gfe-relnote: In BbrSender, always try to get a bandwidth sample even if packet.bytes_acked=0(i.e. packet has been removed from QuicUnackedPacketMap's inflight bytes). Protected by default true flag --gfe2_reloadable_flag_quic_always_get_bw_sample_when_acked.

PiperOrigin-RevId: 242172355
Change-Id: I180592b64915cf213694b1e6d8e41ac561cb2454

quic/core/congestion_control/bbr_sender.cc

diff --git a/quic/core/congestion_control/bbr_sender.cc b/quic/core/congestion_control/bbr_sender.cc
index 5c9e9ea..9de79cd 100644
--- a/quic/core/congestion_control/bbr_sender.cc
+++ b/quic/core/congestion_control/bbr_sender.cc
@@ -137,7 +137,9 @@
       probe_rtt_skipped_if_similar_rtt_(false),
       probe_rtt_disabled_if_app_limited_(false),
       app_limited_since_last_probe_rtt_(false),
-      min_rtt_since_last_probe_rtt_(QuicTime::Delta::Infinite()) {
+      min_rtt_since_last_probe_rtt_(QuicTime::Delta::Infinite()),
+      always_get_bw_sample_when_acked_(
+          GetQuicReloadableFlag(quic_always_get_bw_sample_when_acked)) {
   if (stats_) {
     stats_->slowstart_count = 0;
     stats_->slowstart_start_time = QuicTime::Zero();
@@ -499,12 +501,19 @@
     const AckedPacketVector& acked_packets) {
   QuicTime::Delta sample_min_rtt = QuicTime::Delta::Infinite();
   for (const auto& packet : acked_packets) {
-    if (packet.bytes_acked == 0) {
+    if (!always_get_bw_sample_when_acked_ && packet.bytes_acked == 0) {
       // Skip acked packets with 0 in flight bytes when updating bandwidth.
       continue;
     }
     BandwidthSample bandwidth_sample =
         sampler_.OnPacketAcknowledged(now, packet.packet_number);
+    if (always_get_bw_sample_when_acked_ &&
+        !bandwidth_sample.state_at_send.is_valid) {
+      // From the sampler's perspective, the packet has never been sent, or the
+      // packet has been acked or marked as lost previously.
+      continue;
+    }
+
     last_sample_is_app_limited_ = bandwidth_sample.state_at_send.is_app_limited;
     has_non_app_limited_sample_ |=
         !bandwidth_sample.state_at_send.is_app_limited;

quic/core/congestion_control/bbr_sender.h

diff --git a/quic/core/congestion_control/bbr_sender.h b/quic/core/congestion_control/bbr_sender.h
index 405300f..d01dbfe 100644
--- a/quic/core/congestion_control/bbr_sender.h
+++ b/quic/core/congestion_control/bbr_sender.h
@@ -400,6 +400,9 @@
   bool probe_rtt_disabled_if_app_limited_;
   bool app_limited_since_last_probe_rtt_;
   QuicTime::Delta min_rtt_since_last_probe_rtt_;
+
+  // Latched value of --quic_always_get_bw_sample_when_acked.
+  const bool always_get_bw_sample_when_acked_;
 };
 
 QUIC_EXPORT_PRIVATE std::ostream& operator<<(std::ostream& os,