Ensure pacer state is reset when bbr2 is initialized

Summary:
As title.

Without resetting the pacer, bbr2 inherits the pacer state that was active on the connection until it starts to receive acks. This can happen when bbr2 is enabled using a knob which arrives after another congestion controller was already active.

Reviewed By: ritengupta

Differential Revision: D65922676

fbshipit-source-id: e798eaaa9e1bba4b2e23f9edcac0181eb5d4a4c7

quic/congestion_control/Bbr2.cpp

@@ -61,7 +61,15 @@ Bbr2CongestionController::Bbr2CongestionController(
           conn_.udpSendPacketLen * conn_.transportSettings.initCwndInMss) {
   resetCongestionSignals();
   resetLowerBounds();
-  // Start with the default pacing settings.
+  // If we explicitly don't want to pace the init cwnd, reset the pacing rate.
+  // Otherwise, leave it to the pacer's initial state.
+  if (!conn_.transportSettings.ccaConfig.paceInitCwnd) {
+    if (conn_.pacer) {
+      conn_.pacer->refreshPacingRate(cwndBytes_, 0us);
+    } else {
+      LOG(WARNING) << "BBR2 was initialized on a connection without a pacer";
+    }
+  }
   enterStartup();
 }