Account for leftover fee budget when retrying `PartialFailure`s

diff --git a/lightning/src/ln/outbound_payment.rs b/lightning/src/ln/outbound_payment.rs
index 2f1e3b0dd0e3d4358f7b43b5e01d1706f3ec005f..e06ba8d853982ccbd1c3a0c79e2815586158ffcf 100644 (file)
--- a/lightning/src/ln/outbound_payment.rs
+++ b/lightning/src/ln/outbound_payment.rs
@@ -1322,14 +1322,19 @@ 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();
                        }
@@ -1339,12 +1344,15 @@ impl OutboundPayments {
                                results,
                                payment_id,
                                failed_paths_retry: if pending_amt_unsent != 0 {
-                                       if let Some(payment_params) = route.route_params.as_ref().map(|p| p.payment_params.clone()) {
-                                               Some(RouteParameters {
-                                                       payment_params,
-                                                       final_value_msat: pending_amt_unsent,
-                                                       max_total_routing_fee_msat: None,
-                                               })
+                                       if let Some(route_params) = &route.route_params {
+                                               let mut route_params = route_params.clone();
+                                               // We calculate the leftover fee budget we're allowed to spend by
+                                               // subtracting the used fee from the total fee budget.
+                                               route_params.max_total_routing_fee_msat = route_params
+                                                       .max_total_routing_fee_msat.map(|m| m.saturating_sub(total_ok_fees_msat));
+                                               route_params.final_value_msat = pending_amt_unsent;
+
+                                               Some(route_params)
                                        } else { None }
                                } else { None },
                        })