lnrpc: add first hop custom data to route

lnrpc/routerrpc/router.swagger.json

@@ -1166,6 +1166,16 @@
           "type": "string",
           "format": "int64",
           "description": "The total amount in millisatoshis."
+        },
+        "first_hop_amount_msat": {
+          "type": "string",
+          "format": "int64",
+          "description": "The actual on-chain amount that was sent out to the first hop. This value is\nonly different from the total_amt_msat field if this is a custom channel\npayment and the value transported in the HTLC is different from the BTC\namount in the HTLC. If this value is zero, then this is an old payment that\ndidn't have this value yet and can be ignored."
+        },
+        "custom_channel_data": {
+          "type": "string",
+          "format": "byte",
+          "description": "Custom channel data that might be populated in custom channels."
         }
       },
       "description": "A path through the channel graph which runs over one or more channels in\nsuccession. This struct carries all the information required to craft the\nSphinx onion packet, and send the payment along the first hop in the path. A\nroute is only selected as valid if all the channels have sufficient capacity to\ncarry the initial payment amount after fees are accounted for."

lnrpc/routerrpc/router_backend.go

@@ -578,13 +578,28 @@ func (r *RouterBackend) rpcEdgeToPair(e *lnrpc.EdgeLocator) (
 // MarshallRoute marshalls an internal route to an rpc route struct.
 func (r *RouterBackend) MarshallRoute(route *route.Route) (*lnrpc.Route, error) {
 	resp := &lnrpc.Route{
-		TotalTimeLock: route.TotalTimeLock,
-		TotalFees:     int64(route.TotalFees().ToSatoshis()),
-		TotalFeesMsat: int64(route.TotalFees()),
-		TotalAmt:      int64(route.TotalAmount.ToSatoshis()),
-		TotalAmtMsat:  int64(route.TotalAmount),
-		Hops:          make([]*lnrpc.Hop, len(route.Hops)),
+		TotalTimeLock:      route.TotalTimeLock,
+		TotalFees:          int64(route.TotalFees().ToSatoshis()),
+		TotalFeesMsat:      int64(route.TotalFees()),
+		TotalAmt:           int64(route.TotalAmount.ToSatoshis()),
+		TotalAmtMsat:       int64(route.TotalAmount),
+		Hops:               make([]*lnrpc.Hop, len(route.Hops)),
+		FirstHopAmountMsat: int64(route.FirstHopAmount.Val.Int()),
 	}
+
+	// Encode the route's custom channel data (if available).
+	if len(route.FirstHopWireCustomRecords) > 0 {
+		customData, err := route.FirstHopWireCustomRecords.Serialize()
+		if err != nil {
+			return nil, err
+		}
+
+		resp.CustomChannelData = customData
+
+		// TODO(guggero): Feed the route into the custom data parser
+		// (part 3 of the mega PR series).
+	}
+
 	incomingAmt := route.TotalAmount
 	for i, hop := range route.Hops {
 		fee := route.HopFee(i)