Consider existing commitment transaction feerate when bumping

[rust-lightning] / lightning / src / events / bump_transaction.rs

diff --git a/lightning/src/events/bump_transaction.rs b/lightning/src/events/bump_transaction.rs
index ffd9b22834de567ba1833671af77949abfc19769..f9d956ec29750079587dc979cf8374fea7937bd9 100644 (file)
--- a/lightning/src/events/bump_transaction.rs
+++ b/lightning/src/events/bump_transaction.rs
@@ -14,7 +14,7 @@
 use alloc::collections::BTreeMap;
 use core::ops::Deref;
 
-use crate::chain::chaininterface::{BroadcasterInterface, compute_feerate_sat_per_1000_weight, fee_for_weight};
+use crate::chain::chaininterface::{BroadcasterInterface, compute_feerate_sat_per_1000_weight, fee_for_weight, FEERATE_FLOOR_SATS_PER_KW};
 use crate::chain::ClaimId;
 use crate::io_extras::sink;
 use crate::ln::channel::ANCHOR_OUTPUT_VALUE_SATOSHI;
@@ -700,8 +700,21 @@ where
                &self, claim_id: ClaimId, package_target_feerate_sat_per_1000_weight: u32,
                commitment_tx: &Transaction, commitment_tx_fee_sat: u64, anchor_descriptor: &AnchorDescriptor,
        ) -> Result<(), ()> {
+               // Our commitment transaction already has fees allocated to it, so we should take them into
+               // account. We compute its feerate and subtract it from the package target, using the result
+               // as the target feerate for our anchor transaction. Unfortunately, this results in users
+               // overpaying by a small margin since we don't yet know the anchor transaction size, and
+               // avoiding the small overpayment only makes our API even more complex.
+               let commitment_tx_sat_per_1000_weight: u32 = compute_feerate_sat_per_1000_weight(
+                       commitment_tx_fee_sat, commitment_tx.weight() as u64,
+               );
+               let anchor_target_feerate_sat_per_1000_weight = core::cmp::max(
+                       package_target_feerate_sat_per_1000_weight - commitment_tx_sat_per_1000_weight,
+                       FEERATE_FLOOR_SATS_PER_KW,
+               );
+
                let mut anchor_tx = self.build_anchor_tx(
-                       claim_id, package_target_feerate_sat_per_1000_weight, commitment_tx, anchor_descriptor,
+                       claim_id, anchor_target_feerate_sat_per_1000_weight, commitment_tx, anchor_descriptor,
                )?;
                debug_assert_eq!(anchor_tx.output.len(), 1);