {
+		t.Fatalf("Copy() error = %v, wantErr %v", err, false)
+	}
+	if !reflect.DeepEqual(gotDesc, root) {
+		t.Errorf("Copy() = %v, want %v", gotDesc, root)
+	}
+	// copy with new tag
+	gotDesc, err = oras.Copy(ctx, src, ref, dst, newTag, copyOpts)
+	if err != nil {
+		t.Fatalf("Copy() error = %v, wantErr %v", err, false)
+	}
+	if !reflect.DeepEqual(gotDesc, root) {
+		t.Errorf("Copy() = %v, want %v", gotDesc, root)
+	}
+
+	// verify contents
+	for i, desc := range descs {
+		exists, err := dst.Exists(ctx, desc)
+		if err != nil {
+			t.Fatalf("dst.Exists(%d) error = %v", i, err)
+		}
+		if !exists {
+			t.Errorf("dst.Exists(%d) = %v, want %v", i, exists, true)
+		}
+	}
+
+	// verify src tag
+	gotDesc, err = dst.Resolve(ctx, ref)
+	if err != nil {
+		t.Fatal("dst.Resolve() error =", err)
+	}
+	if !reflect.DeepEqual(gotDesc, root) {
+		t.Errorf("dst.Resolve() = %v, want %v", gotDesc, root)
+	}
+	// verify new tag
+	gotDesc, err = dst.Resolve(ctx, newTag)
+	if err != nil {
+		t.Fatal("dst.Resolve() error =", err)
+	}
+	if !reflect.DeepEqual(gotDesc, root) {
+		t.Errorf("dst.Resolve() = %v, want %v", gotDesc, root)
+	}
+	// verify invocation of onCopySkipped()
+	if got, want := skippedCount, int64(1); got != want {
+		t.Errorf("count(OnCopySkipped()) = %v, want %v", got, want)
+	}
+}
+
 func TestCopyGraph_FullCopy(t *testing.T) {
 	src := cas.NewMemory()
 	dst := cas.NewMemory()