io/fs: add ReadDir and ReadDirFS

Add ReadDir helper function, ReadDirFS interface, and test.
Add ReadDir method to fstest.MapFS.
Add testing of ReadDir method to fstest.TestFS.

For #41190.

Change-Id: Ib860770ec7433ba77b29e626682b238f1b3bf54f
Reviewed-on: https://go-review.googlesource.com/c/go/+/243914
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

2 files changed, 82 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
+}
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)
+}