Add constant and percentage-based fee limits to payments

- Extend SendRequest and QueryRoutesRequest protos
- newRoute function takes fee limit and cuts off routes that exceed it
- queryRoutes, payInvoice and sendPayment commands take the feeLimit inputs and pass them down to newRoute
- When no feeLimit is included, don't enforce any feeLimits at all (by setting feeLimit to maxValue)

routing/pathfind.go

@@ -250,7 +250,7 @@ func (r *Route) ToHopPayloads() []sphinx.HopData {
 // the source to the target node of the path finding attempt.
 func newRoute(amtToSend lnwire.MilliSatoshi, sourceVertex Vertex,
 	pathEdges []*ChannelHop, currentHeight uint32,
-	finalCLTVDelta uint16) (*Route, error) {
+	finalCLTVDelta uint16, feeLimit btcutil.Amount) (*Route, error) {
 
 	// First, we'll create a new empty route with enough hops to match the
 	// amount of path edges. We set the TotalTimeLock to the current block
@@ -265,9 +265,6 @@ func newRoute(amtToSend lnwire.MilliSatoshi, sourceVertex Vertex,
 		prevHopMap:    make(map[Vertex]*ChannelHop),
 	}
 
-	// TODO(roasbeef): need to do sanity check to ensure we don't make a
-	// "dust" payment: over x% of money sending to fees
-
 	// We'll populate the next hop map for the _source_ node with the
 	// information for the first hop so the mapping is sound.
 	route.nextHopMap[sourceVertex] = pathEdges[0]
@@ -342,6 +339,14 @@ func newRoute(amtToSend lnwire.MilliSatoshi, sourceVertex Vertex,
 
 		route.TotalFees += nextHop.Fee
 
+		// If the total fees exceed the fee limit established for this
+		// payment, stop hopping the route and return
+		if route.TotalFees.ToSatoshis() > feeLimit {
+			err := fmt.Sprintf("Route fee exceeded fee limit of %v",
+				feeLimit)
+			return nil, newErrf(ErrFeeCutoffExceeded, err)
+		}
+
 		// As a sanity check, we ensure that the selected channel has
 		// enough capacity to forward the required amount which
 		// includes the fee dictated at each hop.