f clarify that / 0 cant happen

Author: TheBlueMatt

lightning/src/routing/router.rs

@@ -2126,10 +2126,12 @@ impl<'a> PaymentPath<'a> {
 	/// can/will send over the path. This is also the heap score during our Dijkstra's walk.
 	fn get_cost_per_msat(&self) -> u128 {
 		let fee_cost = self.get_cost_msat();
-		if fee_cost == u64::MAX {
+		let value_msat = self.get_value_msat();
+		debug_assert!(value_msat > 0, "Paths should always send more than 0 msat");
+		if fee_cost == u64::MAX || value_msat == 0 {
 			u64::MAX.into()
 		} else {
-			((fee_cost as u128) << 64) / self.get_value_msat() as u128
+			((fee_cost as u128) << 64) / value_msat as u128
 		}
 	}
 
@@ -2800,8 +2802,6 @@ where L::Target: Logger {
 							*used_liquidity_msat
 						});
 
-					// 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
 						+ if $candidate.blinded_hint_idx().is_some() { 0 } else { 1 };
@@ -2813,6 +2813,8 @@ where L::Target: Logger {
 					let exceeds_cltv_delta_limit = hop_total_cltv_delta > max_total_cltv_expiry_delta as u32;
 
 					let value_contribution_msat = cmp::min(available_value_contribution_msat, $next_hops_value_contribution);
+					// Verify the liquidity offered by this channel complies to the minimal contribution.
+					let contributes_sufficient_value = value_contribution_msat >= minimal_value_contribution_msat;
 					// Includes paying fees for the use of the following channels.
 					let amount_to_transfer_over_msat: u64 = match value_contribution_msat.checked_add($next_hops_fee_msat) {
 						Some(result) => result,
@@ -3020,6 +3022,8 @@ where L::Target: Logger {
 									u128::MAX
 								};
 								let new_cost = if new_fee_cost != u64::MAX {
+									// value_contribution_msat is always >= 1, checked above via
+									// `contributes_sufficient_value`.
 									((new_fee_cost as u128) << 64) / value_contribution_msat as u128
 								} else {
 									u128::MAX