routing: add new interface AdditionalEdge.

A new interface AdditionalEdge is introduced which allows us to
enhance the CachedEdgePolicy with additional information. We
currently only need a PayloadSize function which is then used to
determine the exact payload size when building a route to adhere
to the sphinx package size limit (BOLT04).

routing/additional_edge.go

@@ -0,0 +1,107 @@
+package routing
+
+import (
+	"errors"
+
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/lightningnetwork/lnd/channeldb/models"
+	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/lightningnetwork/lnd/routing/route"
+)
+
+var (
+	// ErrNoPayLoadSizeFunc is returned when no payload size function is
+	// definied.
+	ErrNoPayLoadSizeFunc = errors.New("no payloadSizeFunc defined for " +
+		"additional edge")
+)
+
+// AdditionalEdge is an interface which specifies additional edges which can
+// be appended to an existing route. Compared to normal edges of a route they
+// provide an explicit payload size function and are introduced because blinded
+// paths differ in their payload structure.
+type AdditionalEdge interface {
+	// IntermediatePayloadSize returns the size of the payload for the
+	// additional edge when being an intermediate hop in a route NOT the
+	// final hop.
+	IntermediatePayloadSize(amount lnwire.MilliSatoshi, expiry uint32,
+		legacy bool, channelID uint64) uint64
+
+	// EdgePolicy returns the policy of the additional edge.
+	EdgePolicy() *models.CachedEdgePolicy
+}
+
+// PayloadSizeFunc defines the interface for the payload size function.
+type PayloadSizeFunc func(amount lnwire.MilliSatoshi, expiry uint32,
+	legacy bool, channelID uint64) uint64
+
+// PrivateEdge implements the AdditionalEdge interface. As the name implies it
+// is used for private route hints that the receiver adds for example to an
+// invoice.
+type PrivateEdge struct {
+	policy *models.CachedEdgePolicy
+}
+
+// EdgePolicy return the policy of the PrivateEdge.
+func (p *PrivateEdge) EdgePolicy() *models.CachedEdgePolicy {
+	return p.policy
+}
+
+// IntermediatePayloadSize returns the sphinx payload size defined in BOLT04 if
+// this edge were to be included in a route.
+func (p *PrivateEdge) IntermediatePayloadSize(amount lnwire.MilliSatoshi,
+	expiry uint32, legacy bool, channelID uint64) uint64 {
+
+	hop := route.Hop{
+		AmtToForward:     amount,
+		OutgoingTimeLock: expiry,
+		LegacyPayload:    legacy,
+	}
+
+	return hop.PayloadSize(channelID)
+}
+
+// BlindedEdge implements the AdditionalEdge interface. Blinded hops are viewed
+// as additional edges because they are appened at the end of a normal route.
+type BlindedEdge struct {
+	policy        *models.CachedEdgePolicy
+	cipherText    []byte
+	blindingPoint *btcec.PublicKey
+}
+
+// EdgePolicy return the policy of the BlindedEdge.
+func (b *BlindedEdge) EdgePolicy() *models.CachedEdgePolicy {
+	return b.policy
+}
+
+// IntermediatePayloadSize returns the sphinx payload size defined in BOLT04 if
+// this edge were to be included in a route.
+func (b *BlindedEdge) IntermediatePayloadSize(_ lnwire.MilliSatoshi, _ uint32,
+	_ bool, _ uint64) uint64 {
+
+	hop := route.Hop{
+		BlindingPoint: b.blindingPoint,
+		LegacyPayload: false,
+		EncryptedData: b.cipherText,
+	}
+
+	// For blinded paths the next chanID is in the encrypted data tlv.
+	return hop.PayloadSize(0)
+}
+
+// Compile-time constraints to ensure the PrivateEdge and the BlindedEdge
+// implement the AdditionalEdge interface.
+var _ AdditionalEdge = (*PrivateEdge)(nil)
+var _ AdditionalEdge = (*BlindedEdge)(nil)
+
+// defaultHopPayloadSize is the default payload size of a normal (not-blinded)
+// hop in the route.
+func defaultHopPayloadSize(amount lnwire.MilliSatoshi, expiry uint32,
+	legacy bool, channelID uint64) uint64 {
+
+	// The payload size of a cleartext intermediate hop is equal to the
+	// payload size of a private edge therefore we reuse its size function.
+	edge := PrivateEdge{}
+
+	return edge.IntermediatePayloadSize(amount, expiry, legacy, channelID)
+}

routing/additional_edge_test.go

@@ -0,0 +1,136 @@
+package routing
+
+import (
+	"bytes"
+	"encoding/binary"
+	"encoding/hex"
+	"testing"
+
+	"github.com/btcsuite/btcd/btcec/v2"
+	sphinx "github.com/lightningnetwork/lightning-onion"
+	"github.com/lightningnetwork/lnd/routing/route"
+	"github.com/stretchr/testify/require"
+)
+
+// TestIntermediatePayloadSize tests the payload size functions of the
+// PrivateEdge and the BlindedEdge.
+func TestIntermediatePayloadSize(t *testing.T) {
+	t.Parallel()
+
+	testPrivKeyBytes, _ := hex.DecodeString("e126f68f7eafcc8b74f54d269fe" +
+		"206be715000f94dac067d1c04a8ca3b2db734")
+	_, blindedPoint := btcec.PrivKeyFromBytes(testPrivKeyBytes)
+
+	testCases := []struct {
+		name    string
+		hop     route.Hop
+		nextHop uint64
+		edge    AdditionalEdge
+	}{
+		{
+			name: "Legacy payload private edge",
+			hop: route.Hop{
+				AmtToForward:     1000,
+				OutgoingTimeLock: 600000,
+				ChannelID:        3432483437438,
+				LegacyPayload:    true,
+			},
+			nextHop: 1,
+			edge:    &PrivateEdge{},
+		},
+		{
+			name: "Tlv payload private edge",
+			hop: route.Hop{
+				AmtToForward:     1000,
+				OutgoingTimeLock: 600000,
+				ChannelID:        3432483437438,
+				LegacyPayload:    false,
+			},
+			nextHop: 1,
+			edge:    &PrivateEdge{},
+		},
+		{
+			name: "Blinded edge",
+			hop: route.Hop{
+				EncryptedData: []byte{12, 13},
+			},
+			edge: &BlindedEdge{
+				cipherText: []byte{12, 13},
+			},
+		},
+		{
+			name: "Blinded edge - introduction point",
+			hop: route.Hop{
+				EncryptedData: []byte{12, 13},
+				BlindingPoint: blindedPoint,
+			},
+			edge: &BlindedEdge{
+				cipherText:    []byte{12, 13},
+				blindingPoint: blindedPoint,
+			},
+		},
+	}
+
+	for _, testCase := range testCases {
+		testCase := testCase
+
+		t.Run(testCase.name, func(t *testing.T) {
+			t.Parallel()
+
+			payLoad, err := createHopPayload(
+				testCase.hop, testCase.nextHop, false,
+			)
+			require.NoErrorf(t, err, "failed to create hop payload")
+
+			expectedPayloadSize := testCase.edge.
+				IntermediatePayloadSize(
+					testCase.hop.AmtToForward,
+					testCase.hop.OutgoingTimeLock,
+					testCase.hop.LegacyPayload,
+					testCase.nextHop,
+				)
+
+			require.Equal(
+				t, expectedPayloadSize,
+				uint64(payLoad.NumBytes()),
+			)
+		})
+	}
+}
+
+// createHopPayload creates the hop payload of the sphinx package to facilitate
+// the testing of the payload size.
+func createHopPayload(hop route.Hop, nextHop uint64,
+	finalHop bool) (sphinx.HopPayload, error) {
+
+	// If this is the legacy payload, then we can just include the
+	// hop data as normal.
+	if hop.LegacyPayload {
+		// Before we encode this value, we'll pack the next hop
+		// into the NextAddress field of the hop info to ensure
+		// we point to the right now.
+		hopData := sphinx.HopData{
+			ForwardAmount: uint64(hop.AmtToForward),
+			OutgoingCltv:  hop.OutgoingTimeLock,
+		}
+		binary.BigEndian.PutUint64(
+			hopData.NextAddress[:], nextHop,
+		)
+
+		return sphinx.NewLegacyHopPayload(&hopData)
+	}
+
+	// For non-legacy payloads, we'll need to pack the
+	// routing information, along with any extra TLV
+	// information into the new per-hop payload format.
+	// We'll also pass in the chan ID of the hop this
+	// channel should be forwarded to so we can construct a
+	// valid payload.
+	var b bytes.Buffer
+	err := hop.PackHopPayload(&b, nextHop, finalHop)
+	if err != nil {
+		return sphinx.HopPayload{}, err
+	}
+
+	return sphinx.NewTLVHopPayload(b.Bytes())
+}

routing/mocks.go

@@ -0,0 +1,32 @@
+package routing
+
+import (
+	"github.com/lightningnetwork/lnd/channeldb/models"
+	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/stretchr/testify/mock"
+)
+
+// mockAdditionalEdge is a mock of the AdditionalEdge interface.
+type mockAdditionalEdge struct{ mock.Mock }
+
+// IntermediatePayloadSize returns the sphinx payload size defined in BOLT04 if
+// this edge were to be included in a route.
+func (m *mockAdditionalEdge) IntermediatePayloadSize(amount lnwire.MilliSatoshi,
+	expiry uint32, legacy bool, channelID uint64) uint64 {
+
+	args := m.Called(amount, expiry, legacy, channelID)
+
+	return args.Get(0).(uint64)
+}
+
+// EdgePolicy return the policy of the mockAdditionalEdge.
+func (m *mockAdditionalEdge) EdgePolicy() *models.CachedEdgePolicy {
+	args := m.Called()
+
+	edgePolicy := args.Get(0)
+	if edgePolicy == nil {
+		return nil
+	}
+
+	return edgePolicy.(*models.CachedEdgePolicy)
+}