Only set one QUIC sending alarm at once. Flag protected by --quic_only_one_sending_alarm.

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 de6515b723a6d5e0600b94e7743889a06794f5ba..4132cec8ba6fb5e9c4b44dfcb58eb0da8aeba2f3 100644
--- a/net/quic/quic_connection.cc
+++ b/net/quic/quic_connection.cc
@@ -1364,7 +1364,11 @@ void QuicConnection::OnCanWrite() {
     // We're not write blocked, but some stream didn't write out all of its
     // bytes. Register for 'immediate' resumption so we'll keep writing after
     // other connections and events have had a chance to use the thread.
-    resume_writes_alarm_->Set(clock_->ApproximateNow());
+    if (FLAGS_quic_only_one_sending_alarm) {
+      send_alarm_->Update(clock_->ApproximateNow(), QuicTime::Delta::Zero());
+    } else {
+      resume_writes_alarm_->Set(clock_->ApproximateNow());
+    }
   }
 }
 
@@ -1855,6 +1859,10 @@ void QuicConnection::OnRetransmissionTimeout() {
     return;
   }
 
+  // Cancel the send alarm to ensure TimeUntilSend is re-evaluated.
+  if (FLAGS_quic_only_one_sending_alarm) {
+    send_alarm_->Cancel();
+  }
   sent_packet_manager_.OnRetransmissionTimeout();
   WriteIfNotBlocked();
 
@@ -2176,6 +2184,19 @@ void QuicConnection::SetRetransmissionAlarm() {
     pending_retransmission_alarm_ = true;
     return;
   }
+  // Once the handshake has been confirmed, the retransmission alarm should
+  // never fire before the send alarm.
+  if (FLAGS_quic_only_one_sending_alarm &&
+      sent_packet_manager_.handshake_confirmed() && send_alarm_->IsSet()) {
+    DCHECK(!sent_packet_manager_.GetRetransmissionTime().IsInitialized() ||
+           sent_packet_manager_.GetRetransmissionTime() >=
+               send_alarm_->deadline())
+        << " retransmission_time:"
+        << sent_packet_manager_.GetRetransmissionTime().ToDebuggingValue()
+        << " send_alarm:" << send_alarm_->deadline().ToDebuggingValue();
+    retransmission_alarm_->Cancel();
+    return;
+  }
   QuicTime retransmission_time = sent_packet_manager_.GetRetransmissionTime();
   retransmission_alarm_->Update(retransmission_time,
                                 QuicTime::Delta::FromMilliseconds(1));