lnrpc: set a zero value endorsement signal on sender outgoing htlc

Before we have sufficient signaling in the network to relay this
signal, set a zero value experimental endorsement value on the sender's
outgoing htlc. Once the network is relaying this signal and a flag day
has been set, we'll be able to set a non-zero value here.

itest/lnd_forward_interceptor_test.go

@@ -515,12 +515,9 @@ func testForwardInterceptorWireRecords(ht *lntest.HarnessTest) {
 	// all intercepted packets. These packets are held to simulate a
 	// pending payment.
 	packet := ht.ReceiveHtlcInterceptor(bobInterceptor)
-
-	require.Len(ht, packet.InWireCustomRecords, 1)
-
-	val, ok := packet.InWireCustomRecords[65537]
-	require.True(ht, ok, "expected custom record")
-	require.Equal(ht, []byte("test"), val)
+	require.Equal(ht, lntest.CustomRecordsWithUnendorsed(
+		customRecords,
+	), packet.InWireCustomRecords)
 
 	// Just resume the payment on Bob.
 	err := bobInterceptor.Send(&routerrpc.ForwardHtlcInterceptResponse{
@@ -569,7 +566,9 @@ func testForwardInterceptorWireRecords(ht *lntest.HarnessTest) {
 		func(p *lnrpc.Payment) error {
 			recordsEqual := reflect.DeepEqual(
 				p.FirstHopCustomRecords,
-				sendReq.FirstHopCustomRecords,
+				lntest.CustomRecordsWithUnendorsed(
+					customRecords,
+				),
 			)
 			if !recordsEqual {
 				return fmt.Errorf("expected custom records to "+
@@ -643,9 +642,9 @@ func testForwardInterceptorRestart(ht *lntest.HarnessTest) {
 	// all intercepted packets. These packets are held to simulate a
 	// pending payment.
 	packet := ht.ReceiveHtlcInterceptor(bobInterceptor)
-
-	require.Len(ht, packet.InWireCustomRecords, 1)
-	require.Equal(ht, customRecords, packet.InWireCustomRecords)
+	require.Equal(ht, lntest.CustomRecordsWithUnendorsed(
+		customRecords,
+	), packet.InWireCustomRecords)
 
 	// We accept the payment at Bob and resume it, so it gets to Carol.
 	// This means the HTLC should now be fully locked in on Alice's side and
@@ -681,8 +680,9 @@ func testForwardInterceptorRestart(ht *lntest.HarnessTest) {
 	// We should get another notification about the held HTLC.
 	packet = ht.ReceiveHtlcInterceptor(bobInterceptor)
 
-	require.Len(ht, packet.InWireCustomRecords, 1)
-	require.Equal(ht, customRecords, packet.InWireCustomRecords)
+	require.Len(ht, packet.InWireCustomRecords, 2)
+	require.Equal(ht, lntest.CustomRecordsWithUnendorsed(customRecords),
+		packet.InWireCustomRecords)
 
 	err = carolInterceptor.Send(&routerrpc.ForwardHtlcInterceptResponse{
 		IncomingCircuitKey: packet.IncomingCircuitKey,
@@ -690,9 +690,10 @@ func testForwardInterceptorRestart(ht *lntest.HarnessTest) {
 	})
 	require.NoError(ht, err, "failed to send request")
 
-	// And now we forward the payment at Carol.
+	// And now we forward the payment at Carol, expecting only an
+	// endorsement signal in our incoming custom records.
 	packet = ht.ReceiveHtlcInterceptor(carolInterceptor)
-	require.Len(ht, packet.InWireCustomRecords, 0)
+	require.Len(ht, packet.InWireCustomRecords, 1)
 	err = carolInterceptor.Send(&routerrpc.ForwardHtlcInterceptResponse{
 		IncomingCircuitKey: packet.IncomingCircuitKey,
 		Action:             actionResume,
@@ -704,8 +705,9 @@ func testForwardInterceptorRestart(ht *lntest.HarnessTest) {
 		alice, preimage, lnrpc.Payment_SUCCEEDED,
 		func(p *lnrpc.Payment) error {
 			recordsEqual := reflect.DeepEqual(
-				p.FirstHopCustomRecords,
-				sendReq.FirstHopCustomRecords,
+				lntest.CustomRecordsWithUnendorsed(
+					sendReq.FirstHopCustomRecords,
+				), p.FirstHopCustomRecords,
 			)
 			if !recordsEqual {
 				return fmt.Errorf("expected custom records to "+

lnrpc/routerrpc/router_backend.go

@@ -110,6 +110,10 @@ type RouterBackend struct {
 	// ParseCustomChannelData is a function that can be used to parse custom
 	// channel data from the first hop of a route.
 	ParseCustomChannelData func(message proto.Message) error
+
+	// ShouldSetExpEndorsement returns a boolean indicating whether the
+	// experimental endorsement bit should be set.
+	ShouldSetExpEndorsement func() bool
 }
 
 // MissionControl defines the mission control dependencies of routerrpc.
@@ -891,6 +895,23 @@ func (r *RouterBackend) extractIntentFromSendRequest(
 	}
 	payIntent.FirstHopCustomRecords = firstHopRecords
 
+	// If the experimental endorsement signal is not already set, propagate
+	// a zero value field if configured to set this signal.
+	if r.ShouldSetExpEndorsement() {
+		if payIntent.FirstHopCustomRecords == nil {
+			payIntent.FirstHopCustomRecords = make(
+				map[uint64][]byte,
+			)
+		}
+
+		t := uint64(lnwire.ExperimentalEndorsementType)
+		if _, set := payIntent.FirstHopCustomRecords[t]; !set {
+			payIntent.FirstHopCustomRecords[t] = []byte{
+				lnwire.ExperimentalUnendorsed,
+			}
+		}
+	}
+
 	payIntent.PayAttemptTimeout = time.Second *
 		time.Duration(rpcPayReq.TimeoutSeconds)
 

rpcserver.go

@@ -44,6 +44,7 @@ import (
 	"github.com/lightningnetwork/lnd/channeldb/graphsession"
 	"github.com/lightningnetwork/lnd/channeldb/models"
 	"github.com/lightningnetwork/lnd/channelnotifier"
+	"github.com/lightningnetwork/lnd/clock"
 	"github.com/lightningnetwork/lnd/contractcourt"
 	"github.com/lightningnetwork/lnd/discovery"
 	"github.com/lightningnetwork/lnd/feature"
@@ -758,6 +759,11 @@ func (r *rpcServer) addDeps(s *server, macService *macaroons.Service,
 
 			return nil
 		},
+		ShouldSetExpEndorsement: func() bool {
+			return clock.NewDefaultClock().Now().Before(
+				EndorsementExperimentEnd,
+			)
+		},
 	}
 
 	genInvoiceFeatures := func() *lnwire.FeatureVector {