routing/route: define route.ReceiverAmt() method

routing/route/route.go

@@ -308,7 +308,16 @@ func (r *Route) TotalFees() lnwire.MilliSatoshi {
 		return 0
 	}
 
-	return r.TotalAmount - r.Hops[len(r.Hops)-1].AmtToForward
+	return r.TotalAmount - r.ReceiverAmt()
+}
+
+// ReceiverAmt is the amount received by the final hop of this route.
+func (r *Route) ReceiverAmt() lnwire.MilliSatoshi {
+	if len(r.Hops) == 0 {
+		return 0
+	}
+
+	return r.Hops[len(r.Hops)-1].AmtToForward
 }
 
 // NewRouteFromHops creates a new Route structure from the minimally required

routing/route/route_test.go

@@ -20,15 +20,24 @@ var (
 func TestRouteTotalFees(t *testing.T) {
 	t.Parallel()
 
-	// Make sure empty route returns a 0 fee.
+	// Make sure empty route returns a 0 fee, and zero amount.
 	r := &Route{}
 	if r.TotalFees() != 0 {
 		t.Fatalf("expected 0 fees, got %v", r.TotalFees())
 	}
+	if r.ReceiverAmt() != 0 {
+		t.Fatalf("expected 0 amt, got %v", r.ReceiverAmt())
+	}
+
+	// Make sure empty route won't be allowed in the constructor.
+	amt := lnwire.MilliSatoshi(1000)
+	_, err := NewRouteFromHops(amt, 100, Vertex{}, []*Hop{})
+	if err != ErrNoRouteHopsProvided {
+		t.Fatalf("expected ErrNoRouteHopsProvided, got %v", err)
+	}
 
 	// For one-hop routes the fee should be 0, since the last node will
 	// receive the full amount.
-	amt := lnwire.MilliSatoshi(1000)
 	hops := []*Hop{
 		{
 			PubKeyBytes:      Vertex{},
@@ -37,7 +46,7 @@ func TestRouteTotalFees(t *testing.T) {
 			AmtToForward:     amt,
 		},
 	}
-	r, err := NewRouteFromHops(amt, 100, Vertex{}, hops)
+	r, err = NewRouteFromHops(amt, 100, Vertex{}, hops)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -46,6 +55,10 @@ func TestRouteTotalFees(t *testing.T) {
 		t.Fatalf("expected 0 fees, got %v", r.TotalFees())
 	}
 
+	if r.ReceiverAmt() != amt {
+		t.Fatalf("expected %v amt, got %v", amt, r.ReceiverAmt())
+	}
+
 	// Append the route with a node, making the first one take a fee.
 	fee := lnwire.MilliSatoshi(100)
 	hops = append(hops, &Hop{
@@ -64,6 +77,10 @@ func TestRouteTotalFees(t *testing.T) {
 	if r.TotalFees() != fee {
 		t.Fatalf("expected %v fees, got %v", fee, r.TotalFees())
 	}
+
+	if r.ReceiverAmt() != amt-fee {
+		t.Fatalf("expected %v amt, got %v", amt-fee, r.ReceiverAmt())
+	}
 }
 
 var (

routing/router.go

@@ -1746,7 +1746,7 @@ func (r *ChannelRouter) SendToRoute(hash lntypes.Hash, route *route.Route) (
 	paySession := r.cfg.SessionSource.NewPaymentSessionForRoute(route)
 
 	// Calculate amount paid to receiver.
-	amt := route.TotalAmount - route.TotalFees()
+	amt := route.ReceiverAmt()
 
 	// Record this payment hash with the ControlTower, ensuring it is not
 	// already in-flight.