QUIC - bugfix for forced head of line blocking experiment. Guarded by ENABLED --FLAGS_quic_bugfix_f…

net/quic/core/quic_spdy_session.cc

Index: net/quic/core/quic_spdy_session.cc
diff --git a/net/quic/core/quic_spdy_session.cc b/net/quic/core/quic_spdy_session.cc
index 444c35c45bc49ac3b37fb5db0f0a486829c96a82..b2bb42ffca2a7efdfb03bc0c757f530cc05e6fe8 100644
--- a/net/quic/core/quic_spdy_session.cc
+++ b/net/quic/core/quic_spdy_session.cc
@@ -121,12 +121,24 @@ void QuicSpdySession::OnConfigNegotiated() {
   if (FLAGS_quic_enable_force_hol_blocking && version > QUIC_VERSION_35 &&
       config()->ForceHolBlocking(perspective())) {
     force_hol_blocking_ = true;
-    // Autotuning makes sure that the headers stream flow control does
-    // not get in the way, and normal stream and connection level flow
-    // control are active anyway. This is really only for the client
-    // side (and mainly there just in tests and toys), where
-    // autotuning and/or large buffers are not enabled by default.
-    headers_stream_->flow_controller()->set_auto_tune_receive_window(true);
+    if (!FLAGS_quic_bugfix_fhol_writev_fin_only_v2) {
+      // Autotuning makes sure that the headers stream flow control does
+      // not get in the way, and normal stream and connection level flow
+      // control are active anyway. This is really only for the client
+      // side (and mainly there just in tests and toys), where
+      // autotuning and/or large buffers are not enabled by default.
+      headers_stream_->flow_controller()->set_auto_tune_receive_window(true);
+    } else {
+      // Since all streams are tunneled through the headers stream, it
+      // is important that headers stream never flow control blocks.
+      // Otherwise, busy-loop behaviour can ensue where data streams
+      // data try repeatedly to write data not realizing that the
+      // tunnel through the headers stream is blocked.
+      headers_stream_->flow_controller()->UpdateReceiveWindowSize(
+          kStreamReceiveWindowLimit);
+      headers_stream_->flow_controller()->UpdateSendWindowOffset(
+          kStreamReceiveWindowLimit);
+    }
   }
 
   if (version > QUIC_VERSION_34) {