content: remove Provider.Reader

After some analysis, it was found that Content.Reader was generally
redudant to an io.ReaderAt. This change removes `Content.Reader` in
favor of a `Content.ReaderAt`. In general, `ReaderAt` can perform better
over interfaces with indeterminant latency because it avoids remote
state for reads. Where a reader is required, a helper is provided to
convert it into an `io.SectionReader`.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

services/content/reader.go

@@ -7,54 +7,17 @@ import (
 	digest "github.com/opencontainers/go-digest"
 )
 
-type remoteReader struct {
-	client contentapi.Content_ReadClient
-	extra  []byte
-}
-
-func (rr *remoteReader) Read(p []byte) (n int, err error) {
-	n += copy(p, rr.extra)
-	if n >= len(p) {
-		if n <= len(rr.extra) {
-			rr.extra = rr.extra[n:]
-		} else {
-			rr.extra = rr.extra[:0]
-		}
-		return
-	}
-	rr.extra = rr.extra[:0]
-
-	p = p[n:]
-	for len(p) > 0 {
-		var resp *contentapi.ReadContentResponse
-		// fill our buffer up until we can fill p.
-		resp, err = rr.client.Recv()
-		if err != nil {
-			return
-		}
-
-		copied := copy(p, resp.Data)
-		n += copied
-		p = p[copied:]
-
-		if len(p) == 0 {
-			rr.extra = append(rr.extra, resp.Data[copied:]...)
-		}
-	}
-
-	return
-}
-
-func (rr *remoteReader) Close() error {
-	return rr.client.CloseSend()
-}
-
 type remoteReaderAt struct {
 	ctx    context.Context
 	digest digest.Digest
+	size   int64
 	client contentapi.ContentClient
 }
 
+func (ra *remoteReaderAt) Size() int64 {
+	return ra.size
+}
+
 func (ra *remoteReaderAt) ReadAt(p []byte, off int64) (n int, err error) {
 	rr := &contentapi.ReadContentRequest{
 		Digest: ra.digest,
@@ -80,3 +43,7 @@ func (ra *remoteReaderAt) ReadAt(p []byte, off int64) (n int, err error) {
 	}
 	return n, nil
 }
+
+func (rr *remoteReaderAt) Close() error {
+	return nil
+}

services/content/service.go

@@ -168,20 +168,11 @@ func (s *Service) Read(req *api.ReadContentRequest, session api.Content_ReadServ
 		return errdefs.ToGRPC(err)
 	}
 
-	rc, err := s.store.Reader(session.Context(), req.Digest)
+	ra, err := s.store.ReaderAt(session.Context(), req.Digest)
 	if err != nil {
 		return errdefs.ToGRPC(err)
 	}
-	defer rc.Close() // TODO(stevvooe): Cache these file descriptors for performance.
-
-	ra, ok := rc.(io.ReaderAt)
-	if !ok {
-		// TODO(stevvooe): Need to set this up to get correct behavior across
-		// board. May change interface to store to just return ReaderAtCloser.
-		// Possibly, we could just return io.ReaderAt and handle file
-		// descriptors internally.
-		return errors.New("content service only supports content stores that return ReaderAt")
-	}
+	defer ra.Close()
 
 	var (
 		offset = req.Offset

services/content/store.go

@@ -70,21 +70,16 @@ func (rs *remoteStore) Delete(ctx context.Context, dgst digest.Digest) error {
 	return nil
 }
 
-func (rs *remoteStore) Reader(ctx context.Context, dgst digest.Digest) (io.ReadCloser, error) {
-	client, err := rs.client.Read(ctx, &contentapi.ReadContentRequest{Digest: dgst})
+func (rs *remoteStore) ReaderAt(ctx context.Context, dgst digest.Digest) (content.ReaderAt, error) {
+	i, err := rs.Info(ctx, dgst)
 	if err != nil {
 		return nil, err
 	}
 
-	return &remoteReader{
-		client: client,
-	}, nil
-}
-
-func (rs *remoteStore) ReaderAt(ctx context.Context, dgst digest.Digest) (io.ReaderAt, error) {
 	return &remoteReaderAt{
 		ctx:    ctx,
 		digest: dgst,
+		size:   i.Size,
 		client: rs.client,
 	}, nil
 }

services/tasks/service.go

@@ -100,11 +100,11 @@ func (s *Service) Create(ctx context.Context, r *api.CreateTaskRequest) (*api.Cr
 		if r.Checkpoint.MediaType != images.MediaTypeContainerd1Checkpoint {
 			return nil, fmt.Errorf("unsupported checkpoint type %q", r.Checkpoint.MediaType)
 		}
-		reader, err := s.store.Reader(ctx, r.Checkpoint.Digest)
+		reader, err := s.store.ReaderAt(ctx, r.Checkpoint.Digest)
 		if err != nil {
 			return nil, err
 		}
-		_, err = archive.Apply(ctx, checkpointPath, reader)
+		_, err = archive.Apply(ctx, checkpointPath, content.NewReader(reader))
 		reader.Close()
 		if err != nil {
 			return nil, err