Handle duplicated Flight 4

Author: JoTurk

internal/handshake/fsm13.go

@@ -12,6 +12,7 @@ import (
 	dtlsflight "github.com/pion/dtls/v3/internal/flight"
 	dtlsflight13 "github.com/pion/dtls/v3/internal/flight/flight13"
 	dtlsstate "github.com/pion/dtls/v3/internal/state"
+	"github.com/pion/dtls/v3/pkg/protocol"
 	"github.com/pion/dtls/v3/pkg/protocol/alert"
 	"github.com/pion/dtls/v3/pkg/protocol/handshake"
 )
@@ -347,7 +348,7 @@ func (s *fsm13) finish(ctx context.Context, conn Conn) (State, error) {
 	return StateFinished, nil
 }
 
-func (s *fsm13) handleReceivedFlight(
+func (s *fsm13) handleReceivedFlight( //nolint:cyclop
 	ctx context.Context,
 	conn Conn,
 	received RecvHandshakeState,
@@ -362,6 +363,9 @@ func (s *fsm13) handleReceivedFlight(
 	if !received.HasHandshake && len(received.ACKs) != 0 {
 		return s.transitionAfterACK(ackResult, false), nil
 	}
+	if received.HasHandshake && received.IsRetransmit && s.currentFlight.IsLastSendFlight() {
+		return s.handlePreviousFlightRetransmit(ctx, conn, received.RecordsToACK, ackResult)
+	}
 
 	nextFlight, err := s.parseReceivedFlight(ctx, conn, s.currentFlight)
 	if err != nil {
@@ -386,6 +390,23 @@ func (s *fsm13) handleReceivedFlight(
 	return transition, nil
 }
 
+func (s *fsm13) handlePreviousFlightRetransmit(
+	ctx context.Context,
+	conn Conn,
+	recordsToACK []protocol.RecordNumber,
+	ackResult ACKResult,
+) (receivedFlightTransition, error) {
+	// A duplicate peer flight means it didn't receive our response.
+	// ACK the duplicate and retransmit the pending final flight.
+	// check WAIT exit 3:
+	// https://datatracker.ietf.org/doc/html/rfc9147#section-5.8.1
+	if err := sendACK(ctx, conn, s.state.LocalEpoch(), recordsToACK); err != nil {
+		return receivedFlightTransition{}, err
+	}
+
+	return s.transitionAfterACK(ackResult, true), nil
+}
+
 func (s *fsm13) prepareFlightACKTracking(flights []*dtlsflight.Packet, retransmit bool) {
 	s.flightACK.reset()
 	if retransmit {

internal/handshake/fsm_test.go

@@ -894,6 +894,38 @@ func TestHandshakeFSM13ClientFlight5GeneratesFinished(t *testing.T) {
 	assert.Equal(t, dtlsflight13.EpochHandshake, conn.localEpoch)
 }
 
+func TestHandshakeFSM13ClientFlight5HandlesPreviousFlightRetransmit(t *testing.T) {
+	fixture := newNoHRRFlight13Fixture(t)
+	fsm := clientFSMThroughServerFlight13(t, fixture)
+	conn := &flightTestConn{}
+	nextState, err := fsm.prepare(context.Background(), conn)
+	require.NoError(t, err)
+	require.Equal(t, StateSending, nextState)
+	require.Equal(t, dtlsflight13.Flight5, fsm.currentFlight)
+
+	fixture.clientState.SetLocalEpoch(dtlsflight13.EpochHandshake)
+	record := protocol.RecordNumber{Epoch: uint64(dtlsflight13.EpochHandshake), SequenceNumber: 11}
+	retransmit := RecvHandshakeState{
+		Done:         make(chan struct{}),
+		HasHandshake: true,
+		IsRetransmit: true,
+		RecordsToACK: []protocol.RecordNumber{record},
+	}
+	transition, err := fsm.handleReceivedFlight(context.Background(), conn, retransmit)
+	require.NoError(t, err)
+	assert.Equal(t, StateSending, transition.state)
+	assert.Equal(t, dtlsflight13.Flight5, fsm.currentFlight)
+	require.Len(t, conn.writtenPackets, 1)
+	ackPacket := conn.writtenPackets[0]
+	assert.True(t, ackPacket.ShouldEncrypt)
+	assert.Equal(t, dtlsflight13.EpochHandshake, ackPacket.Record.Header.Epoch)
+	ack, ok := ackPacket.Record.Content.(*protocol.ACK)
+	require.True(t, ok)
+	assert.Equal(t, []protocol.RecordNumber{record}, ack.Records)
+	fsm.received.release()
+	assertFlight13RecvDoneClosed(t, retransmit)
+}
+
 func TestHandshakeFSM13SendErrorReleasesReader(t *testing.T) {
 	tests := []struct {
 		name     string