render: skip default serializer after Renderer writes

render.go

@@ -24,22 +24,51 @@ func Bind(r *http.Request, v Binder) error {
 	return binder(r, v)
 }
 
+// trackingWriter notes whether the Renderer already wrote a response so
+// Render/RenderList can skip the default serializer (see #56).
+type trackingWriter struct {
+	http.ResponseWriter
+	wrote bool
+}
+
+func (w *trackingWriter) Write(b []byte) (int, error) {
+	w.wrote = true
+	return w.ResponseWriter.Write(b)
+}
+
+func (w *trackingWriter) WriteHeader(code int) {
+	w.wrote = true
+	w.ResponseWriter.WriteHeader(code)
+}
+
+func (w *trackingWriter) Unwrap() http.ResponseWriter {
+	return w.ResponseWriter
+}
+
 // Render renders a single payload and respond to the client request.
 func Render(w http.ResponseWriter, r *http.Request, v Renderer) error {
-	if err := renderer(w, r, v); err != nil {
+	tw := &trackingWriter{ResponseWriter: w}
+	if err := renderer(tw, r, v); err != nil {
 		return err
 	}
+	if tw.wrote {
+		return nil
+	}
 	Respond(w, r, v)
 	return nil
 }
 
 // RenderList renders a slice of payloads and responds to the client request.
 func RenderList(w http.ResponseWriter, r *http.Request, l []Renderer) error {
+	tw := &trackingWriter{ResponseWriter: w}
 	for _, v := range l {
-		if err := renderer(w, r, v); err != nil {
+		if err := renderer(tw, r, v); err != nil {
 			return err
 		}
 	}
+	if tw.wrote {
+		return nil
+	}
 	Respond(w, r, l)
 	return nil
 }

render_test.go

@@ -0,0 +1,84 @@
+package render
+
+import (
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+type (
+	profile struct {
+		Name string
+		Age  int
+	}
+	publicProfile profile
+	profileJSON   struct {
+		Name string `json:"name"`
+	}
+	prepareOnly struct {
+		Name string `json:"name"`
+	}
+)
+
+func (p publicProfile) Render(w http.ResponseWriter, r *http.Request) error {
+	Status(r, http.StatusNoContent)
+	JSON(w, r, profileJSON{Name: p.Name})
+	return nil
+}
+
+func (p prepareOnly) Render(http.ResponseWriter, *http.Request) error {
+	return nil
+}
+
+func TestRenderSkipsDefaultWhenRendererWrote(t *testing.T) {
+	w := httptest.NewRecorder()
+	r := httptest.NewRequest(http.MethodGet, "/", nil)
+	if err := Render(w, r, publicProfile{Name: "John", Age: 42}); err != nil {
+		t.Fatal(err)
+	}
+	body, _ := io.ReadAll(w.Body)
+	got := strings.TrimSpace(string(body))
+	if w.Code != http.StatusNoContent {
+		t.Fatalf("status=%d want %d", w.Code, http.StatusNoContent)
+	}
+	if got != `{"name":"John"}` {
+		t.Fatalf("body=%q, extra default JSON dump?", got)
+	}
+	if strings.Contains(got, `"Age"`) {
+		t.Fatalf("leaked aliased fields: %s", got)
+	}
+}
+
+func TestRenderStillSerializesPrepareOnlyRenderer(t *testing.T) {
+	w := httptest.NewRecorder()
+	r := httptest.NewRequest(http.MethodGet, "/", nil)
+	if err := Render(w, r, prepareOnly{Name: "Ada"}); err != nil {
+		t.Fatal(err)
+	}
+	body, _ := io.ReadAll(w.Body)
+	got := strings.TrimSpace(string(body))
+	if w.Code != http.StatusOK {
+		t.Fatalf("status=%d want %d", w.Code, http.StatusOK)
+	}
+	if got != `{"name":"Ada"}` {
+		t.Fatalf("body=%q", got)
+	}
+}
+
+func TestRenderListSkipsDefaultWhenRendererWrote(t *testing.T) {
+	w := httptest.NewRecorder()
+	r := httptest.NewRequest(http.MethodGet, "/", nil)
+	list := []Renderer{
+		publicProfile{Name: "John", Age: 42},
+	}
+	if err := RenderList(w, r, list); err != nil {
+		t.Fatal(err)
+	}
+	body, _ := io.ReadAll(w.Body)
+	got := strings.TrimSpace(string(body))
+	if got != `{"name":"John"}` {
+		t.Fatalf("body=%q", got)
+	}
+}