reverse proxy: Support more h2 stream scenarios (caddyserver#3556)

modules/caddyhttp/reverseproxy/reverseproxy.go

@@ -604,6 +604,13 @@ func (h *Handler) reverseProxy(rw http.ResponseWriter, req *http.Request, di Dia
 
 	rw.WriteHeader(res.StatusCode)
 
+	// for some apps which need a correct response header to start http2 streaming,
+	// it's important to explicit flush headers to the client before copy streaming data.
+	if req.ProtoMajor == 2 && res.ContentLength == -1 {
+		if wf, ok := rw.(http.Flusher); ok {
+			wf.Flush()
+		}
+	}
 	err = h.copyResponse(rw, res.Body, h.flushInterval(req, res))
 	res.Body.Close() // close now, instead of defer, to populate res.Trailer
 	if err != nil {

modules/caddyhttp/reverseproxy/streaming.go

@@ -96,6 +96,11 @@ func (h Handler) flushInterval(req *http.Request, res *http.Response) time.Durat
 		return -1 // negative means immediately
 	}
 
+	// for h2 and h2c upstream streaming data to client
+	if req.ProtoMajor == 2 && res.ContentLength == -1 {
+		return -1
+	}
+
 	// TODO: more specific cases? e.g. res.ContentLength == -1? (this TODO is from the std lib)
 	return time.Duration(h.FlushInterval)
 }