ackhandler: fix panic in probe packet tracking logic (#4998)

Under certain circumstances (loss and acknowledgment patterns), the
probe packet tracking logic could run into a nil-pointer dereference.

internal/ackhandler/sent_packet_handler.go

@@ -460,10 +460,10 @@ func (h *sentPacketHandler) detectAndRemoveAckedPackets(ack *wire.AckFrame, encL
 		}
 		if p.isPathProbePacket {
 			probePacket := pnSpace.history.RemovePathProbe(p.PacketNumber)
-			if probePacket == nil {
-				panic(fmt.Sprintf("path probe doesn't exist: %d", p.PacketNumber))
+			// the probe packet might already have been declared lost
+			if probePacket != nil {
+				h.ackedPackets = append(h.ackedPackets, probePacket)
 			}
-			h.ackedPackets = append(h.ackedPackets, probePacket)
 			continue
 		}
 		h.ackedPackets = append(h.ackedPackets, p)
@@ -658,7 +658,6 @@ func (h *sentPacketHandler) detectLostPathProbes(now time.Time) {
 		for _, f := range p.Frames {
 			f.Handler.OnLost(f.Frame)
 		}
-		h.appDataPackets.history.Remove(p.PacketNumber)
 		h.appDataPackets.history.RemovePathProbe(p.PacketNumber)
 	}
 }

internal/ackhandler/sent_packet_handler_test.go

@@ -1,7 +1,9 @@
 package ackhandler
 
 import (
+	"encoding/binary"
 	"fmt"
+	"math/rand/v2"
 	"slices"
 	"testing"
 	"time"
@@ -39,6 +41,11 @@ type packetTracker struct {
 	Lost  []protocol.PacketNumber
 }
 
+func (t *packetTracker) Reset() {
+	t.Acked = nil
+	t.Lost = nil
+}
+
 func (t *packetTracker) NewPingFrame(pn protocol.PacketNumber) Frame {
 	return Frame{
 		Frame: &wire.PingFrame{},
@@ -1227,3 +1234,91 @@ func TestSentPacketHandlerPathProbeAckAndLoss(t *testing.T) {
 
 	require.Equal(t, t2.Add(pathProbePacketLossTimeout), sph.GetLossDetectionTimeout())
 }
+
+// The packet tracking logic is pretty complex.
+// We test it with a randomized approach, to make sure that it doesn't panic under any circumstances.
+func TestSentPacketHandlerRandomized(t *testing.T) {
+	seed := uint64(time.Now().UnixNano())
+	for i := range 5 {
+		t.Run(fmt.Sprintf("run %d (seed %d)", i+1, seed), func(t *testing.T) {
+			testSentPacketHandlerRandomized(t, seed)
+		})
+		seed++
+	}
+}
+
+func testSentPacketHandlerRandomized(t *testing.T, seed uint64) {
+	var b [32]byte
+	binary.BigEndian.PutUint64(b[:], seed)
+	r := rand.New(rand.NewChaCha8(b))
+
+	var rttStats utils.RTTStats
+	rtt := []time.Duration{10 * time.Millisecond, 100 * time.Millisecond, 1000 * time.Millisecond}[r.IntN(3)]
+	t.Logf("rtt: %dms", rtt.Milliseconds())
+	rttStats.UpdateRTT(rtt, 0) // RTT of the original path
+
+	randDuration := func(min, max time.Duration) time.Duration {
+		return time.Duration(rand.Int64N(int64(max-min))) + min
+	}
+
+	sph := newSentPacketHandler(
+		0,
+		1200,
+		&rttStats,
+		true,
+		false,
+		protocol.PerspectiveClient,
+		nil,
+		utils.DefaultLogger,
+	)
+	sph.DropPackets(protocol.EncryptionInitial, time.Now())
+	sph.DropPackets(protocol.EncryptionHandshake, time.Now())
+
+	var packets packetTracker
+	sendPacket := func(ti time.Time, isPathProbe bool) protocol.PacketNumber {
+		pn := sph.PopPacketNumber(protocol.Encryption1RTT)
+		sph.SentPacket(ti, pn, protocol.InvalidPacketNumber, nil, []Frame{packets.NewPingFrame(pn)}, protocol.Encryption1RTT, protocol.ECNNon, 1200, false, isPathProbe)
+		return pn
+	}
+
+	now := time.Now()
+	start := now
+	var pns []protocol.PacketNumber
+	for range 4 {
+		isProbe := r.Int()%2 == 0
+		pn := sendPacket(now, isProbe)
+		t.Logf("t=%dms: sending packet %d (probe packet: %t)", now.Sub(start).Milliseconds(), pn, isProbe)
+		pns = append(pns, pn)
+		now = now.Add(randDuration(0, 500*time.Millisecond))
+		if r.Int()%3 == 0 {
+			sph.OnLossDetectionTimeout(now)
+			t.Logf("t=%dms: loss detection timeout (lost: %v)", now.Sub(start).Milliseconds(), packets.Lost)
+			packets.Reset()
+			now = now.Add(randDuration(0, 500*time.Millisecond))
+		}
+		if r.Int()%3 == 0 {
+			// acknowledge up to 2 random packet numbers from the pns slice
+			var ackPns []protocol.PacketNumber
+			if len(pns) > 0 {
+				numToAck := min(1+r.IntN(2), len(pns))
+				for range numToAck {
+					ackPns = append(ackPns, pns[r.IntN(len(pns))])
+				}
+			}
+			if len(ackPns) > 1 {
+				slices.Sort(ackPns)
+				ackPns = slices.Compact(ackPns)
+			}
+			sph.ReceivedAck(&wire.AckFrame{AckRanges: ackRanges(ackPns...)}, protocol.Encryption1RTT, now)
+			t.Logf("t=%dms: received ACK for packets %v (acked: %v, lost: %v)", now.Sub(start).Milliseconds(), ackPns, packets.Acked, packets.Lost)
+			packets.Reset()
+			now = now.Add(randDuration(0, 500*time.Millisecond))
+		}
+		if r.Int()%10 == 0 {
+			sph.MigratedPath(now, 1200)
+			now = now.Add(randDuration(0, 500*time.Millisecond))
+		}
+	}
+	t.Logf("t=%dms: loss detection timeout (lost: %v)", now.Sub(start).Milliseconds(), packets.Lost)
+	sph.OnLossDetectionTimeout(now)
+}