Record the last packet send time before we start sending the packet

net/quic/quic_connection.cc

Index: net/quic/quic_connection.cc
diff --git a/net/quic/quic_connection.cc b/net/quic/quic_connection.cc
index 44fe48b8f407085155abf4e9f49cd5a504eb6f61..61fd3c140ae4c0eb1654d40f4bff94c147a5f820 100644
--- a/net/quic/quic_connection.cc
+++ b/net/quic/quic_connection.cc
@@ -1449,6 +1449,13 @@ bool QuicConnection::WritePacketInner(QueuedPacket* packet) {
            << QuicUtils::StringToHexASCIIDump(
                packet->serialized_packet.packet->AsStringPiece());
 
+  QuicTime packet_send_time = QuicTime::Zero();
+  if (FLAGS_quic_record_send_time_before_write) {
+    // Measure the RTT from before the write begins to avoid underestimating the
+    // min_rtt_, especially in cases where the thread blocks or gets swapped out
+    // during the WritePacket below.
+    packet_send_time = clock_->Now();
+  }
   WriteResult result = writer_->WritePacket(encrypted->data(),
                                             encrypted->length(),
                                             self_address().address(),
@@ -1467,7 +1474,16 @@ bool QuicConnection::WritePacketInner(QueuedPacket* packet) {
       return false;
     }
   }
-  QuicTime now = clock_->Now();
+  if (!FLAGS_quic_record_send_time_before_write) {
+    packet_send_time = clock_->Now();
+  }
+  if (!packet_send_time.IsInitialized()) {
+    // TODO(jokulik): This is only needed because of the two code paths for
+    // initializing packet_send_time.  Once "quic_record_send_time_before_write"
+    // is deprecated, this check can be removed.
+    LOG(DFATAL) << "The packet send time should never be zero. "
+                << "This is a programming bug, please report it.";
+  }
   if (result.status != WRITE_STATUS_ERROR && debug_visitor_.get() != nullptr) {
     // Pass the write result to the visitor.
     debug_visitor_->OnPacketSent(packet->serialized_packet,
@@ -1475,14 +1491,17 @@ bool QuicConnection::WritePacketInner(QueuedPacket* packet) {
                                  packet->encryption_level,
                                  packet->transmission_type,
                                  *encrypted,
-                                 now);
+                                 packet_send_time);
   }
   if (packet->transmission_type == NOT_RETRANSMISSION) {
-    time_of_last_sent_new_packet_ = now;
+    time_of_last_sent_new_packet_ = packet_send_time;
   }
   SetPingAlarm();
-  DVLOG(1) << ENDPOINT << "time of last sent packet: "
-           << now.ToDebuggingValue();
+  DVLOG(1) << ENDPOINT << "time "
+           << (FLAGS_quic_record_send_time_before_write ?
+               "we began writing " : "we finished writing ")
+           << "last sent packet: "
+           << packet_send_time.ToDebuggingValue();
 
   // TODO(ianswett): Change the sequence number length and other packet creator
   // options by a more explicit API than setting a struct value directly,
@@ -1494,7 +1513,7 @@ bool QuicConnection::WritePacketInner(QueuedPacket* packet) {
   bool reset_retransmission_alarm = sent_packet_manager_.OnPacketSent(
       &packet->serialized_packet,
       packet->original_sequence_number,
-      now,
+      packet_send_time,
       encrypted->length(),
       packet->transmission_type,
       IsRetransmittable(*packet));