ackhandler: track lost packets and detect spurious losses (#5355)

* ackhandler: implement a lost packet tracker

* logging: add ConnectionTracer.DetectedSpuriousLoss event

* ackhandler: detect spurious losses

* qlog: add support for logging spurious packet loss

* ackhandler: delete lost packets after detecting spurios loss

* guard against slice index underflow

internal/ackhandler/lost_packet_tracker_test.go

@@ -0,0 +1,75 @@
+package ackhandler
+
+import (
+	"maps"
+	"testing"
+	"time"
+
+	"github.com/quic-go/quic-go/internal/monotime"
+	"github.com/quic-go/quic-go/internal/protocol"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestLostPacketTracker(t *testing.T) {
+	lt := newLostPacketTracker(4)
+
+	start := monotime.Now()
+	lt.Add(1, start)
+	lt.Add(5, start.Add(time.Second))
+	lt.Add(8, start.Add(2*time.Second))
+	require.Equal(t, map[protocol.PacketNumber]monotime.Time{
+		1: start,
+		5: start.Add(time.Second),
+		8: start.Add(2 * time.Second),
+	}, maps.Collect(lt.All()))
+
+	// Lose 2 more packets. The first one should be removed.
+	lt.Add(10, start.Add(3*time.Second))
+	lt.Add(11, start.Add(4*time.Second))
+	require.Equal(t, map[protocol.PacketNumber]monotime.Time{
+		5:  start.Add(time.Second),
+		8:  start.Add(2 * time.Second),
+		10: start.Add(3 * time.Second),
+		11: start.Add(4 * time.Second),
+	}, maps.Collect(lt.All()))
+
+	lt.Delete(5)
+	lt.Delete(10)
+	require.Equal(t, map[protocol.PacketNumber]monotime.Time{
+		8:  start.Add(2 * time.Second),
+		11: start.Add(4 * time.Second),
+	}, maps.Collect(lt.All()))
+}
+
+func TestLostPacketTrackerDeleteBefore(t *testing.T) {
+	lt := newLostPacketTracker(4)
+
+	trackedPackets := func(lt *lostPacketTracker) []protocol.PacketNumber {
+		var pns []protocol.PacketNumber
+		for pn := range lt.All() {
+			pns = append(pns, pn)
+		}
+		return pns
+	}
+
+	start := monotime.Now()
+	lt.Add(1, start)
+	lt.Add(5, start.Add(time.Second))
+	lt.Add(8, start.Add(2*time.Second))
+	lt.Add(10, start.Add(3*time.Second))
+
+	require.Equal(t, []protocol.PacketNumber{1, 5, 8, 10}, trackedPackets(lt))
+
+	lt.DeleteBefore(start) // this should be a no-op
+	require.Equal(t, []protocol.PacketNumber{1, 5, 8, 10}, trackedPackets(lt))
+
+	lt.DeleteBefore(start.Add(2 * time.Second))
+	require.Equal(t, []protocol.PacketNumber{8, 10}, trackedPackets(lt))
+
+	lt.DeleteBefore(start.Add(time.Second * 5 / 2))
+	require.Equal(t, []protocol.PacketNumber{10}, trackedPackets(lt))
+
+	lt.DeleteBefore(start.Add(time.Hour))
+	require.Empty(t, trackedPackets(lt))
+}