Account for leftover fee budget when retrying PartialFailures

Author: tnull

lightning/src/ln/outbound_payment.rs

@@ -1075,8 +1075,14 @@ impl OutboundPayments {
 				payment_id,
 				failed_paths_retry: if pending_amt_unsent != 0 {
 					if let Some(payment_params) = &route.payment_params {
+						let mut payment_params = payment_params.clone();
+						// We calculate the leftover fee budget we're allowed to spend by
+						// subtracting the used fee from the total fee budget. 
+						payment_params.max_total_routing_fee_msat = payment_params
+							.max_total_routing_fee_msat.map(|m| m.saturating_sub(route.get_total_fees()));
+
 						Some(RouteParameters {
-							payment_params: payment_params.clone(),
+							payment_params,
 							final_value_msat: pending_amt_unsent,
 						})
 					} else { None }