middleware: ResponseRecorder now is an http.Flusher (fixes #677)

Flush every 250ms. This should keep latency somewhat low but there is
still buffering; maybe in the future we can make this configurable.

middleware/proxy/reverseproxy.go

@@ -111,7 +111,7 @@ func NewSingleHostReverseProxy(target *url.URL, without string) *ReverseProxy {
 			req.URL.Path = strings.TrimPrefix(req.URL.Path, without)
 		}
 	}
-	rp := &ReverseProxy{Director: director}
+	rp := &ReverseProxy{Director: director, FlushInterval: 250 * time.Millisecond} // flushing good for streaming & server-sent events
 	if target.Scheme == "unix" {
 		rp.Transport = &http.Transport{
 			Dial: socketDial(target.String()),

middleware/recorder.go

@@ -77,3 +77,11 @@ func (r *ResponseRecorder) Hijack() (net.Conn, *bufio.ReadWriter, error) {
 	}
 	return nil, nil, errors.New("not a Hijacker")
 }
+
+// Flush implements http.Flusher. It simply wraps the underlying
+// ResponseWriter's Flush method if there is one, or does nothing.
+func (r *ResponseRecorder) Flush() {
+	if f, ok := r.ResponseWriter.(http.Flusher); ok {
+		f.Flush()
+	}
+}