Fixed integer signing bug in cubic bytes/packet implementation

net/quic/core/congestion_control/cubic.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/quic/core/congestion_control/cubic.h"
 
 #include <stdint.h>
 #include <algorithm>
 #include <cmath>
 
@@ skipping 26 matching lines @@
 // new concurrent flows and speed up convergence.
 const float kBetaLastMax = 0.85f;
 
 }  // namespace
 
 Cubic::Cubic(const QuicClock* clock)
     : clock_(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();
 }
 
 void Cubic::SetNumConnections(int num_connections) {
   num_connections_ = num_connections;
 }
 
 float Cubic::Alpha() const {
   // TCPFriendly alpha is described in Section 3.3 of the CUBIC paper. Note that
   // beta here is a cwnd multiplier, and is equal to 1-beta from the paper.
@@ skipping 15 matching lines @@
   app_limited_start_time_ = QuicTime::Zero();
   last_update_time_ = QuicTime::Zero();  // Reset time.
   last_congestion_window_ = 0;
   last_max_congestion_window_ = 0;
   acked_packets_count_ = 0;
   epoch_packets_count_ = 0;
   estimated_tcp_congestion_window_ = 0;
   origin_point_congestion_window_ = 0;
   time_to_origin_point_ = 0;
   last_target_congestion_window_ = 0;
+  fix_convex_mode_ = false;
 }
 
 void Cubic::OnApplicationLimited() {
   // When sender is not using the available congestion window, Cubic's epoch
   // should not continue growing. Reset the epoch when in such a period.
   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_) {
     // We never reached the old max, so assume we are competing with another
     // flow. Use our extra back off factor to allow the other flow to go up.
     last_max_congestion_window_ =
         static_cast<int>(kBetaLastMax * current_congestion_window);
   } else {
     last_max_congestion_window_ = current_congestion_window;
   }
@@ skipping 31 matching lines @@
       time_to_origin_point_ = static_cast<uint32_t>(
           cbrt(kCubeFactor *
                (last_max_congestion_window_ - current_congestion_window)));
       origin_point_congestion_window_ = last_max_congestion_window_;
     }
   }
 
   // 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.
   target_congestion_window =
       min(target_congestion_window,
           current_congestion_window + (epoch_packets_count_ + 1) / 2);
 
   DCHECK_LT(0u, estimated_tcp_congestion_window_);
   // With dynamic beta/alpha based on number of active streams, it is possible
   // for the required_ack_count to become much lower than acked_packets_count_
@@ skipping 17 matching lines @@
   // congestion_window, use highest (fastest).
   if (target_congestion_window < estimated_tcp_congestion_window_) {
     target_congestion_window = estimated_tcp_congestion_window_;
   }
 
   DVLOG(1) << "Final target congestion_window: " << target_congestion_window;
   return target_congestion_window;
 }
 
 }  // namespace net