Allow image layers to use any custom media type (#1136)

Workaround for #1133

Co-authored-by: Ryan Miller <rvmiller@google.com>

pkg/v1/layout/image.go

@@ -93,17 +93,10 @@ func (li *layoutImage) LayerByDigest(h v1.Hash) (partial.CompressedLayer, error)
 
 	for _, desc := range manifest.Layers {
 		if h == desc.Digest {
-			switch desc.MediaType {
-			case types.OCILayer, types.DockerLayer:
-				return &compressedBlob{
-					path: li.path,
-					desc: desc,
-				}, nil
-			default:
-				// TODO: We assume everything is a compressed blob, but that might not be true.
-				// TODO: Handle foreign layers.
-				return nil, fmt.Errorf("unexpected media type: %v for layer: %v", desc.MediaType, desc.Digest)
-			}
+			return &compressedBlob{
+				path: li.path,
+				desc: desc,
+			}, nil
 		}
 	}
 
@@ -131,6 +124,11 @@ func (b *compressedBlob) MediaType() (types.MediaType, error) {
 	return b.desc.MediaType, nil
 }
 
+// Descriptor implements partial.withDescriptor.
+func (b *compressedBlob) Descriptor() (*v1.Descriptor, error) {
+	return &b.desc, nil
+}
+
 // See partial.Exists.
 func (b *compressedBlob) Exists() (bool, error) {
 	_, err := os.Stat(b.path.blobPath(b.desc.Digest))

pkg/v1/layout/image_test.go

@@ -41,9 +41,15 @@ var (
 		Algorithm: "sha256",
 		Hex:       "deadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeef",
 	}
-	bogusPath        = filepath.Join("testdata", "does_not_exist")
-	testPath         = filepath.Join("testdata", "test_index")
-	testPathOneImage = filepath.Join("testdata", "test_index_one_image")
+	customManifestDigest = v1.Hash{
+		Algorithm: "sha256",
+		Hex:       "b544f71ecd82372bc9a3c0dbef378abfd2734fe437df81ff6e242a0d720d8e3e",
+	}
+	bogusPath                         = filepath.Join("testdata", "does_not_exist")
+	testPath                          = filepath.Join("testdata", "test_index")
+	testPathOneImage                  = filepath.Join("testdata", "test_index_one_image")
+	testPathMediaType                 = filepath.Join("testdata", "test_index_media_type")
+	customMediaType   types.MediaType = "application/tar+gzip"
 )
 
 func TestImage(t *testing.T) {
@@ -145,3 +151,31 @@ func TestImageErrors(t *testing.T) {
 		t.Errorf("Image(%s, %s) = nil, expected err", bogusPath, bogusDigest)
 	}
 }
+
+func TestImageCustomMediaType(t *testing.T) {
+	lp, err := FromPath(testPathMediaType)
+	if err != nil {
+		t.Fatalf("FromPath() = %v", err)
+	}
+	img, err := lp.Image(customManifestDigest)
+	if err != nil {
+		t.Fatalf("Image() = %v", err)
+	}
+	mt, err := img.MediaType()
+	if err != nil {
+		t.Errorf("MediaType() = %v", err)
+	} else if got, want := mt, types.OCIManifestSchema1; got != want {
+		t.Errorf("MediaType(); want: %v got: %v", want, got)
+	}
+	layers, err := img.Layers()
+	if err != nil {
+		t.Fatalf("img.Layers() = %v", err)
+	}
+	mediaType, err := layers[0].MediaType()
+	if err != nil {
+		t.Fatalf("img.Layers() = %v", err)
+	}
+	if got, want := mediaType, customMediaType; got != want {
+		t.Fatalf("MediaType(); want: %q got: %q", want, got)
+	}
+}

pkg/v1/layout/testdata/test_index_media_type/blobs/sha256/b544f71ecd82372bc9a3c0dbef378abfd2734fe437df81ff6e242a0d720d8e3e

@@ -0,0 +1,15 @@
+{
+  "schemaVersion": 2,
+  "config": {
+    "mediaType": "application/vnd.cncf.helm.config.v1+json",
+    "digest": "sha256:ca3d163bab055381827226140568f3bef7eaac187cebd76878e0b63e9e442356",
+    "size": 3
+  },
+  "layers": [
+    {
+      "mediaType": "application/tar+gzip",
+      "digest": "sha256:dc52c6e48a1d51a96047b059f16889bc889c4b4c28f3b36b3f93187f62fc0b2b",
+      "size": 167
+    }
+  ]
+}

pkg/v1/layout/testdata/test_index_media_type/blobs/sha256/ca3d163bab055381827226140568f3bef7eaac187cebd76878e0b63e9e442356

@@ -0,0 +1 @@
+{}

pkg/v1/layout/testdata/test_index_media_type/index.json

@@ -0,0 +1,10 @@
+{
+  "schemaVersion": 2,
+  "manifests": [
+    {
+      "mediaType": "application/vnd.oci.image.manifest.v1+json",
+      "size": 391,
+      "digest": "sha256:b544f71ecd82372bc9a3c0dbef378abfd2734fe437df81ff6e242a0d720d8e3e"
+    }
+  ]
+}

pkg/v1/layout/testdata/test_index_media_type/oci-layout

@@ -0,0 +1,3 @@
+{
+    "imageLayoutVersion": "1.0.0"
+}