net/http: make ServeContent errors return more specific HTTP status codes

Previously all errors were 404 errors, even if the real error had
nothing to do with a file being non-existent.

Fixes #10283

Change-Id: I5b08b471a9064c347510cfcf8557373704eef7c0
Reviewed-on: https://go-review.googlesource.com/9200Reviewed-by: Daniel Morsing <daniel.morsing@gmail.com>

src/net/http/fs.go

@@ -358,16 +358,16 @@ func serveFile(w ResponseWriter, r *Request, fs FileSystem, name string, redirec
 
 	f, err := fs.Open(name)
 	if err != nil {
-		// TODO expose actual error?
-		NotFound(w, r)
+		msg, code := toHTTPError(err)
+		Error(w, msg, code)
 		return
 	}
 	defer f.Close()
 
 	d, err1 := f.Stat()
 	if err1 != nil {
-		// TODO expose actual error?
-		NotFound(w, r)
+		msg, code := toHTTPError(err)
+		Error(w, msg, code)
 		return
 	}
 
@@ -417,6 +417,22 @@ func serveFile(w ResponseWriter, r *Request, fs FileSystem, name string, redirec
 	serveContent(w, r, d.Name(), d.ModTime(), sizeFunc, f)
 }
 
+// toHTTPError returns a non-specific HTTP error message and status code
+// for a given non-nil error value. It's important that toHTTPError does not
+// actually return err.Error(), since msg and httpStatus are returned to users,
+// and historically Go's ServeContent always returned just "404 Not Found" for
+// all errors. We don't want to start leaking information in error messages.
+func toHTTPError(err error) (msg string, httpStatus int) {
+	if os.IsNotExist(err) {
+		return "404 page not found", StatusNotFound
+	}
+	if os.IsPermission(err) {
+		return "403 Forbidden", StatusForbidden
+	}
+	// Default:
+	return "500 Internal Server Error", StatusInternalServerError
+}
+
 // 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) {

src/net/http/fs_test.go

@@ -497,6 +497,7 @@ type fakeFileInfo struct {
 	modtime  time.Time
 	ents     []*fakeFileInfo
 	contents string
+	err      error
 }
 
 func (f *fakeFileInfo) Name() string       { return f.basename }
@@ -549,6 +550,9 @@ func (fs fakeFS) Open(name string) (File, error) {
 	if !ok {
 		return nil, os.ErrNotExist
 	}
+	if f.err != nil {
+		return nil, f.err
+	}
 	return &fakeFile{ReadSeeker: strings.NewReader(f.contents), fi: f, path: name}, nil
 }
 
@@ -803,6 +807,31 @@ func TestServeContent(t *testing.T) {
 	}
 }
 
+func TestServeContentErrorMessages(t *testing.T) {
+	defer afterTest(t)
+	fs := fakeFS{
+		"/500": &fakeFileInfo{
+			err: errors.New("random error"),
+		},
+		"/403": &fakeFileInfo{
+			err: &os.PathError{Err: os.ErrPermission},
+		},
+	}
+	ts := httptest.NewServer(FileServer(fs))
+	defer ts.Close()
+	for _, code := range []int{403, 404, 500} {
+		res, err := DefaultClient.Get(fmt.Sprintf("%s/%d", ts.URL, code))
+		if err != nil {
+			t.Errorf("Error fetching /%d: %v", code, err)
+			continue
+		}
+		if res.StatusCode != code {
+			t.Errorf("For /%d, status code = %d; want %d", code, res.StatusCode, code)
+		}
+		res.Body.Close()
+	}
+}
+
 // verifies that sendfile is being used on Linux
 func TestLinuxSendfile(t *testing.T) {
 	defer afterTest(t)