Fixed integer signing bug in cubic bytes/packet implementation

net/quic/core/congestion_control/cubic.cc

Index: net/quic/core/congestion_control/cubic.cc
diff --git a/net/quic/core/congestion_control/cubic.cc b/net/quic/core/congestion_control/cubic.cc
index 7b7d9f8325ba4f219173b65d92583ddf137a23f1..a273b39dcb596d320ae0c73962c0354b4cb384ae 100644
--- a/net/quic/core/congestion_control/cubic.cc
+++ b/net/quic/core/congestion_control/cubic.cc
@@ -44,7 +44,8 @@ Cubic::Cubic(const QuicClock* clock)
       num_connections_(kDefaultNumConnections),
       epoch_(QuicTime::Zero()),
       app_limited_start_time_(QuicTime::Zero()),
-      last_update_time_(QuicTime::Zero()) {
+      last_update_time_(QuicTime::Zero()),
+      fix_convex_mode_(false) {
   Reset();
 }
 
@@ -80,6 +81,7 @@ void Cubic::Reset() {
   origin_point_congestion_window_ = 0;
   time_to_origin_point_ = 0;
   last_target_congestion_window_ = 0;
+  fix_convex_mode_ = false;
 }
 
 void Cubic::OnApplicationLimited() {
@@ -88,6 +90,10 @@ void Cubic::OnApplicationLimited() {
   epoch_ = QuicTime::Zero();
 }
 
+void Cubic::SetFixConvexMode(bool fix_convex_mode) {
+  fix_convex_mode_ = fix_convex_mode;
+}
+
 QuicPacketCount Cubic::CongestionWindowAfterPacketLoss(
     QuicPacketCount current_congestion_window) {
   if (current_congestion_window < last_max_congestion_window_) {
@@ -139,16 +145,29 @@ QuicPacketCount Cubic::CongestionWindowAfterAck(
   // Change the time unit from microseconds to 2^10 fractions per second. Take
   // the round trip time in account. This is done to allow us to use shift as a
   // divide operator.
-  int64_t elapsed_time =
+  const int64_t elapsed_time =
       ((current_time + delay_min - epoch_).ToMicroseconds() << 10) /
       kNumMicrosPerSecond;
+  DCHECK_GE(elapsed_time, 0);
 
   int64_t offset = time_to_origin_point_ - elapsed_time;
+  if (fix_convex_mode_) {
+    // Right-shifts of negative, signed numbers have
+    // implementation-dependent behavior.  Force the offset to be
+    // positive, similar to the kernel implementation.
+    offset = labs(time_to_origin_point_ - elapsed_time);
+  }
+
   QuicPacketCount delta_congestion_window =
       (kCubeCongestionWindowScale * offset * offset * offset) >> kCubeScale;
 
+  const bool add_delta = elapsed_time > time_to_origin_point_;
+  DCHECK(add_delta ||
+         (origin_point_congestion_window_ > delta_congestion_window));
   QuicPacketCount target_congestion_window =
-      origin_point_congestion_window_ - delta_congestion_window;
+      (fix_convex_mode_ && add_delta)
+          ? origin_point_congestion_window_ + delta_congestion_window
+          : origin_point_congestion_window_ - delta_congestion_window;
 
   // Limit the CWND increase to half the acked packets rounded up to the
   // nearest packet.