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>

content/content.go

@@ -22,6 +22,7 @@ import (
 	"time"
 
 	"github.com/opencontainers/go-digest"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
 )
 
 // ReaderAt extends the standard io.ReaderAt interface with reporting of Size and io.Closer
@@ -33,12 +34,16 @@ type ReaderAt interface {
 
 // Provider provides a reader interface for specific content
 type Provider interface {
-	ReaderAt(ctx context.Context, dgst digest.Digest) (ReaderAt, error)
+	// ReaderAt only requires desc.Digest to be set.
+	// Other fields in the descriptor may be used internally for resolving
+	// the location of the actual data.
+	ReaderAt(ctx context.Context, dec ocispec.Descriptor) (ReaderAt, error)
 }
 
 // Ingester writes content
 type Ingester interface {
-	Writer(ctx context.Context, ref string, size int64, expected digest.Digest) (Writer, error)
+	// Some implementations require WithRef to be included in opts.
+	Writer(ctx context.Context, opts ...WriterOpt) (Writer, error)
 }
 
 // Info holds content specific information
@@ -142,3 +147,33 @@ func WithLabels(labels map[string]string) Opt {
 		return nil
 	}
 }
+
+// WriterOpts is internally used by WriterOpt.
+type WriterOpts struct {
+	Ref  string
+	Desc ocispec.Descriptor
+}
+
+// WriterOpt is used for passing options to Ingester.Writer.
+type WriterOpt func(*WriterOpts) error
+
+// WithDescriptor specifies an OCI descriptor.
+// Writer may optionally use the descriptor internally for resolving
+// the location of the actual data.
+// Write does not require any field of desc to be set.
+// If the data size is unknown, desc.Size should be set to 0.
+// Some implementations may also accept negative values as "unknown".
+func WithDescriptor(desc ocispec.Descriptor) WriterOpt {
+	return func(opts *WriterOpts) error {
+		opts.Desc = desc
+		return nil
+	}
+}
+
+// WithRef specifies a ref string.
+func WithRef(ref string) WriterOpt {
+	return func(opts *WriterOpts) error {
+		opts.Ref = ref
+		return nil
+	}
+}