http: fix serving from CWD with http.ServeFile

http: make Dir("") equivalent to Dir(".")

Fixes #2471.

R=golang-dev, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/5370061

src/pkg/net/http/fs.go

@@ -22,13 +22,19 @@ import (
 
 // A Dir implements http.FileSystem using the native file
 // system restricted to a specific directory tree.
+//
+// An empty Dir is treated as ".".
 type Dir string
 
 func (d Dir) Open(name string) (File, error) {
 	if filepath.Separator != '/' && strings.IndexRune(name, filepath.Separator) >= 0 {
 		return nil, errors.New("http: invalid character in file path")
 	}
-	f, err := os.Open(filepath.Join(string(d), filepath.FromSlash(path.Clean("/"+name))))
+	dir := string(d)
+	if dir == "" {
+		dir = "."
+	}
+	f, err := os.Open(filepath.Join(dir, filepath.FromSlash(path.Clean("/"+name))))
 	if err != nil {
 		return nil, err
 	}

src/pkg/net/http/fs_test.go

@@ -208,6 +208,20 @@ func TestDirJoin(t *testing.T) {
 	test(Dir("/etc/hosts"), "../")
 }
 
+func TestEmptyDirOpenCWD(t *testing.T) {
+	test := func(d Dir) {
+		name := "fs_test.go"
+		f, err := d.Open(name)
+		if err != nil {
+			t.Fatalf("open of %s: %v", name, err)
+		}
+		defer f.Close()
+	}
+	test(Dir(""))
+	test(Dir("."))
+	test(Dir("./"))
+}
+
 func TestServeFileContentType(t *testing.T) {
 	const ctype = "icecream/chocolate"
 	override := false
@@ -247,6 +261,20 @@ func TestServeFileMimeType(t *testing.T) {
 	}
 }
 
+func TestServeFileFromCWD(t *testing.T) {
+	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		ServeFile(w, r, "fs_test.go")
+	}))
+	defer ts.Close()
+	r, err := Get(ts.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if r.StatusCode != 200 {
+		t.Fatalf("expected 200 OK, got %s", r.Status)
+	}
+}
+
 func TestServeFileWithContentEncoding(t *testing.T) {
 	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
 		w.Header().Set("Content-Encoding", "foo")