Add ReaderAt support to content store

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

content/content.go

@@ -30,6 +30,7 @@ var (
 
 type Provider interface {
 	Reader(ctx context.Context, dgst digest.Digest) (io.ReadCloser, error)
+	ReaderAt(ctx context.Context, dgst digest.Digest) (io.ReaderAt, error)
 }
 
 type Ingester interface {

content/readerat.go

@@ -0,0 +1,26 @@
+package content
+
+import (
+	"io"
+	"os"
+)
+
+// readerat implements io.ReaderAt in a completely stateless manner by opening
+// the referenced file for each call to ReadAt.
+type readerAt struct {
+	f string
+}
+
+func (ra readerAt) ReadAt(p []byte, offset int64) (int, error) {
+	fp, err := os.Open(ra.f)
+	if err != nil {
+		return 0, err
+	}
+	defer fp.Close()
+
+	if _, err := fp.Seek(offset, io.SeekStart); err != nil {
+		return 0, err
+	}
+
+	return fp.Read(p)
+}

content/store.go

@@ -58,11 +58,7 @@ func (s *store) info(dgst digest.Digest, fi os.FileInfo) Info {
 	}
 }
 
-// Open returns an io.ReadCloser for the blob.
-//
-// TODO(stevvooe): This would work much better as an io.ReaderAt in practice.
-// Right now, we are doing type assertion to tease that out, but it won't scale
-// well.
+// Reader returns an io.ReadCloser for the blob.
 func (s *store) Reader(ctx context.Context, dgst digest.Digest) (io.ReadCloser, error) {
 	fp, err := os.Open(s.blobPath(dgst))
 	if err != nil {
@@ -75,6 +71,11 @@ func (s *store) Reader(ctx context.Context, dgst digest.Digest) (io.ReadCloser,
 	return fp, nil
 }
 
+// ReaderAt returns an io.ReaderAt for the blob.
+func (s *store) ReaderAt(ctx context.Context, dgst digest.Digest) (io.ReaderAt, error) {
+	return readerAt{f: s.blobPath(dgst)}, nil
+}
+
 // Delete removes a blob by its digest.
 //
 // While this is safe to do concurrently, safe exist-removal logic must hold