1 files changed, 47 insertions, 0 deletions
diff --git a/src/io/fs/readdir.go b/src/io/fs/readdir.go
new file mode 100644
index 0000000000..3a5aa6d86a
--- /dev/null
+++ b/src/io/fs/readdir.go
@@ -0,0 +1,47 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fs
+
+import (
+	"errors"
+	"sort"
+)
+
+// ReadDirFS is the interface implemented by a file system
+// that provides an optimized implementation of ReadDir.
+type ReadDirFS interface {
+	FS
+
+	// ReadDir reads the named directory
+	// and returns a list of directory entries sorted by filename.
+	ReadDir(name string) ([]DirEntry, error)
+}
+
+// ReadDir reads the named directory
+// and returns a list of directory entries sorted by filename.
+//
+// If fs implements ReadDirFS, ReadDir calls fs.ReadDir.
+// Otherwise ReadDir calls fs.Open and uses ReadDir and Close
+// on the returned file.
+func ReadDir(fsys FS, name string) ([]DirEntry, error) {
+	if fsys, ok := fsys.(ReadDirFS); ok {
+		return fsys.ReadDir(name)
+	}
+
+	file, err := fsys.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	defer file.Close()
+
+	dir, ok := file.(ReadDirFile)
+	if !ok {
+		return nil, &PathError{Op: "readdir", Path: name, Err: errors.New("not implemented")}
+	}
+
+	list, err := dir.ReadDir(-1)
+	sort.Slice(list, func(i, j int) bool { return list[i].Name() < list[j].Name() })
+	return list, err
+}