Include any recipient overpayment amounts in the route fee limit

If the user told us to limit their total fee exposure, we should
do so including any potential overpayment to the recipient, which
is ultimately a part of the "fee" as far as the user is concerned.

Author: TheBlueMatt

lightning/src/routing/router.rs

@@ -2628,21 +2628,22 @@ where L::Target: Logger {
 	// Make sure we would never create a route with more paths than we allow.
 	debug_assert!(paths.len() <= payment_params.max_path_count.into());
 
-	// Make sure we would never create a route whose total fees exceed max_total_routing_fee_msat.
-	if let Some(max_total_routing_fee_msat) = route_params.max_total_routing_fee_msat {
-		if paths.iter().map(|p| p.fee_msat()).sum::<u64>() > max_total_routing_fee_msat {
-			return Err(LightningError{err: format!("Failed to find route that adheres to the maximum total fee limit of {}msat",
-				max_total_routing_fee_msat), action: ErrorAction::IgnoreError});
-		}
-	}
-
 	if let Some(node_features) = payment_params.payee.node_features() {
 		for path in paths.iter_mut() {
 			path.hops.last_mut().unwrap().node_features = node_features.clone();
 		}
 	}
 
 	let route = Route { paths, route_params: Some(route_params.clone()) };
+
+	// Make sure we would never create a route whose total fees exceed max_total_routing_fee_msat.
+	if let Some(max_total_routing_fee_msat) = route_params.max_total_routing_fee_msat {
+		if route.get_total_fees() > max_total_routing_fee_msat {
+			return Err(LightningError{err: format!("Failed to find route that adheres to the maximum total fee limit of {}msat",
+				max_total_routing_fee_msat), action: ErrorAction::IgnoreError});
+		}
+	}
+
 	log_info!(logger, "Got route: {}", log_route!(route));
 	Ok(route)
 }
@@ -3266,11 +3267,22 @@ mod tests {
 			excess_data: Vec::new()
 		});
 
-		// Now check that we'll find a path if the htlc_minimum is overrun substantially.
+		// Now check that we'll fail to find a path if we fail to find a path if the htlc_minimum
+		// is overrun. Note that the fees are actually calculated on 3*payment amount as that's
+		// what we try to find a route for, so this test only just happens to work out to exactly
+		// the fee limit.
 		let mut route_params = RouteParameters::from_payment_params_and_value(
 			payment_params.clone(), 5_000);
-		// TODO: This can even overrun the fee limit set by the recipient!
 		route_params.max_total_routing_fee_msat = Some(9_999);
+		if let Err(LightningError{err, action: ErrorAction::IgnoreError}) = get_route(&our_id,
+			&route_params, &network_graph.read_only(), None, Arc::clone(&logger), &scorer,
+			&Default::default(), &random_seed_bytes) {
+				assert_eq!(err, "Failed to find route that adheres to the maximum total fee limit of 9999msat");
+		} else { panic!(); }
+
+		let mut route_params = RouteParameters::from_payment_params_and_value(
+			payment_params.clone(), 5_000);
+		route_params.max_total_routing_fee_msat = Some(10_000);
 		let route = get_route(&our_id, &route_params, &network_graph.read_only(), None,
 			Arc::clone(&logger), &scorer, &Default::default(), &random_seed_bytes).unwrap();
 		assert_eq!(route.get_total_fees(), 10_000);