Implement pathfinding for onion messages

We *could* reuse router::get_route, but it's better to start from scratch
because get_route includes a lot of payment-specific computations that impact
performance.

Author: valentinewallace

lightning/src/routing/mod.rs

@@ -10,6 +10,7 @@
 //! Structs and impls for receiving messages about the network and storing the topology live here.
 
 pub mod gossip;
+pub mod onion_message;
 pub mod router;
 pub mod scoring;
 #[cfg(test)]

lightning/src/routing/onion_message.rs

@@ -0,0 +1,167 @@
+use bitcoin::secp256k1::{self, PublicKey};
+
+use routing::gossip::{NetworkGraph, NodeId};
+use util::logger::Logger;
+
+use alloc::collections::BinaryHeap;
+use core::cmp;
+use core::ops::Deref;
+use prelude::*;
+
+//! Onion message pathfinding lives here.
+
+///
+pub fn find_path<L: Deref, GL: Deref>(
+	our_node_pubkey: &PublicKey, destination: &PublicKey, network_graph: &NetworkGraph<GL>, first_hops: Option<&[&PublicKey]>, logger: L
+) -> Result<Vec<PublicKey>, Error> where L::Target: Logger, GL::Target: Logger
+{
+	log_trace!(logger, "Searching for an onion message path from origin {} to destination {} and {} first hops {}overriding the network graph", our_node_pubkey, destination, first_hops.map(|hops| hops.len()).unwrap_or(0), if first_hops.is_some() { "" } else { "not " });
+	let graph_lock = network_graph.read_only();
+	let network_channels = graph_lock.channels();
+	let network_nodes = graph_lock.nodes();
+	let our_node_id = NodeId::from_pubkey(our_node_pubkey);
+	let dest_node_id = NodeId::from_pubkey(destination);
+
+	// Add our start and first-hops to `frontier`.
+	let start = NodeId::from_pubkey(&our_node_pubkey);
+	let mut frontier = BinaryHeap::new();
+	frontier.push(PathBuildingHop { cost: 0, node_id: start, parent_node_id: start });
+	if let Some(first_hops) = first_hops {
+		for hop in first_hops {
+			let node_id = NodeId::from_pubkey(&hop);
+			frontier.push(PathBuildingHop { cost: 1, node_id, parent_node_id: start });
+		}
+	}
+
+	let mut visited = HashMap::new();
+	while !frontier.is_empty() {
+		let PathBuildingHop { cost, node_id, parent_node_id } = frontier.pop().unwrap();
+		if visited.contains_key(&node_id) { continue; }
+		visited.insert(node_id, parent_node_id);
+		if node_id == dest_node_id {
+			return Ok(reverse_path(visited, our_node_id, dest_node_id, logger)?)
+		}
+		if let Some(node_info) = network_nodes.get(&node_id) {
+			for scid in &node_info.channels {
+				if let Some(chan_info) = network_channels.get(&scid) {
+					if let Some((_, successor)) = chan_info.as_directed_from(&node_id) {
+						// We may push a given successor multiple times, but the heap should sort its best entry
+						// to the top. We do this because there is no way to adjust the priority of an existing
+						// entry in `BinaryHeap`.
+						frontier.push(PathBuildingHop {
+							cost: cost + 1,
+							node_id: *successor,
+							parent_node_id: node_id,
+						});
+					}
+				}
+			}
+		}
+	}
+
+	Err(Error::PathNotFound)
+}
+
+#[derive(Debug, PartialEq)]
+/// Errored running [`find_path`].
+pub enum Error {
+	/// No path exists to the destination.
+	PathNotFound,
+	/// We failed to convert this node id into a [`PublicKey`].
+	InvalidNodeId(secp256k1::Error),
+}
+
+#[derive(Eq, PartialEq)]
+struct PathBuildingHop {
+	cost: u64,
+	node_id: NodeId,
+	parent_node_id: NodeId,
+}
+
+impl PartialOrd for PathBuildingHop {
+	fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> {
+		// We need a min-heap, whereas `BinaryHeap`s are a max-heap, so compare the costs in reverse.
+		other.cost.partial_cmp(&self.cost)
+	}
+}
+
+impl Ord for PathBuildingHop {
+	fn cmp(&self, other: &Self) -> cmp::Ordering {
+		self.partial_cmp(other).unwrap()
+	}
+}
+
+fn reverse_path<L: Deref>(
+	parents: HashMap<NodeId, NodeId>, our_node_id: NodeId, destination: NodeId, logger: L
+)-> Result<Vec<PublicKey>, Error> where L::Target: Logger
+{
+	let mut path = Vec::new();
+	let mut curr = destination;
+	loop {
+		match PublicKey::from_slice(curr.as_slice()) {
+			Ok(pk) => path.push(pk),
+			Err(e) => return Err(Error::InvalidNodeId(e))
+		}
+		match parents.get(&curr) {
+			None => return Err(Error::PathNotFound),
+			Some(parent) => {
+				if *parent == our_node_id { break; }
+				curr = *parent;
+			}
+		}
+	}
+
+	path.reverse();
+	log_info!(logger, "Got route to {:?}: {:?}", destination, path);
+	Ok(path)
+}
+
+#[cfg(test)]
+mod tests {
+	use routing::test_utils;
+
+	use sync::Arc;
+
+	#[test]
+	fn one_hop() {
+		let (secp_ctx, network_graph, _, _, logger) = test_utils::build_graph();
+		let (_, our_id, _, node_pks) = test_utils::get_nodes(&secp_ctx);
+
+		let path = super::find_path(&our_id, &node_pks[0], &network_graph, None, Arc::clone(&logger)).unwrap();
+		assert_eq!(path.len(), 1);
+		assert!(path[0] == node_pks[0]);
+	}
+
+	#[test]
+	fn two_hops() {
+		let (secp_ctx, network_graph, _, _, logger) = test_utils::build_graph();
+		let (_, our_id, _, node_pks) = test_utils::get_nodes(&secp_ctx);
+
+		let path = super::find_path(&our_id, &node_pks[2], &network_graph, None, Arc::clone(&logger)).unwrap();
+		assert_eq!(path.len(), 2);
+		// See test_utils::build_graph ASCII graph, the first hop can be any of these
+		assert!(path[0] == node_pks[1] || path[0] == node_pks[7] || path[0] == node_pks[0]);
+		assert_eq!(path[1], node_pks[2]);
+	}
+
+	#[test]
+	fn three_hops() {
+		let (secp_ctx, network_graph, _, _, logger) = test_utils::build_graph();
+		let (_, our_id, _, node_pks) = test_utils::get_nodes(&secp_ctx);
+
+		let mut path = super::find_path(&our_id, &node_pks[5], &network_graph, None, Arc::clone(&logger)).unwrap();
+		assert_eq!(path.len(), 3);
+		assert!(path[0] == node_pks[1] || path[0] == node_pks[7] || path[0] == node_pks[0]);
+		path.remove(0);
+		assert_eq!(path, vec![node_pks[2], node_pks[5]]);
+	}
+
+	#[test]
+	fn long_path() {
+		let (secp_ctx, network_graph, _, _, logger) = test_utils::build_line_graph();
+		let (_, our_id, _, node_pks) = test_utils::get_nodes(&secp_ctx);
+
+		let path = super::find_path(&our_id, &node_pks[18], &network_graph, None, Arc::clone(&logger)).unwrap();
+		assert_eq!(path.len(), 19);
+	}
+}