content: change Writer/ReaderAt to take OCI

This change allows implementations to resolve the location of the actual data
using OCI descriptor fields such as MediaType.

No OCI descriptor field is written to the store.

No change on gRPC API.

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

remotes/handlers.go

@@ -81,7 +81,7 @@ func FetchHandler(ingester content.Ingester, fetcher Fetcher) images.HandlerFunc
 func fetch(ctx context.Context, ingester content.Ingester, fetcher Fetcher, desc ocispec.Descriptor) error {
 	log.G(ctx).Debug("fetch")
 
-	cw, err := content.OpenWriter(ctx, ingester, MakeRefKey(ctx, desc), desc.Size, desc.Digest)
+	cw, err := content.OpenWriter(ctx, ingester, content.WithRef(MakeRefKey(ctx, desc)), content.WithDescriptor(desc))
 	if err != nil {
 		if errdefs.IsAlreadyExists(err) {
 			return nil
@@ -141,7 +141,7 @@ func push(ctx context.Context, provider content.Provider, pusher Pusher, desc oc
 	}
 	defer cw.Close()
 
-	ra, err := provider.ReaderAt(ctx, desc.Digest)
+	ra, err := provider.ReaderAt(ctx, desc)
 	if err != nil {
 		return err
 	}