delivery rate: fix where on_packet_sent() hook is called

Fix where `delivery_rate.on_packet_sent()` is called to be
after updating the time to send the packet. It will reduce the
chance that `rate_sample.interval` is shorter than `min_rtt`,
especailly useful for low latency (<1ms) network.

Also, when updating `first_time_sent` when the network is idle
(bytes_in_flight == 0), use the packet's timestamp. `now` may
be in the future.

quiche/src/recovery/delivery_rate.rs

diff --git a/quiche/src/recovery/delivery_rate.rs b/quiche/src/recovery/delivery_rate.rs
index 37c2aaf..59abf81 100644
--- a/quiche/src/recovery/delivery_rate.rs
+++ b/quiche/src/recovery/delivery_rate.rs
@@ -79,13 +79,11 @@
 }
 
 impl Rate {
-    pub fn on_packet_sent(
-        &mut self, pkt: &mut Sent, bytes_in_flight: usize, now: Instant,
-    ) {
-        // No packets in flight yet?
+    pub fn on_packet_sent(&mut self, pkt: &mut Sent, bytes_in_flight: usize) {
+        // No packets in flight.
         if bytes_in_flight == 0 {
-            self.first_sent_time = now;
-            self.delivered_time = now;
+            self.first_sent_time = pkt.time_sent;
+            self.delivered_time = pkt.time_sent;
         }
 
         pkt.first_sent_time = self.first_sent_time;

quiche/src/recovery/mod.rs

diff --git a/quiche/src/recovery/mod.rs b/quiche/src/recovery/mod.rs
index 4011fe3..901c10e 100644
--- a/quiche/src/recovery/mod.rs
+++ b/quiche/src/recovery/mod.rs
@@ -273,9 +273,6 @@
         self.largest_sent_pkt[epoch] =
             cmp::max(self.largest_sent_pkt[epoch], pkt_num);
 
-        self.delivery_rate
-            .on_packet_sent(&mut pkt, self.bytes_in_flight, now);
-
         if in_flight {
             if ack_eliciting {
                 self.time_of_last_sent_ack_eliciting_pkt[epoch] = Some(now);
@@ -315,6 +312,10 @@
 
         pkt.time_sent = self.get_packet_send_time();
 
+        // bytes_in_flight is already updated. Use previous value.
+        self.delivery_rate
+            .on_packet_sent(&mut pkt, self.bytes_in_flight - sent_bytes);
+
         self.sent[epoch].push_back(pkt);
 
         self.bytes_sent += sent_bytes;
@@ -1125,11 +1126,11 @@
 impl std::fmt::Debug for Sent {
     fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
         write!(f, "pkt_num={:?} ", self.pkt_num)?;
-        write!(f, "pkt_sent_time={:?} ", self.time_sent.elapsed())?;
+        write!(f, "pkt_sent_time={:?} ", self.time_sent)?;
         write!(f, "pkt_size={:?} ", self.size)?;
         write!(f, "delivered={:?} ", self.delivered)?;
-        write!(f, "delivered_time={:?} ", self.delivered_time.elapsed())?;
-        write!(f, "first_sent_time={:?} ", self.first_sent_time.elapsed())?;
+        write!(f, "delivered_time={:?} ", self.delivered_time)?;
+        write!(f, "first_sent_time={:?} ", self.first_sent_time)?;
         write!(f, "is_app_limited={} ", self.is_app_limited)?;
         write!(f, "has_data={} ", self.has_data)?;