Work around docker v25 tarballs (google#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>

Author: jonjohnsonjr

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",