Allow building of a route from given hops

Implements `build_route_from_hops`, which provides a simple way to build
a route from us (payer) to the target node (payee) via the given hops
(which should exclude the payer, but include the payee). This may be
useful, e.g., for probing the chosen path.

Author: tnull

lightning/src/routing/router.rs

@@ -19,7 +19,7 @@ use ln::features::{ChannelFeatures, InvoiceFeatures, NodeFeatures};
 use ln::msgs::{DecodeError, ErrorAction, LightningError, MAX_VALUE_MSAT};
 use routing::scoring::{ChannelUsage, Score};
 use routing::network_graph::{DirectedChannelInfoWithUpdate, EffectiveCapacity, NetworkGraph, ReadOnlyNetworkGraph, NodeId, RoutingFees};
-use util::ser::{Writeable, Readable};
+use util::ser::{Writeable, Readable, Writer};
 use util::logger::{Level, Logger};
 use util::chacha20::ChaCha20;
 
@@ -151,8 +151,8 @@ impl Readable for Route {
 
 /// Parameters needed to find a [`Route`].
 ///
-/// Passed to [`find_route`] and also provided in [`Event::PaymentPathFailed`] for retrying a failed
-/// payment path.
+/// Passed to [`find_route`] and [`build_route_from_hops`], but also provided in
+/// [`Event::PaymentPathFailed`] for retrying a failed payment path.
 ///
 /// [`Event::PaymentPathFailed`]: crate::util::events::Event::PaymentPathFailed
 #[derive(Clone, Debug)]
@@ -676,16 +676,11 @@ pub fn find_route<L: Deref, S: Score>(
 ) -> Result<Route, LightningError>
 where L::Target: Logger {
 	let network_graph = network.read_only();
-	match get_route(
-		our_node_pubkey, &route_params.payment_params, &network_graph, first_hops, route_params.final_value_msat,
-		route_params.final_cltv_expiry_delta, logger, scorer, random_seed_bytes
-	) {
-		Ok(mut route) => {
-			add_random_cltv_offset(&mut route, &route_params.payment_params, &network_graph, random_seed_bytes);
-			Ok(route)
-		},
-		Err(err) => Err(err),
-	}
+	let mut route = get_route(our_node_pubkey, &route_params.payment_params, &network_graph, first_hops,
+		route_params.final_value_msat, route_params.final_cltv_expiry_delta, logger, scorer,
+		random_seed_bytes)?;
+	add_random_cltv_offset(&mut route, &route_params.payment_params, &network_graph, random_seed_bytes);
+	Ok(route)
 }
 
 pub(crate) fn get_route<L: Deref, S: Score>(
@@ -1703,7 +1698,9 @@ where L::Target: Logger {
 // destination, if the remaining CLTV expiry delta exactly matches a feasible path in the network
 // graph. In order to improve privacy, this method obfuscates the CLTV expiry deltas along the
 // payment path by adding a randomized 'shadow route' offset to the final hop.
-fn add_random_cltv_offset(route: &mut Route, payment_params: &PaymentParameters, network_graph: &ReadOnlyNetworkGraph, random_seed_bytes: &[u8; 32]) {
+fn add_random_cltv_offset(route: &mut Route, payment_params: &PaymentParameters,
+	network_graph: &ReadOnlyNetworkGraph, random_seed_bytes: &[u8; 32]
+) {
 	let network_channels = network_graph.channels();
 	let network_nodes = network_graph.nodes();
 
@@ -1785,10 +1782,84 @@ fn add_random_cltv_offset(route: &mut Route, payment_params: &PaymentParameters,
 	}
 }
 
+/// Construct a route from us (payer) to the target node (payee) via the given hops (which should
+/// exclude the payer, but include the payee). This may be useful, e.g., for probing the chosen path.
+///
+/// Re-uses logic from `find_route`, so the restrictions described there also apply here.
+pub fn build_route_from_hops<L: Deref>(
+	our_node_pubkey: &PublicKey, hops: &[PublicKey], route_params: &RouteParameters, network: &NetworkGraph,
+	logger: L, random_seed_bytes: &[u8; 32]
+) -> Result<Route, LightningError>
+where L::Target: Logger {
+	let network_graph = network.read_only();
+	let mut route = build_route_from_hops_internal(
+		our_node_pubkey, hops, &route_params.payment_params, &network_graph,
+		route_params.final_value_msat, route_params.final_cltv_expiry_delta, logger, random_seed_bytes)?;
+	add_random_cltv_offset(&mut route, &route_params.payment_params, &network_graph, random_seed_bytes);
+	Ok(route)
+}
+
+fn build_route_from_hops_internal<L: Deref>(
+	our_node_pubkey: &PublicKey, hops: &[PublicKey], payment_params: &PaymentParameters,
+	network_graph: &ReadOnlyNetworkGraph, final_value_msat: u64, final_cltv_expiry_delta: u32,
+	logger: L, random_seed_bytes: &[u8; 32]
+) -> Result<Route, LightningError> where L::Target: Logger {
+
+	struct HopScorer {
+		our_node_id: NodeId,
+		hop_ids: [Option<NodeId>; MAX_PATH_LENGTH_ESTIMATE as usize],
+	}
+
+	impl Score for HopScorer {
+		fn channel_penalty_msat(&self, _short_channel_id: u64, source: &NodeId, target: &NodeId,
+			_usage: ChannelUsage) -> u64
+		{
+			let mut cur_id = self.our_node_id;
+			for i in 0..self.hop_ids.len() {
+				if let Some(next_id) = self.hop_ids[i] {
+					if cur_id == *source && next_id == *target {
+						return 0;
+					}
+					cur_id = next_id;
+				} else {
+					break;
+				}
+			}
+			u64::max_value()
+		}
+
+		fn payment_path_failed(&mut self, _path: &[&RouteHop], _short_channel_id: u64) {}
+
+		fn payment_path_successful(&mut self, _path: &[&RouteHop]) {}
+	}
+
+	impl<'a> Writeable for HopScorer {
+		#[inline]
+		fn write<W: Writer>(&self, _w: &mut W) -> Result<(), io::Error> {
+			unreachable!();
+		}
+	}
+
+	if hops.len() > MAX_PATH_LENGTH_ESTIMATE.into() {
+		return Err(LightningError{err: "Cannot build a route exceeding the maximum path length.".to_owned(), action: ErrorAction::IgnoreError});
+	}
+
+	let our_node_id = NodeId::from_pubkey(our_node_pubkey);
+	let mut hop_ids = [None; MAX_PATH_LENGTH_ESTIMATE as usize];
+	for i in 0..hops.len() {
+		hop_ids[i] = Some(NodeId::from_pubkey(&hops[i]));
+	}
+
+	let scorer = HopScorer { our_node_id, hop_ids };
+
+	get_route(our_node_pubkey, payment_params, network_graph, None, final_value_msat,
+		final_cltv_expiry_delta, logger, &scorer, random_seed_bytes)
+}
+
 #[cfg(test)]
 mod tests {
 	use routing::network_graph::{NetworkGraph, NetGraphMsgHandler, NodeId};
-	use routing::router::{get_route, add_random_cltv_offset, default_node_features,
+	use routing::router::{get_route, build_route_from_hops_internal, add_random_cltv_offset, default_node_features,
 		PaymentParameters, Route, RouteHint, RouteHintHop, RouteHop, RoutingFees,
 		DEFAULT_MAX_TOTAL_CLTV_EXPIRY_DELTA, MAX_PATH_LENGTH_ESTIMATE};
 	use routing::scoring::{ChannelUsage, Score};
@@ -5486,6 +5557,26 @@ mod tests {
 		assert!(path_plausibility.iter().all(|x| *x));
 	}
 
+	#[test]
+	fn builds_correct_path_from_hops() {
+		let (secp_ctx, network, _, _, logger) = build_graph();
+		let (_, our_id, _, nodes) = get_nodes(&secp_ctx);
+		let network_graph = network.read_only();
+
+		let keys_manager = test_utils::TestKeysInterface::new(&[0u8; 32], Network::Testnet);
+		let random_seed_bytes = keys_manager.get_secure_random_bytes();
+
+		let payment_params = PaymentParameters::from_node_id(nodes[3]);
+		let hops = [nodes[1], nodes[2], nodes[4], nodes[3]];
+		let route = build_route_from_hops_internal(&our_id, &hops, &payment_params,
+			 &network_graph, 100, 0, Arc::clone(&logger), &random_seed_bytes).unwrap();
+		let route_hop_pubkeys = route.paths[0].iter().map(|hop| hop.pubkey).collect::<Vec<_>>();
+		assert_eq!(hops.len(), route.paths[0].len());
+		for (idx, hop_pubkey) in hops.iter().enumerate() {
+			assert!(*hop_pubkey == route_hop_pubkeys[idx]);
+		}
+	}
+
 	#[cfg(not(feature = "no-std"))]
 	pub(super) fn random_init_seed() -> u64 {
 		// Because the default HashMap in std pulls OS randomness, we can use it as a (bad) RNG.