Stream docker export directly to consumer

Rather than queueing up into a `bytes.Buffer`.

In my test case (building kube master image) this reduces Maximum RSS (as
measured by time(1)) compared with the previous patch from 2.8G to 110M. The
tar output case goes from 2.1G to 110M also. Overall allocations are ~715M in
both cases.

Signed-off-by: Ian Campbell <ijc@docker.com>

src/moby/docker.go

@@ -4,7 +4,6 @@ package moby
 // and also using the Docker API not shelling out
 
 import (
-	"bytes"
 	"errors"
 	"fmt"
 	"io"
@@ -81,25 +80,18 @@ func dockerCreate(image string) (string, error) {
 	return respBody.ID, nil
 }
 
-func dockerExport(container string) ([]byte, error) {
+func dockerExport(container string) (io.ReadCloser, error) {
 	log.Debugf("docker export: %s", container)
 	cli, err := dockerClient()
 	if err != nil {
-		return []byte{}, errors.New("could not initialize Docker API client")
+		return nil, errors.New("could not initialize Docker API client")
 	}
 	responseBody, err := cli.ContainerExport(context.Background(), container)
 	if err != nil {
-		return []byte{}, err
-	}
-	defer responseBody.Close()
-
-	output := bytes.NewBuffer(nil)
-	_, err = io.Copy(output, responseBody)
-	if err != nil {
-		return []byte{}, err
+		return nil, err
 	}
 
-	return output.Bytes(), nil
+	return responseBody, err
 }
 
 func dockerRm(container string) error {

src/moby/image.go

@@ -119,15 +119,16 @@ func ImageTar(ref *reference.Spec, prefix string, tw tarWriter, trust bool, pull
 	if err != nil {
 		return fmt.Errorf("Failed to docker export container from container %s: %v", container, err)
 	}
+	defer contents.Close()
+
 	err = dockerRm(container)
 	if err != nil {
 		return fmt.Errorf("Failed to docker rm container %s: %v", container, err)
 	}
 
 	// now we need to filter out some files from the resulting tar archive
 
-	r := bytes.NewReader(contents)
-	tr := tar.NewReader(r)
+	tr := tar.NewReader(contents)
 
 	for {
 		hdr, err := tr.Next()