diff --git a/godoc/vfs/namespace_test.go b/godoc/vfs/namespace_test.go
new file mode 100644
index 0000000..edf3bc7
--- /dev/null
+++ b/godoc/vfs/namespace_test.go
@@ -0,0 +1,137 @@
+// Copyright 2016 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 vfs_test
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+	"time"
+
+	"golang.org/x/tools/godoc/vfs"
+	"golang.org/x/tools/godoc/vfs/mapfs"
+)
+
+func TestNewNameSpace(t *testing.T) {
+
+	// We will mount this filesystem under /fs1
+	mount := mapfs.New(map[string]string{"fs1file": "abcdefgh"})
+
+	// Existing process. This should give error on Stat("/")
+	t1 := vfs.NameSpace{}
+	t1.Bind("/fs1", mount, "/", vfs.BindReplace)
+
+	// using NewNameSpace. This should work fine.
+	t2 := vfs.NewNameSpace()
+	t2.Bind("/fs1", mount, "/", vfs.BindReplace)
+
+	testcases := map[string][]bool{
+		"/":            {false, true},
+		"/fs1":         {true, true},
+		"/fs1/fs1file": {true, true},
+	}
+
+	fss := []vfs.FileSystem{t1, t2}
+
+	for j, fs := range fss {
+		for k, v := range testcases {
+			_, err := fs.Stat(k)
+			result := err == nil
+			if result != v[j] {
+				t.Errorf("fs: %d, testcase: %s, want: %v, got: %v, err: %s", j, k, v[j], result, err)
+			}
+		}
+	}
+
+	fi, err := t2.Stat("/")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if fi.Name() != "/" {
+		t.Errorf("t2.Name() : want:%s got:%s", "/", fi.Name())
+	}
+
+	if !fi.ModTime().IsZero() {
+		t.Errorf("t2.ModTime() : want:%v got:%v", time.Time{}, fi.ModTime())
+	}
+}
+
+func TestReadDirUnion(t *testing.T) {
+	for _, tc := range []struct {
+		desc       string
+		ns         vfs.NameSpace
+		path, want string
+	}{
+		{
+			desc: "no_go_files",
+			ns: func() vfs.NameSpace {
+				rootFs := mapfs.New(map[string]string{
+					"doc/a.txt":       "1",
+					"doc/b.txt":       "1",
+					"doc/dir1/d1.txt": "",
+				})
+				docFs := mapfs.New(map[string]string{
+					"doc/a.txt":       "22",
+					"doc/dir2/d2.txt": "",
+				})
+				ns := vfs.NameSpace{}
+				ns.Bind("/", rootFs, "/", vfs.BindReplace)
+				ns.Bind("/doc", docFs, "/doc", vfs.BindBefore)
+				return ns
+			}(),
+			path: "/doc",
+			want: "a.txt:2,b.txt:1,dir1:0,dir2:0",
+		}, {
+			desc: "have_go_files",
+			ns: func() vfs.NameSpace {
+				a := mapfs.New(map[string]string{
+					"src/x/a.txt":        "",
+					"src/x/suba/sub.txt": "",
+				})
+				b := mapfs.New(map[string]string{
+					"src/x/b.go":         "package b",
+					"src/x/subb/sub.txt": "",
+				})
+				c := mapfs.New(map[string]string{
+					"src/x/c.txt":        "",
+					"src/x/subc/sub.txt": "",
+				})
+				ns := vfs.NameSpace{}
+				ns.Bind("/", a, "/", vfs.BindReplace)
+				ns.Bind("/", b, "/", vfs.BindAfter)
+				ns.Bind("/", c, "/", vfs.BindAfter)
+				return ns
+			}(),
+			path: "/src/x",
+			want: "b.go:9,suba:0,subb:0,subc:0",
+		}, {
+			desc: "empty_mount",
+			ns: func() vfs.NameSpace {
+				ns := vfs.NameSpace{}
+				ns.Bind("/empty", mapfs.New(nil), "/empty", vfs.BindReplace)
+				return ns
+			}(),
+			path: "/",
+			want: "empty:0",
+		},
+	} {
+		t.Run(tc.desc, func(t *testing.T) {
+			fis, err := tc.ns.ReadDir(tc.path)
+			if err != nil {
+				t.Fatal(err)
+			}
+			buf := &strings.Builder{}
+			sep := ""
+			for _, fi := range fis {
+				fmt.Fprintf(buf, "%s%s:%d", sep, fi.Name(), fi.Size())
+				sep = ","
+			}
+			if got := buf.String(); got != tc.want {
+				t.Errorf("got %q; want %q", got, tc.want)
+			}
+		})
+	}
+}