Move MediaType into manifest.Versioned

This makes content type sniffing cleaner. The document just needs to be
decoded into a manifest.Versioned structure. It's no longer a two-step
process.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

manifest/manifestlist/manifestlist.go

@@ -17,6 +17,7 @@ const MediaTypeManifestList = "application/vnd.docker.distribution.manifest.list
 // packages version of the manifest.
 var SchemaVersion = manifest.Versioned{
 	SchemaVersion: 2,
+	MediaType:     MediaTypeManifestList,
 }
 
 func init() {
@@ -68,10 +69,6 @@ type ManifestDescriptor struct {
 type ManifestList struct {
 	manifest.Versioned
 
-	// MediaType is the media type of this document. It should always
-	// be set to MediaTypeManifestList.
-	MediaType string `json:"mediaType"`
-
 	// Config references the image configuration as a blob.
 	Manifests []ManifestDescriptor `json:"manifests"`
 }
@@ -102,7 +99,6 @@ type DeserializedManifestList struct {
 func FromDescriptors(descriptors []ManifestDescriptor) (*DeserializedManifestList, error) {
 	m := ManifestList{
 		Versioned: SchemaVersion,
-		MediaType: MediaTypeManifestList,
 	}
 
 	m.Manifests = make([]ManifestDescriptor, len(descriptors), len(descriptors))

manifest/schema2/builder.go

@@ -4,7 +4,6 @@ import (
 	"github.com/docker/distribution"
 	"github.com/docker/distribution/context"
 	"github.com/docker/distribution/digest"
-	"github.com/docker/distribution/manifest"
 )
 
 // builder is a type for constructing manifests.
@@ -36,10 +35,7 @@ func NewManifestBuilder(bs distribution.BlobService, configJSON []byte) distribu
 // Build produces a final manifest from the given references.
 func (mb *builder) Build(ctx context.Context) (distribution.Manifest, error) {
 	m := Manifest{
-		Versioned: manifest.Versioned{
-			SchemaVersion: 2,
-		},
-		MediaType: MediaTypeManifest,
+		Versioned: SchemaVersion,
 		Layers:    make([]distribution.Descriptor, len(mb.layers)),
 	}
 	copy(m.Layers, mb.layers)

manifest/schema2/manifest.go

@@ -27,6 +27,7 @@ var (
 	// packages version of the manifest.
 	SchemaVersion = manifest.Versioned{
 		SchemaVersion: 2,
+		MediaType:     MediaTypeManifest,
 	}
 )
 
@@ -50,7 +51,6 @@ func init() {
 // Manifest defines a schema2 manifest.
 type Manifest struct {
 	manifest.Versioned
-	MediaType string `json:"mediaType"`
 
 	// Config references the image configuration as a blob.
 	Config distribution.Descriptor `json:"config"`

manifest/schema2/manifest_test.go

@@ -29,7 +29,6 @@ var expectedManifestSerialization = []byte(`{
 func TestManifest(t *testing.T) {
 	manifest := Manifest{
 		Versioned: SchemaVersion,
-		MediaType: MediaTypeManifest,
 		Config: distribution.Descriptor{
 			Digest:    "sha256:1a9ec845ee94c202b2d5da74a24f0ed2058318bfa9879fa541efaecba272e86b",
 			Size:      985,

manifest/versioned.go

@@ -1,9 +1,12 @@
 package manifest
 
-// Versioned provides a struct with just the manifest schemaVersion. Incoming
+// Versioned provides a struct with the manifest schemaVersion and . Incoming
 // content with unknown schema version can be decoded against this struct to
 // check the version.
 type Versioned struct {
 	// SchemaVersion is the image manifest schema that this image follows
 	SchemaVersion int `json:"schemaVersion"`
+
+	// MediaType is the media type of this schema.
+	MediaType string `json:"mediaType,omitempty"`
 }