Consider maximum path length during path finding.

Author: tnull

lightning/src/routing/router.rs

@@ -829,6 +829,20 @@ where L::Target: Logger {
 	// - when we want to stop looking for new paths.
 	let mut already_collected_value_msat = 0;
 
+	// We only consider paths shorter than our maximum length estimate.
+	// In the legacy onion format, the maximum number of hops used to be a fixed value of 20.
+	// However, in the TLV onion format, there is no fixed maximum length, but the `hop_payloads`
+	// field is always 1300 bytes. As the `tlv_payload` for each hop may vary in length, we have to
+	// estimate how many hops the route may have so that it actually fits the `hop_payloads` field.
+	//
+	// We estimate 3+32 (payload length and HMAC) + 1+8 (amt_to_forward) + 1+4 (outgoing_cltv_value) +
+	// 1+8 (short_channel_id) = 58 bytes for each intermediate hop and 3+32
+	// (payload length and HMAC) + 1+8 (amt_to_forward) + 1+4 (outgoing_cltv_value) + 1+32+8
+	// (payment_secret and total_msat) = 90 bytes for the final hop.
+	// Since the length of the potentially included `payment_metadata` is unkown to us, we round
+	// down from (1300-90) / 58 = 20.86... just to arrive again at a conservative estimate of 20.
+	const MAX_PATH_LENGTH_ESTIMATE: usize = 20;
+
 	for (_, channels) in first_hop_targets.iter_mut() {
 		// Sort the first_hops channels to the same node(s) in priority order of which channel we'd
 		// most like to use.
@@ -1360,6 +1374,12 @@ where L::Target: Logger {
 						break 'path_walk;
 					}
 
+					// If the path reaches the estimated maximum feasible length, skip this path
+					// and continue path construction.
+					if ordered_hops.len() == MAX_PATH_LENGTH_ESTIMATE {
+						continue 'path_construction;
+					}
+
 					new_entry = match dist.remove(&ordered_hops.last().unwrap().0.node_id) {
 						Some(payment_hop) => payment_hop,
 						// We can't arrive at None because, if we ever add an entry to targets,