Account for leftover fee budget when retrying PartialFailures

Author: tnull

lightning/src/ln/outbound_payment.rs

@@ -1057,14 +1057,19 @@ impl OutboundPayments {
 		let mut has_ok = false;
 		let mut has_err = false;
 		let mut pending_amt_unsent = 0;
+		let mut total_ok_fees_msat = 0;
 		for (res, path) in results.iter().zip(route.paths.iter()) {
-			if res.is_ok() { has_ok = true; }
+			if res.is_ok() {
+				has_ok = true;
+				total_ok_fees_msat += path.fee_msat();
+			}
 			if res.is_err() { has_err = true; }
 			if let &Err(APIError::MonitorUpdateInProgress) = res {
 				// MonitorUpdateInProgress is inherently unsafe to retry, so we call it a
 				// PartialFailure.
 				has_err = true;
 				has_ok = true;
+				total_ok_fees_msat += path.fee_msat();
 			} else if res.is_err() {
 				pending_amt_unsent += path.final_value_msat();
 			}
@@ -1075,8 +1080,14 @@ 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(total_ok_fees_msat));
+
 						Some(RouteParameters {
-							payment_params: payment_params.clone(),
+							payment_params,
 							final_value_msat: pending_amt_unsent,
 						})
 					} else { None }