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/9200
Reviewed-by: Daniel Morsing <daniel.morsing@gmail.com>

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)