Work around docker v25 tarballs (#1872)

Previously, we'd only seen compressed layers have LayerSources. As of
v25, docker puts all layers in here, uncompressed, which exposed a bug
in how Descriptor interacts with partial's uncompressed layer handling.

Signed-off-by: Jon Johnson <jon.johnson@chainguard.dev>

pkg/v1/tarball/image.go

@@ -299,18 +299,29 @@ func (i *uncompressedImage) LayerByDiffID(h v1.Hash) (partial.UncompressedLayer,
 			// v1.Layer doesn't force consumers to care about whether the layer is compressed
 			// we should be fine returning the DockerLayer media type
 			mt := types.DockerLayer
-			if bd, ok := i.imgDescriptor.LayerSources[h]; ok {
-				// Overwrite the mediaType for foreign layers.
-				return &foreignUncompressedLayer{
-					uncompressedLayerFromTarball: uncompressedLayerFromTarball{
-						diffID:    diffID,
-						mediaType: bd.MediaType,
-						opener:    i.opener,
-						filePath:  i.imgDescriptor.Layers[idx],
-					},
-					desc: bd,
-				}, nil
+			bd, ok := i.imgDescriptor.LayerSources[h]
+			if ok {
+				// This is janky, but we don't want to implement Descriptor for
+				// uncompressed layers because it breaks a bunch of assumptions in partial.
+				// See https://github.com/google/go-containerregistry/issues/1870
+				docker25workaround := bd.MediaType == types.DockerUncompressedLayer || bd.MediaType == types.OCIUncompressedLayer
+
+				if !docker25workaround {
+					// Overwrite the mediaType for foreign layers.
+					return &foreignUncompressedLayer{
+						uncompressedLayerFromTarball: uncompressedLayerFromTarball{
+							diffID:    diffID,
+							mediaType: bd.MediaType,
+							opener:    i.opener,
+							filePath:  i.imgDescriptor.Layers[idx],
+						},
+						desc: bd,
+					}, nil
+				}
+
+				// Intentional fall through.
 			}
+
 			return &uncompressedLayerFromTarball{
 				diffID:    diffID,
 				mediaType: mt,

pkg/v1/tarball/image_test.go

@@ -70,6 +70,16 @@ func TestBundleSingle(t *testing.T) {
 	}
 }
 
+func TestDocker25(t *testing.T) {
+	img, err := ImageFromPath("testdata/hello-world-v25.tar", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := validate.Image(img); err != nil {
+		t.Fatal(err)
+	}
+}
+
 func TestBundleMultiple(t *testing.T) {
 	for _, imgName := range []string{
 		"test_image_1",