Account for max path length payment parameter in router.

We now only limit the number of unblinded intermediate hops. Previously we
limited the total number of hops, including the final hop and any blinded hops.

Author: valentinewallace

lightning/src/routing/router.rs

@@ -1453,6 +1453,20 @@ impl<'a> CandidateRouteHop<'a> {
 			CandidateRouteHop::OneHopBlinded(_) => None,
 		}
 	}
+	/// Whether this candidate is an unblinded forwarding hop along the path, i.e. not the final
+	/// hop or part of a blinded path.
+	pub fn is_intermediate_hop(&self, payer_node_id: &NodeId, payee_node_id_opt: &Option<NodeId>) -> bool {
+		match self {
+			// Never count first hops because we can end up with a path of FirstHopCandidate ->
+			// BlindedPathCandidate, in which case there are 0 intermediate hops.
+			CandidateRouteHop::FirstHop(_) => false,
+			CandidateRouteHop::PublicHop(_) =>
+				&self.source() != payer_node_id || &self.target() != payee_node_id_opt,
+			CandidateRouteHop::PrivateHop(_) => &self.target() != payee_node_id_opt,
+			CandidateRouteHop::Blinded(_) => false,
+			CandidateRouteHop::OneHopBlinded(_) => false,
+		}
+	}
 }
 
 /// A unique(ish) identifier for a specific [`CandidateRouteHop`].
@@ -1860,6 +1874,7 @@ pub(crate) fn get_route<L: Deref, S: ScoreLookUp>(
 where L::Target: Logger {
 
 	let payment_params = &route_params.payment_params;
+	let max_intermediate_hops = core::cmp::min(payment_params.max_intermediate_hops, MAX_PATH_LENGTH_ESTIMATE);
 	let final_value_msat = route_params.final_value_msat;
 	// If we're routing to a blinded recipient, we won't have their node id. Therefore, keep the
 	// unblinded payee id as an option. We also need a non-optional "payee id" for path construction,
@@ -2155,8 +2170,10 @@ where L::Target: Logger {
 					// Verify the liquidity offered by this channel complies to the minimal contribution.
 					let contributes_sufficient_value = available_value_contribution_msat >= minimal_value_contribution_msat;
 					// Do not consider candidate hops that would exceed the maximum path length.
-					let path_length_to_node = $next_hops_path_length + 1;
-					let exceeds_max_path_length = path_length_to_node > MAX_PATH_LENGTH_ESTIMATE;
+					let intermediate_path_length_to_node = $next_hops_path_length
+						+ if $candidate.is_intermediate_hop(&our_node_id, &payee_node_id_opt) { 1 } else { 0 };
+
+					let exceeds_max_path_length = intermediate_path_length_to_node > max_intermediate_hops;
 
 					// Do not consider candidates that exceed the maximum total cltv expiry limit.
 					// In order to already account for some of the privacy enhancing random CLTV
@@ -2367,7 +2384,7 @@ where L::Target: Logger {
 										score: cmp::max(total_fee_msat, path_htlc_minimum_msat).saturating_add(path_penalty_msat),
 										total_cltv_delta: hop_total_cltv_delta,
 										value_contribution_msat,
-										path_length_to_node,
+										path_length_to_node: intermediate_path_length_to_node,
 									};
 									targets.push(new_graph_node);
 									old_entry.next_hops_fee_msat = $next_hops_fee_msat;
@@ -2610,9 +2627,8 @@ where L::Target: Logger {
 					};
 					let path_min = candidate.htlc_minimum_msat().saturating_add(
 						compute_fees_saturating(candidate.htlc_minimum_msat(), candidate.fees()));
-					add_entry!(&first_hop_candidate, blinded_path_fee,
-						path_contribution_msat, path_min, 0_u64, candidate.cltv_expiry_delta(),
-						candidate.blinded_path().map_or(1, |bp| bp.blinded_hops.len() as u8));
+					add_entry!(&first_hop_candidate, blinded_path_fee, path_contribution_msat, path_min,
+						0_u64, candidate.cltv_expiry_delta(), 0);
 				}
 			}
 		}