Track when QUIC connections are application limited. Protected by quic_enable_app_limited_check.

net/quic/core/quic_connection.cc

Index: net/quic/core/quic_connection.cc
diff --git a/net/quic/core/quic_connection.cc b/net/quic/core/quic_connection.cc
index add7d673302b4dc795154abb96051a2ab4293be0..e205e31dd9359785c64ff85a70181837844ef71e 100644
--- a/net/quic/core/quic_connection.cc
+++ b/net/quic/core/quic_connection.cc
@@ -2306,6 +2306,28 @@ QuicConnection::ScopedPacketBundler::~ScopedPacketBundler() {
   if (!already_in_batch_mode_) {
     DVLOG(2) << "Leaving Batch Mode.";
     connection_->packet_generator_.FinishBatchOperations();
+
+    // Once all transmissions are done, check if there is any outstanding data
+    // to send and notify the congestion controller if not.
+    //
+    // Note that this means that the application limited check will happen as
+    // soon as the last bundler gets destroyed, which is typically after a
+    // single stream write is finished.  This means that if all the data from a
+    // single write goes through the connection, the application-limited signal
+    // will fire even if the caller does a write operation immediately after.
+    // There are two important approaches to remedy this situation:
+    // (1) Instantiate ScopedPacketBundler before performing multiple subsequent
+    //     writes, thus deferring this check until all writes are done.
+    // (2) Write data in chunks sufficiently large so that they cause the
+    //     connection to be limited by the congestion control.  Typically, this
+    //     would mean writing chunks larger than the product of the current
+    //     pacing rate and the pacer granularity.  So, for instance, if the
+    //     pacing rate of the connection is 1 Gbps, and the pacer granularity is
+    //     1 ms, the caller should send at least 125k bytes in order to not
+    //     be marked as application-limited.
+    if (FLAGS_quic_enable_app_limited_check) {
+      connection_->CheckIfApplicationLimited();
+    }
   }
   DCHECK_EQ(already_in_batch_mode_,
             connection_->packet_generator_.InBatchMode());
@@ -2522,4 +2544,12 @@ const QuicTime::Delta QuicConnection::DelayedAckTime() {
       min(kMaxDelayedAckTimeMs, kMinRetransmissionTimeMs / 2));
 }
 
+void QuicConnection::CheckIfApplicationLimited() {
+  if (queued_packets_.empty() &&
+      !sent_packet_manager_->HasPendingRetransmissions() &&
+      !visitor_->WillingAndAbleToWrite()) {
+    sent_packet_manager_->OnApplicationLimited();
+  }
+}
+
 }  // namespace net