From 5c00df8afadd06cc5be530dde00fe6d9fa4a2e4a Mon Sep 17 00:00:00 2001
From: Denis Galeev <11516397+dengaleev@users.noreply.github.com>
Date: Sat, 28 Feb 2026 09:07:31 +0700
Subject: [PATCH] fix(render): write content length in Data.Render (#4206)

* init test

* fix test

* fix assert.EqualValues usage

---------

Co-authored-by: Bo-Yi Wu <appleboy.tw@gmail.com>
---
 render/data.go        |  8 +++++++-
 render/render_test.go | 31 +++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/render/data.go b/render/data.go
index a653ea30..2c0ad5e3 100644
--- a/render/data.go
+++ b/render/data.go
@@ -4,7 +4,10 @@
 
 package render
 
-import "net/http"
+import (
+	"net/http"
+	"strconv"
+)
 
 // Data contains ContentType and bytes data.
 type Data struct {
@@ -15,6 +18,9 @@ type Data struct {
 // Render (Data) writes data with custom ContentType.
 func (r Data) Render(w http.ResponseWriter) (err error) {
 	r.WriteContentType(w)
+	if len(r.Data) > 0 {
+		w.Header().Set("Content-Length", strconv.Itoa(len(r.Data)))
+	}
 	_, err = w.Write(r.Data)
 	return
 }
diff --git a/render/render_test.go b/render/render_test.go
index 40e7e7be..9c3019eb 100644
--- a/render/render_test.go
+++ b/render/render_test.go
@@ -8,6 +8,7 @@ import (
 	"encoding/xml"
 	"errors"
 	"html/template"
+	"io"
 	"net"
 	"net/http"
 	"net/http/httptest"
@@ -453,6 +454,36 @@ func TestRenderData(t *testing.T) {
 	require.NoError(t, err)
 	assert.Equal(t, "#!PNG some raw data", w.Body.String())
 	assert.Equal(t, "image/png", w.Header().Get("Content-Type"))
+	assert.Equal(t, "19", w.Header().Get("Content-Length"))
+}
+
+func TestRenderDataContentLength(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		size, err := strconv.Atoi(r.URL.Query().Get("size"))
+		assert.NoError(t, err)
+
+		data := Data{
+			ContentType: "application/octet-stream",
+			Data:        make([]byte, size),
+		}
+		assert.NoError(t, data.Render(w))
+	}))
+	t.Cleanup(srv.Close)
+
+	for _, size := range []int{0, 1, 100, 100_000} {
+		t.Run(strconv.Itoa(size), func(t *testing.T) {
+			resp, err := http.Get(srv.URL + "?size=" + strconv.Itoa(size))
+			require.NoError(t, err)
+			defer resp.Body.Close()
+
+			assert.Equal(t, "application/octet-stream", resp.Header.Get("Content-Type"))
+			assert.Equal(t, strconv.Itoa(size), resp.Header.Get("Content-Length"))
+
+			actual, err := io.Copy(io.Discard, resp.Body)
+			require.NoError(t, err)
+			assert.EqualValues(t, size, actual)
+		})
+	}
 }
 
 func TestRenderString(t *testing.T) {