implement io.ReaderFrom for gin.ResponseWriter

This causes io.Copy, which calls io.copyBuffer, to automatically use the
sendfile syscall by calling the underlying http.ResponseWriter's
ReadFrom implementation [1]. This can be a significant performance improvement.

[1] https://github.com/golang/go/blob/7eeec1f6e4/src/io/io.go#L410-L413

Co-authored-by: Alex Guerra <alex@heyimalex.com>

response_writer.go

@@ -22,6 +22,7 @@ type ResponseWriter interface {
 	http.Hijacker
 	http.Flusher
 	http.CloseNotifier
+	io.ReaderFrom
 
 	// Status returns the HTTP response status code of the current request.
 	Status() int
@@ -87,6 +88,15 @@ func (w *responseWriter) WriteString(s string) (n int, err error) {
 	return
 }
 
+// ReadFrom implements the io.ReaderFrom interface, allowing Go to automatically
+// use the sendfile syscall in methods such as http.ServeFile
+func (w *responseWriter) ReadFrom(src io.Reader) (n int64, err error) {
+	w.WriteHeaderNow()
+	n, err = io.Copy(w.ResponseWriter, src)
+	w.size += int(n)
+	return
+}
+
 func (w *responseWriter) Status() int {
 	return w.status
 }

response_writer_test.go

@@ -5,8 +5,10 @@
 package gin
 
 import (
+	"io"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -132,3 +134,23 @@ func TestResponseWriterFlush(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Equal(t, http.StatusInternalServerError, resp.StatusCode)
 }
+
+func TestResponseWriterReadFrom(t *testing.T) {
+	testWriter := httptest.NewRecorder()
+	writer := &responseWriter{}
+	writer.reset(testWriter)
+	w := ResponseWriter(writer)
+
+	n, err := io.Copy(w, strings.NewReader("hola"))
+	assert.Equal(t, int64(4), n)
+	assert.Equal(t, 4, w.Size())
+	assert.Equal(t, http.StatusOK, w.Status())
+	assert.Equal(t, http.StatusOK, testWriter.Code)
+	assert.Equal(t, "hola", testWriter.Body.String())
+	assert.NoError(t, err)
+	n, err = writer.ReadFrom(strings.NewReader(" adios"))
+	assert.Equal(t, int64(6), n)
+	assert.Equal(t, 10, w.Size())
+	assert.Equal(t, testWriter.Body.String(), "hola adios")
+	assert.NoError(t, err)
+}