services/content: move service client into package

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

services/content/provider.go

@@ -0,0 +1,76 @@
+package content
+
+import (
+	"context"
+	"io"
+
+	contentapi "github.com/docker/containerd/api/services/content"
+	"github.com/docker/containerd/content"
+	digest "github.com/opencontainers/go-digest"
+)
+
+func NewProviderFromClient(client contentapi.ContentClient) content.Provider {
+	return &remoteProvider{
+		client: client,
+	}
+}
+
+type remoteProvider struct {
+	client contentapi.ContentClient
+}
+
+func (rp *remoteProvider) Reader(ctx context.Context, dgst digest.Digest) (io.ReadCloser, error) {
+	client, err := rp.client.Read(ctx, &contentapi.ReadRequest{Digest: dgst})
+	if err != nil {
+		return nil, err
+	}
+
+	return &remoteReader{
+		client: client,
+	}, nil
+}
+
+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
+	}
+
+	p = p[n:]
+	for len(p) > 0 {
+		var resp *contentapi.ReadResponse
+		// 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 copied < len(p) {
+			continue
+		}
+
+		rr.extra = append(rr.extra, resp.Data[copied:]...)
+	}
+
+	return
+}
+
+// TODO(stevvooe): Implemente io.ReaderAt.
+
+func (rr *remoteReader) Close() error {
+	return rr.client.CloseSend()
+}