reverse_proxy: flush HTTP/2 response when ContentLength is unknown (#3561)

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

* reverse proxy: add integration test for better h2 stream (#3556)

* reverse proxy: adjust comments as francislavoie suggests

* link to issue #3556 in the comments

modules/caddyhttp/reverseproxy/reverseproxy.go

@@ -611,6 +611,14 @@ func (h *Handler) reverseProxy(rw http.ResponseWriter, req *http.Request, di Dia
 
 	rw.WriteHeader(res.StatusCode)
 
+	// some apps need the response headers before starting to stream content with http2,
+	// so it's important to explicitly flush the headers to the client before streaming the data.
+	// (see https://github.com/caddyserver/caddy/issues/3556 for use case)
+	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 (issue #3556)
+	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)
 }