multi: explicitly signal final hop in pack hop payload

Previously, we'd use the value of nextChanID to infer whether a payload
was for the final hop in a route. This commit updates our packing logic
to explicitly signal to account for blinded routes, which allow zero
value nextChanID in intermediate hops. This is a preparatory commit
that allows us to more thoroughly validate payloads.

htlcswitch/hop/fuzz_test.go

@@ -121,7 +121,7 @@ func fuzzPayload(f *testing.F, finalPayload bool) {
 
 		var b bytes.Buffer
 		hop, nextChanID := hopFromPayload(payload1)
-		err = hop.PackHopPayload(&b, nextChanID)
+		err = hop.PackHopPayload(&b, nextChanID, finalPayload)
 		if errors.Is(err, route.ErrAMPMissingMPP) {
 			// PackHopPayload refuses to encode an AMP record
 			// without an MPP record. However, NewPayloadFromReader

routing/route/route.go

@@ -172,11 +172,15 @@ func (h *Hop) Copy() *Hop {
 // PackHopPayload writes to the passed io.Writer, the series of byes that can
 // be placed directly into the per-hop payload (EOB) for this hop. This will
 // include the required routing fields, as well as serializing any of the
-// passed optional TLVRecords.  nextChanID is the unique channel ID that
+// passed optional TLVRecords. nextChanID is the unique channel ID that
-// references the _outgoing_ channel ID that follows this hop. This field
+// references the _outgoing_ channel ID that follows this hop. The lastHop bool
-// follows the same semantics as the NextAddress field in the onion: it should
+// is used to signal whether this hop is the final hop in a route. Previously,
-// be set to zero to indicate the terminal hop.
+// a zero nextChanID would be used for this purpose, but with the addition of
-func (h *Hop) PackHopPayload(w io.Writer, nextChanID uint64) error {
+// blinded routes which allow zero nextChanID values for intermediate hops we
+// add an explicit signal.
+func (h *Hop) PackHopPayload(w io.Writer, nextChanID uint64,
+	finalHop bool) error {
+
 	// If this is a legacy payload, then we'll exit here as this method
 	// shouldn't be called.
 	if h.LegacyPayload == true {
@@ -218,7 +222,7 @@ func (h *Hop) PackHopPayload(w io.Writer, nextChanID uint64) error {
 	// attach it to the final hop. Otherwise the route was constructed
 	// incorrectly.
 	if h.MPP != nil {
-		if nextChanID == 0 {
+		if finalHop {
 			records = append(records, h.MPP.Record())
 		} else {
 			return ErrIntermediateMPPHop
@@ -559,7 +563,8 @@ func (r *Route) ToSphinxPath() (*sphinx.PaymentPath, error) {
 
 		// If we aren't on the last hop, then we set the "next address"
 		// field to be the channel that directly follows it.
-		if i != len(r.Hops)-1 {
+		finalHop := i == len(r.Hops)-1
+		if !finalHop {
 			nextHop = r.Hops[i+1].ChannelID
 		}
 
@@ -591,7 +596,7 @@ func (r *Route) ToSphinxPath() (*sphinx.PaymentPath, error) {
 			// channel should be forwarded to so we can construct a
 			// valid payload.
 			var b bytes.Buffer
-			err := hop.PackHopPayload(&b, nextHop)
+			err := hop.PackHopPayload(&b, nextHop, finalHop)
 			if err != nil {
 				return nil, err
 			}

routing/route/route_test.go

@@ -105,7 +105,7 @@ func TestMPPHop(t *testing.T) {
 	// Encoding an MPP record to an intermediate hop should result in a
 	// failure.
 	var b bytes.Buffer
-	err := hop.PackHopPayload(&b, 2)
+	err := hop.PackHopPayload(&b, 2, false)
 	if err != ErrIntermediateMPPHop {
 		t.Fatalf("expected err: %v, got: %v",
 			ErrIntermediateMPPHop, err)
@@ -113,7 +113,7 @@ func TestMPPHop(t *testing.T) {
 
 	// Encoding an MPP record to a final hop should be successful.
 	b.Reset()
-	err = hop.PackHopPayload(&b, 0)
+	err = hop.PackHopPayload(&b, 0, true)
 	if err != nil {
 		t.Fatalf("expected err: %v, got: %v", nil, err)
 	}
@@ -135,7 +135,7 @@ func TestAMPHop(t *testing.T) {
 	// Encoding an AMP record to an intermediate hop w/o an MPP record
 	// should result in a failure.
 	var b bytes.Buffer
-	err := hop.PackHopPayload(&b, 2)
+	err := hop.PackHopPayload(&b, 2, false)
 	if err != ErrAMPMissingMPP {
 		t.Fatalf("expected err: %v, got: %v",
 			ErrAMPMissingMPP, err)
@@ -144,7 +144,7 @@ func TestAMPHop(t *testing.T) {
 	// Encoding an AMP record to a final hop w/o an MPP record should result
 	// in a failure.
 	b.Reset()
-	err = hop.PackHopPayload(&b, 0)
+	err = hop.PackHopPayload(&b, 0, true)
 	if err != ErrAMPMissingMPP {
 		t.Fatalf("expected err: %v, got: %v",
 			ErrAMPMissingMPP, err)
@@ -154,7 +154,7 @@ func TestAMPHop(t *testing.T) {
 	// successful.
 	hop.MPP = record.NewMPP(testAmt, testAddr)
 	b.Reset()
-	err = hop.PackHopPayload(&b, 0)
+	err = hop.PackHopPayload(&b, 0, true)
 	if err != nil {
 		t.Fatalf("expected err: %v, got: %v", nil, err)
 	}
@@ -170,7 +170,7 @@ func TestNoForwardingParams(t *testing.T) {
 	}
 
 	var b bytes.Buffer
-	err := hop.PackHopPayload(&b, 2)
+	err := hop.PackHopPayload(&b, 2, false)
 	require.NoError(t, err)
 }