Update storage package to use StorageDriver.Stat

This change updates the backend storage package that consumes StorageDriver to
use the new Stat call, over CurrentSize. It also makes minor updates for using
WriteStream and ReadStream.

storage/filereader.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"time"
 
 	"github.com/docker/docker-registry/storagedriver"
 )
@@ -16,8 +17,9 @@ type fileReader struct {
 	driver storagedriver.StorageDriver
 
 	// identifying fields
-	path string
-	size int64 // size is the total layer size, must be set.
+	path    string
+	size    int64 // size is the total layer size, must be set.
+	modtime time.Time
 
 	// mutable fields
 	rc     io.ReadCloser // remote read closer
@@ -28,16 +30,21 @@ type fileReader struct {
 
 func newFileReader(driver storagedriver.StorageDriver, path string) (*fileReader, error) {
 	// Grab the size of the layer file, ensuring existence.
-	size, err := driver.CurrentSize(path)
+	fi, err := driver.Stat(path)
 
 	if err != nil {
 		return nil, err
 	}
 
+	if fi.IsDir() {
+		return nil, fmt.Errorf("cannot read a directory")
+	}
+
 	return &fileReader{
-		driver: driver,
-		path:   path,
-		size:   int64(size),
+		driver:  driver,
+		path:    path,
+		size:    fi.Size(),
+		modtime: fi.ModTime(),
 	}, nil
 }
 
@@ -126,7 +133,7 @@ func (fr *fileReader) reader() (io.Reader, error) {
 	}
 
 	// If we don't have a reader, open one up.
-	rc, err := fr.driver.ReadStream(fr.path, uint64(fr.offset))
+	rc, err := fr.driver.ReadStream(fr.path, fr.offset)
 
 	if err != nil {
 		return nil, err