Fix concurrent write bug in reader.Render

If a caller passed in a map retained within the callers context as
extraHeaders to gin.Context.DataFromReader() then a race to write the
"Content-Lenght" header would occur.

// globalHeader is passed to gin.Context.DataFromReader
var globalHeaders = map[string]string{
    "cache-control": "public, max-age=3600",
}

func (c *gin.Context) {
    //...

    // DataFromReader must not write to globalHeaders
    c.DataFromReader(code, contentLength, contentType, reader,
	globalHeaders)
}

render/reader.go

@@ -21,13 +21,8 @@ type Reader struct {
 // Render (Reader) writes data with custom ContentType and headers.
 func (r Reader) Render(w http.ResponseWriter) (err error) {
 	r.WriteContentType(w)
-	if r.ContentLength >= 0 {
-		if r.Headers == nil {
-			r.Headers = map[string]string{}
-		}
-		r.Headers["Content-Length"] = strconv.FormatInt(r.ContentLength, 10)
-	}
 	r.writeHeaders(w, r.Headers)
+
 	_, err = io.Copy(w, r.Reader)
 	return
 }
@@ -45,4 +40,8 @@ func (r Reader) writeHeaders(w http.ResponseWriter, headers map[string]string) {
 			header.Set(k, v)
 		}
 	}
+
+	if r.ContentLength > 0 {
+		header.Set("Content-Length", strconv.FormatInt(r.ContentLength, 10))
+	}
 }

render/reader_test.go

@@ -6,6 +6,7 @@ package render
 
 import (
 	"net/http/httptest"
+	"strconv"
 	"strings"
 	"testing"
 
@@ -21,3 +22,21 @@ func TestReaderRenderNoHeaders(t *testing.T) {
 	err := r.Render(httptest.NewRecorder())
 	require.NoError(t, err)
 }
+
+func TestReaderRenderWithHeaders(t *testing.T) {
+	content := "test"
+	r := Reader{
+		ContentLength: int64(len(content)),
+		Reader:        strings.NewReader(content),
+		Headers: map[string]string{
+			"Test-Content": "test/content",
+		},
+	}
+	recorder := httptest.NewRecorder()
+	err := r.Render(recorder)
+	require.NoError(t, err)
+
+	require.Contains(t, recorder.Header()["Content-Length"], strconv.FormatInt(r.ContentLength, 10))
+
+	require.Contains(t, recorder.Header()["Test-Content"], "test/content")
+}