Add a LOG(DFATAL) to a case where packets are acked before being sent.

net/quic/quic_sent_packet_manager.cc

 // Copyright 2013 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/quic_sent_packet_manager.h"
 
 #include <algorithm>
 
 #include "base/logging.h"
 #include "base/stl_util.h"
@@ skipping 702 matching lines @@
 bool QuicSentPacketManager::MaybeUpdateRTT(
     const QuicAckFrame& ack_frame,
     const QuicTime& ack_receive_time) {
   if (!unacked_packets_.IsUnacked(ack_frame.largest_observed)) {
     return false;
   }
   // We calculate the RTT based on the highest ACKed sequence number, the lower
   // sequence numbers will include the ACK aggregation delay.
   const TransmissionInfo& transmission_info =
       unacked_packets_.GetTransmissionInfo(ack_frame.largest_observed);
-  // Don't update the RTT if it hasn't been sent.
+  // Ensure the packet has a valid sent time.
   if (transmission_info.sent_time == QuicTime::Zero()) {
+    LOG(DFATAL) << "Acked packet has zero sent time, largest_observed:"

[ramant (doing other things), 2014/09/15 19:56:44]

rch, ian: should we make this DLOG(DFATAL) in chromium? Or this should never
happen and we want to crash the browser if it happens.

[Ryan Hamilton, 2014/09/15 21:44:54]

I don't have strong feelings about LOG(DFATAL) vs DLOG(DFATAL). LOG(DFATAL) ==
LOG(ERROR) in non-debug builds so we won't crash in either case.

[ramant (doing other things), 2014/09/15 21:47:50]

Sounds great. 

+                << ack_frame.largest_observed;
     return false;
   }
 
   QuicTime::Delta send_delta =
       ack_receive_time.Subtract(transmission_info.sent_time);
   rtt_stats_.UpdateRtt(
       send_delta, ack_frame.delta_time_largest_observed, ack_receive_time);
   return true;
 }
 
@@ skipping 140 matching lines @@
 
   // Set up a pacing sender with a 5 millisecond alarm granularity.
   using_pacing_ = true;
   send_algorithm_.reset(
       new PacingSender(send_algorithm_.release(),
                        QuicTime::Delta::FromMilliseconds(5),
                        kInitialUnpacedBurst));
 }
 
 }  // namespace net