http: make serveFile redirects relative to work with StripPrefix

serveFile was using absolute redirects, which didn't work under StripPrefix.
Now it uses relative redirects.

R=golang-dev, rsc, bradfitz
CC=golang-dev, kevlar
https://golang.org/cl/4789042

src/pkg/http/fs.go

@@ -102,8 +102,10 @@ func serveFile(w ResponseWriter, r *Request, fs FileSystem, name string, redirec
 	const indexPage = "/index.html"
 
 	// redirect .../index.html to .../
+	// can't use Redirect() because that would make the path absolute,
+	// which would be a problem running under StripPrefix
 	if strings.HasSuffix(r.URL.Path, indexPage) {
-		Redirect(w, r, r.URL.Path[0:len(r.URL.Path)-len(indexPage)+1], StatusMovedPermanently)
+		localRedirect(w, r, "./")
 		return
 	}
 
@@ -128,12 +130,12 @@ func serveFile(w ResponseWriter, r *Request, fs FileSystem, name string, redirec
 		url := r.URL.Path
 		if d.IsDirectory() {
 			if url[len(url)-1] != '/' {
-				Redirect(w, r, url+"/", StatusMovedPermanently)
+				localRedirect(w, r, path.Base(url)+"/")
 				return
 			}
 		} else {
 			if url[len(url)-1] == '/' {
-				Redirect(w, r, url[0:len(url)-1], StatusMovedPermanently)
+				localRedirect(w, r, "../"+path.Base(url))
 				return
 			}
 		}
@@ -221,6 +223,16 @@ func serveFile(w ResponseWriter, r *Request, fs FileSystem, name string, redirec
 	}
 }
 
+// localRedirect gives a Moved Permanently response.
+// It does not convert relative paths to absolute paths like Redirect does.
+func localRedirect(w ResponseWriter, r *Request, newPath string) {
+	if q := r.URL.RawQuery; q != "" {
+		newPath += "?" + q
+	}
+	w.Header().Set("Location", newPath)
+	w.WriteHeader(StatusMovedPermanently)
+}
+
 // ServeFile replies to the request with the contents of the named file or directory.
 func ServeFile(w ResponseWriter, r *Request, name string) {
 	dir, file := filepath.Split(name)

src/pkg/http/fs_test.go

@@ -87,6 +87,30 @@ func TestServeFile(t *testing.T) {
 	}
 }
 
+var fsRedirectTestData = []struct {
+	original, redirect string
+}{
+	{"/test/index.html", "/test/"},
+	{"/test/testdata", "/test/testdata/"},
+	{"/test/testdata/file/", "/test/testdata/file"},
+}
+
+func TestFSRedirect(t *testing.T) {
+	ts := httptest.NewServer(StripPrefix("/test", FileServer(Dir("."))))
+	defer ts.Close()
+
+	for _, data := range fsRedirectTestData {
+		res, err := Get(ts.URL + data.original)
+		if err != nil {
+			t.Fatal(err)
+		}
+		res.Body.Close()
+		if g, e := res.Request.URL.Path, data.redirect; g != e {
+			t.Errorf("redirect from %s: got %s, want %s", data.original, g, e)
+		}
+	}
+}
+
 type testFileSystem struct {
 	open func(name string) (File, os.Error)
 }