Context: json should not send status code before serialization is complete

Author: aldas

context.go

@@ -139,8 +139,8 @@ func (c *Context) Response() http.ResponseWriter {
 	return c.response
 }
 
-// SetResponse sets `*http.ResponseWriter`. Some middleware require that given ResponseWriter implements following
-// method `Unwrap() http.ResponseWriter` which eventually should return echo.Response instance.
+// SetResponse sets `*http.ResponseWriter`. Some context methods and/or middleware require that given ResponseWriter implements following
+// method `Unwrap() http.ResponseWriter` which eventually should return *echo.Response instance.
 func (c *Context) SetResponse(r http.ResponseWriter) {
 	c.response = r
 }
@@ -453,7 +453,16 @@ func (c *Context) jsonPBlob(code int, callback string, i any) (err error) {
 
 func (c *Context) json(code int, i any, indent string) error {
 	c.writeContentType(MIMEApplicationJSON)
-	c.response.WriteHeader(code)
+
+	if r, err := UnwrapResponse(c.response); err == nil {
+		// *echo.Response can delay sending status code until the first Write is called. As serialization can fail, we should delay
+		// sending the status code to the client until serialization is complete (first Write would be an indication it succeeded)
+		// Unsuccessful serialization error needs to go through the error handler and get a proper status code there.
+		r.Status = code
+	} else {
+		return fmt.Errorf("json: response does not unwrap to *echo.Response")
+	}
+
 	return c.echo.JSONSerializer.Serialize(c, i, indent)
 }
 

context_test.go

@@ -138,6 +138,24 @@ func TestContextRenderTemplate(t *testing.T) {
 	}
 }
 
+func TestContextRenderTemplateError(t *testing.T) {
+	// we test that when template rendering fails, no response is sent to the client yet, so the global error handler can decide what to do
+	e := New()
+	req := httptest.NewRequest(http.MethodPost, "/", strings.NewReader(userJSON))
+	rec := httptest.NewRecorder()
+	c := e.NewContext(req, rec)
+
+	tmpl := &Template{
+		templates: template.Must(template.New("hello").Parse("Hello, {{.}}!")),
+	}
+	c.Echo().Renderer = tmpl
+	err := c.Render(http.StatusOK, "not_existing", "Jon Snow")
+
+	assert.EqualError(t, err, `template: no template "not_existing" associated with template "hello"`)
+	assert.Equal(t, http.StatusOK, rec.Code) // status code must not be sent to the client
+	assert.Empty(t, rec.Body.String())       // body must not be sent to the client
+}
+
 func TestContextRenderErrorsOnNoRenderer(t *testing.T) {
 	e := New()
 	req := httptest.NewRequest(http.MethodPost, "/", strings.NewReader(userJSON))
@@ -222,6 +240,9 @@ func TestContextJSONErrorsOut(t *testing.T) {
 
 	err := c.JSON(http.StatusOK, make(chan bool))
 	assert.EqualError(t, err, "json: unsupported type: chan bool")
+
+	assert.Equal(t, http.StatusOK, rec.Code) // status code must not be sent to the client
+	assert.Empty(t, rec.Body.String())       // body must not be sent to the client
 }
 
 func TestContextJSONPretty(t *testing.T) {

response.go

@@ -126,7 +126,7 @@ func UnwrapResponse(rw http.ResponseWriter) (*Response, error) {
 			rw = t.Unwrap()
 			continue
 		default:
-			return nil, errors.New("ResponseWriter does not implement 'Unwrap() http.ResponseWriter' interface")
+			return nil, errors.New("ResponseWriter does not implement 'Unwrap() http.ResponseWriter' interface or unwrap to *echo.Response")
 		}
 	}
 }

response_test.go

@@ -115,3 +115,19 @@ func TestResponse_FlushPanics(t *testing.T) {
 		res.Flush()
 	})
 }
+
+func TestResponse_UnwrapResponse(t *testing.T) {
+	orgRes := NewResponse(httptest.NewRecorder(), nil)
+	res, err := UnwrapResponse(orgRes)
+
+	assert.NotNil(t, res)
+	assert.NoError(t, err)
+}
+
+func TestResponse_UnwrapResponse_error(t *testing.T) {
+	rw := new(testResponseWriter)
+	res, err := UnwrapResponse(rw)
+
+	assert.Nil(t, res)
+	assert.EqualError(t, err, "ResponseWriter does not implement 'Unwrap() http.ResponseWriter' interface or unwrap to *echo.Response")
+}