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
+}