routing/route/route: add optional MPP field to Hop

This commit also modifies the Router serialization to persist the MPP
struct when present, and properly restore it when loading from disk.

routing/route/route.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"encoding/binary"
 	"encoding/hex"
+	"errors"
 	"fmt"
 	"io"
 	"strconv"
@@ -19,9 +20,17 @@ import (
 // VertexSize is the size of the array to store a vertex.
 const VertexSize = 33
 
-// ErrNoRouteHopsProvided is returned when a caller attempts to construct a new
-// sphinx packet, but provides an empty set of hops for each route.
-var ErrNoRouteHopsProvided = fmt.Errorf("empty route hops provided")
+var (
+	// ErrNoRouteHopsProvided is returned when a caller attempts to
+	// construct a new sphinx packet, but provides an empty set of hops for
+	// each route.
+	ErrNoRouteHopsProvided = fmt.Errorf("empty route hops provided")
+
+	// ErrIntermediateMPPHop is returned when a hop tries to deliver an MPP
+	// record to an intermediate hop, only final hops can receive MPP
+	// records.
+	ErrIntermediateMPPHop = errors.New("cannot send MPP to intermediate")
+)
 
 // Vertex is a simple alias for the serialization of a compressed Bitcoin
 // public key.
@@ -94,6 +103,10 @@ type Hop struct {
 	// carries as a fee will be subtracted by the hop.
 	AmtToForward lnwire.MilliSatoshi
 
+	// MPP encapsulates the data required for option_mpp. This field should
+	// only be set for the final hop.
+	MPP *record.MPP
+
 	// TLVRecords if non-nil are a set of additional TLV records that
 	// should be included in the forwarding instructions for this node.
 	TLVRecords []tlv.Record
@@ -140,6 +153,17 @@ func (h *Hop) PackHopPayload(w io.Writer, nextChanID uint64) error {
 		)
 	}
 
+	// If an MPP record is destined for this hop, ensure that we only ever
+	// attach it to the final hop. Otherwise the route was constructed
+	// incorrectly.
+	if h.MPP != nil {
+		if nextChanID == 0 {
+			records = append(records, h.MPP.Record())
+		} else {
+			return ErrIntermediateMPPHop
+		}
+	}
+
 	// Append any custom types destined for this hop.
 	records = append(records, h.TLVRecords...)