Make file interface a bit more interoperable

fs.go

@@ -173,6 +173,17 @@ type File interface {
 	Lock() error
 	// Unlock unlocks the file.
 	Unlock() error
+	// Readdir reads the contents of the directory associated with file and returns a
+	// slice of up to n FileInfo values, as would be returned by Lstat, in directory order.
+	// Subsequent calls on the same file will yield further FileInfos.
+	// If n > 0, Readdir returns at most n FileInfo structures. In this case, if Readdir
+	// returns an empty slice, it will return a non-nil error explaining why. At the end of
+	// a directory, the error is io.EOF.
+	// If n <= 0, Readdir returns all the FileInfo from the directory in a single slice.
+	// In this case, if Readdir succeeds (reads all the way to the end of the directory),
+	// it returns the slice and a nil error. If it encounters an error before the end of the directory,
+	// Readdir returns the FileInfo read until that point and a non-nil error.
+	Readdir(count int) ([]os.FileInfo, error)
 	// Truncate the file.
 	Truncate(size int64) error
 }

memfs/memory.go

@@ -289,6 +289,11 @@ func (f *file) Truncate(size int64) error {
 	return nil
 }
 
+func (f *file) Readdir(count int) ([]os.FileInfo, error) {
+	// Not implemented
+	return nil, nil
+}
+
 func (f *file) Duplicate(filename string, mode os.FileMode, flag int) billy.File {
 	new := &file{
 		name:    filename,

test/mock.go

@@ -135,6 +135,10 @@ func (*FileMock) Truncate(size int64) error {
 	return nil
 }
 
+func (*FileMock) Readdir(count int) ([]os.FileInfo, error) {
+	return nil, nil
+}
+
 type OnlyReadCapFs struct {
 	BasicMock
 }