fixup! restore cwnd rollback only for in-flight packets

Previously we would only rollback cwnd updates due to spurious losses
for in-flight packets (while still updating threshold and stats for all
spurious losses) as that could mean there was no prior state to rollback
to.

The refactor made rollbacks happen for all spurious losses, so this
restores that (we still want to update threshold and stats for not
in-flight spurious losses too).

quiche/src/recovery/mod.rs

@@ -101,6 +101,7 @@ struct AckedDetectionResult {
     spurious_losses: usize,
     spurious_pkt_thresh: Option<u64>,
     has_ack_eliciting: bool,
+    has_in_flight_spurious_loss: bool,
 }
 
 struct LossDetectionResult {
@@ -121,6 +122,7 @@ impl RecoveryEpoch {
         let mut spurious_losses = 0;
         let mut spurious_pkt_thresh = None;
         let mut has_ack_eliciting = false;
+        let mut has_in_flight_spurious_loss = false;
 
         let largest_acked = self.largest_acked_packet.unwrap();
 
@@ -154,6 +156,10 @@ impl RecoveryEpoch {
                     spurious_pkt_thresh
                         .get_or_insert(largest_acked - unacked.pkt_num + 1);
                     unacked.time_acked = Some(now);
+
+                    if unacked.in_flight {
+                        has_in_flight_spurious_loss = true;
+                    }
                 } else {
                     if unacked.in_flight {
                         self.in_flight_count -= 1;
@@ -192,6 +198,7 @@ impl RecoveryEpoch {
             spurious_losses,
             spurious_pkt_thresh,
             has_ack_eliciting,
+            has_in_flight_spurious_loss,
         }
     }
 
@@ -666,6 +673,7 @@ impl Recovery {
             spurious_losses,
             spurious_pkt_thresh,
             has_ack_eliciting,
+            has_in_flight_spurious_loss,
         } = self.epochs[epoch].detect_and_remove_acked_packets(
             now,
             ranges,
@@ -681,7 +689,7 @@ impl Recovery {
         }
 
         // Undo congestion window update.
-        if spurious_losses > 0 {
+        if has_in_flight_spurious_loss {
             (self.cc_ops.rollback)(self);
         }