detect CloseNotify capability in accesslog and metrics

pkg/middlewares/accesslog/capture_response_writer.go

@@ -13,6 +13,20 @@ var (
 	_ middlewares.Stateful = &captureResponseWriter{}
 )
 
+type capturer interface {
+	http.ResponseWriter
+	Size() int64
+	Status() int
+}
+
+func newCaptureResponseWriter(rw http.ResponseWriter) capturer {
+	capt := &captureResponseWriter{rw: rw}
+	if _, ok := rw.(http.CloseNotifier); !ok {
+		return capt
+	}
+	return captureResponseWriterWithCloseNotify{capt}
+}
+
 // captureResponseWriter is a wrapper of type http.ResponseWriter
 // that tracks request status and size
 type captureResponseWriter struct {
@@ -21,6 +35,16 @@ type captureResponseWriter struct {
 	size   int64
 }
 
+type captureResponseWriterWithCloseNotify struct {
+	*captureResponseWriter
+}
+
+// CloseNotify returns a channel that receives at most a
+// single value (true) when the client connection has gone away.
+func (r *captureResponseWriterWithCloseNotify) CloseNotify() <-chan bool {
+	return r.rw.(http.CloseNotifier).CloseNotify()
+}
+
 func (crw *captureResponseWriter) Header() http.Header {
 	return crw.rw.Header()
 }