Refactor to optimize storage driver ApplyDiff()

To avoid an expensive call to archive.ChangesDirs() which walks two directory
trees and compares every entry, archive.ApplyLayer() has been extended to
also return the size of the layer changes.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

chrootarchive/diff.go

@@ -1,6 +1,8 @@
 package chrootarchive
 
 import (
+	"bytes"
+	"encoding/json"
 	"flag"
 	"fmt"
 	"io"
@@ -14,6 +16,10 @@ import (
 	"github.com/docker/docker/pkg/reexec"
 )
 
+type applyLayerResponse struct {
+	LayerSize int64 `json:"layerSize"`
+}
+
 func applyLayer() {
 	runtime.LockOSThread()
 	flag.Parse()
@@ -21,6 +27,7 @@ func applyLayer() {
 	if err := chroot(flag.Arg(0)); err != nil {
 		fatal(err)
 	}
+
 	// We need to be able to set any perms
 	oldmask := syscall.Umask(0)
 	defer syscall.Umask(oldmask)
@@ -28,33 +35,53 @@ func applyLayer() {
 	if err != nil {
 		fatal(err)
 	}
+
 	os.Setenv("TMPDIR", tmpDir)
-	err = archive.UnpackLayer("/", os.Stdin)
+	size, err := archive.UnpackLayer("/", os.Stdin)
 	os.RemoveAll(tmpDir)
 	if err != nil {
 		fatal(err)
 	}
-	os.RemoveAll(tmpDir)
+
+	encoder := json.NewEncoder(os.Stdout)
+	if err := encoder.Encode(applyLayerResponse{size}); err != nil {
+		fatal(fmt.Errorf("unable to encode layerSize JSON: %s", err))
+	}
+
+	flush(os.Stdout)
 	flush(os.Stdin)
 	os.Exit(0)
 }
 
-func ApplyLayer(dest string, layer archive.ArchiveReader) error {
+func ApplyLayer(dest string, layer archive.ArchiveReader) (size int64, err error) {
 	dest = filepath.Clean(dest)
 	decompressed, err := archive.DecompressStream(layer)
 	if err != nil {
-		return err
+		return 0, err
 	}
+
 	defer func() {
 		if c, ok := decompressed.(io.Closer); ok {
 			c.Close()
 		}
 	}()
+
 	cmd := reexec.Command("docker-applyLayer", dest)
 	cmd.Stdin = decompressed
-	out, err := cmd.CombinedOutput()
-	if err != nil {
-		return fmt.Errorf("ApplyLayer %s %s", err, out)
+
+	outBuf, errBuf := new(bytes.Buffer), new(bytes.Buffer)
+	cmd.Stdout, cmd.Stderr = outBuf, errBuf
+
+	if err = cmd.Run(); err != nil {
+		return 0, fmt.Errorf("ApplyLayer %s stdout: %s stderr: %s", err, outBuf, errBuf)
 	}
-	return nil
+
+	// Stdout should be a valid JSON struct representing an applyLayerResponse.
+	response := applyLayerResponse{}
+	decoder := json.NewDecoder(outBuf)
+	if err = decoder.Decode(&response); err != nil {
+		return 0, fmt.Errorf("unable to decode ApplyLayer JSON response: %s", err)
+	}
+
+	return response.LayerSize, nil
 }