storage: enforce sorted traversal during Walk

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

docs/storage/walk.go

@@ -3,6 +3,7 @@ package storage
 import (
 	"errors"
 	"fmt"
+	"sort"
 
 	"github.com/docker/distribution/context"
 	storageDriver "github.com/docker/distribution/registry/storage/driver"
@@ -26,7 +27,12 @@ func Walk(ctx context.Context, driver storageDriver.StorageDriver, from string,
 	if err != nil {
 		return err
 	}
+	sort.Stable(sort.StringSlice(children))
 	for _, child := range children {
+		// TODO(stevvooe): Calling driver.Stat for every entry is quite
+		// expensive when running against backends with a slow Stat
+		// implementation, such as s3. This is very likely a serious
+		// performance bottleneck.
 		fileInfo, err := driver.Stat(ctx, child)
 		if err != nil {
 			return err