4 files changed, 127 insertions, 1 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
+}
diff --git a/src/io/fs/readdir_test.go b/src/io/fs/readdir_test.go
new file mode 100644
index 0000000000..46a4bc2788
--- /dev/null
+++ b/src/io/fs/readdir_test.go
@@ -0,0 +1,35 @@
+// 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_test
+
+import (
+	. "io/fs"
+	"testing"
+)
+
+type readDirOnly struct{ ReadDirFS }
+
+func (readDirOnly) Open(name string) (File, error) { return nil, ErrNotExist }
+
+func TestReadDir(t *testing.T) {
+	check := func(desc string, dirs []DirEntry, err error) {
+		t.Helper()
+		if err != nil || len(dirs) != 1 || dirs[0].Name() != "hello.txt" {
+			var names []string
+			for _, d := range dirs {
+				names = append(names, d.Name())
+			}
+			t.Errorf("ReadDir(%s) = %v, %v, want %v, nil", desc, names, err, []string{"hello.txt"})
+		}
+	}
+
+	// Test that ReadDir uses the method when present.
+	dirs, err := ReadDir(readDirOnly{testFsys}, ".")
+	check("readDirOnly", dirs, err)
+
+	// Test that ReadDir uses Open when the method is not present.
+	dirs, err = ReadDir(openOnly{testFsys}, ".")
+	check("openOnly", dirs, err)
+}
diff --git a/src/testing/fstest/mapfs.go b/src/testing/fstest/mapfs.go
index b01911e589..1eaf8f0040 100644
--- a/src/testing/fstest/mapfs.go
+++ b/src/testing/fstest/mapfs.go
@@ -124,6 +124,10 @@ func (fsys MapFS) Stat(name string) (fs.FileInfo, error) {
 	return fs.Stat(fsOnly{fsys}, name)
 }
 
+func (fsys MapFS) ReadDir(name string) ([]fs.DirEntry, error) {
+	return fs.ReadDir(fsOnly{fsys}, name)
+}
+
 // A mapFileInfo implements fs.FileInfo and fs.DirEntry for a given map file.
 type mapFileInfo struct {
 	name string
diff --git a/src/testing/fstest/testfs.go b/src/testing/fstest/testfs.go
index 290d2596cc..4ea6ed6095 100644
--- a/src/testing/fstest/testfs.go
+++ b/src/testing/fstest/testfs.go
@@ -196,6 +196,36 @@ func (t *fsTester) checkDir(dir string) {
 		}
 	}
 	t.checkDirList(dir, "first Open+ReadDir(-1) vs third Open+ReadDir(1,2) loop", list, list2)
+
+	// If fsys has ReadDir, check that it matches and is sorted.
+	if fsys, ok := t.fsys.(fs.ReadDirFS); ok {
+		list2, err := fsys.ReadDir(dir)
+		if err != nil {
+			t.errorf("%s: fsys.ReadDir: %v", dir, err)
+			return
+		}
+		t.checkDirList(dir, "first Open+ReadDir(-1) vs fsys.ReadDir", list, list2)
+
+		for i := 0; i+1 < len(list2); i++ {
+			if list2[i].Name() >= list2[i+1].Name() {
+				t.errorf("%s: fsys.ReadDir: list not sorted: %s before %s", dir, list2[i].Name(), list2[i+1].Name())
+			}
+		}
+	}
+
+	// Check fs.ReadDir as well.
+	list2, err = fs.ReadDir(t.fsys, dir)
+	if err != nil {
+		t.errorf("%s: fs.ReadDir: %v", dir, err)
+		return
+	}
+	t.checkDirList(dir, "first Open+ReadDir(-1) vs fs.ReadDir", list, list2)
+
+	for i := 0; i+1 < len(list2); i++ {
+		if list2[i].Name() >= list2[i+1].Name() {
+			t.errorf("%s: fs.ReadDir: list not sorted: %s before %s", dir, list2[i].Name(), list2[i+1].Name())
+		}
+	}
 }
 
 // formatEntry formats an fs.DirEntry into a string for error messages and comparison.
@@ -233,12 +263,22 @@ func (t *fsTester) checkStat(path string, entry fs.DirEntry) {
 		t.errorf("%s: mismatch:\n\tentry = %s\n\tfile.Stat() = %s", path, fentry, finfo)
 	}
 
+	einfo, err := entry.Info()
+	if err != nil {
+		t.errorf("%s: entry.Info: %v", path, err)
+		return
+	}
+	fentry = formatInfo(einfo)
+	finfo = formatInfo(info)
+	if fentry != finfo {
+		t.errorf("%s: mismatch:\n\tentry.Info() = %s\n\tfile.Stat() = %s\n", path, fentry, finfo)
+	}
+
 	info2, err := fs.Stat(t.fsys, path)
 	if err != nil {
 		t.errorf("%s: fs.Stat: %v", path, err)
 		return
 	}
-	finfo = formatInfo(info)
 	finfo2 := formatInfo(info2)
 	if finfo2 != finfo {
 		t.errorf("%s: fs.Stat(...) = %s\n\twant %s", path, finfo2, finfo)