nghttpx: Accept incoming bytes at least initial window size

src/shrpx_http2_upstream.cc

@@ -284,8 +284,13 @@ void on_data_chunk_recv_callback(nghttp2_session *session,
     }
     if(upstream->get_flow_control()) {
       downstream->inc_recv_window_size(len);
+      // In case that user specified initial window size is smaller
+      // than default one and avoid stream tear down for the first
+      // request due to race condition, we allow at least default
+      // initial window size.
       if(downstream->get_recv_window_size() >
-         upstream->get_initial_window_size()) {
+         std::max(NGHTTP2_INITIAL_WINDOW_SIZE,
+                  upstream->get_initial_window_size())) {
         if(LOG_ENABLED(INFO)) {
           ULOG(INFO, upstream) << "Flow control error: recv_window_size="
                                << downstream->get_recv_window_size()

src/shrpx_spdy_session.cc

@@ -906,7 +906,9 @@ void on_data_chunk_recv_callback(nghttp2_session *session,
 
   if(spdy->get_flow_control()) {
     sd->dconn->inc_recv_window_size(len);
-    if(sd->dconn->get_recv_window_size() > spdy->get_initial_window_size()) {
+    if(sd->dconn->get_recv_window_size() >
+       std::max(NGHTTP2_INITIAL_WINDOW_SIZE,
+                spdy->get_initial_window_size())) {
       if(LOG_ENABLED(INFO)) {
         SSLOG(INFO, spdy) << "Flow control error: recv_window_size="
                           << sd->dconn->get_recv_window_size()

src/shrpx_spdy_upstream.cc

@@ -248,7 +248,7 @@ void on_data_chunk_recv_callback(spdylay_session *session,
     if(upstream->get_flow_control()) {
       downstream->inc_recv_window_size(len);
       if(downstream->get_recv_window_size() >
-         upstream->get_initial_window_size()) {
+         std::max(65536, upstream->get_initial_window_size())) {
         if(LOG_ENABLED(INFO)) {
           ULOG(INFO, upstream) << "Flow control error: recv_window_size="
                                << downstream->get_recv_window_size()