Sanity check fees on transactions produced by the bump event handler

We add a few debug assertions to ensure we don't overpay fees by 5% more
than expected.

diff --git a/lightning/src/events/bump_transaction.rs b/lightning/src/events/bump_transaction.rs
index ec16a16af4d574c0d7239f12e4e27b96c7818dea..35e9da60544ddc680535983767117e9faf732afd 100644 (file)
--- a/lightning/src/events/bump_transaction.rs
+++ b/lightning/src/events/bump_transaction.rs
@@ -734,6 +734,8 @@ where
                        previous_utxo: anchor_utxo,
                        satisfaction_weight: commitment_tx.weight() as u64 + ANCHOR_INPUT_WITNESS_WEIGHT + EMPTY_SCRIPT_SIG_WEIGHT,
                }];
+               #[cfg(debug_assertions)]
+               let must_spend_amount = must_spend.iter().map(|input| input.previous_utxo.value).sum::<u64>();
 
                log_debug!(self.logger, "Peforming coin selection for commitment package (commitment and anchor transaction) targeting {} sat/kW",
                        package_target_feerate_sat_per_1000_weight);
@@ -747,10 +749,13 @@ where
                        input: vec![anchor_descriptor.unsigned_tx_input()],
                        output: vec![],
                };
+
+               #[cfg(debug_assertions)]
+               let total_satisfaction_weight = ANCHOR_INPUT_WITNESS_WEIGHT + EMPTY_SCRIPT_SIG_WEIGHT +
+                       coin_selection.confirmed_utxos.iter().map(|utxo| utxo.satisfaction_weight).sum::<u64>();
                #[cfg(debug_assertions)]
-               let total_satisfaction_weight =
-                       coin_selection.confirmed_utxos.iter().map(|utxo| utxo.satisfaction_weight).sum::<u64>() +
-                               ANCHOR_INPUT_WITNESS_WEIGHT + EMPTY_SCRIPT_SIG_WEIGHT;
+               let total_input_amount = must_spend_amount +
+                       coin_selection.confirmed_utxos.iter().map(|utxo| utxo.output.value).sum::<u64>();
 
                self.process_coin_selection(&mut anchor_tx, coin_selection);
                let anchor_txid = anchor_tx.txid();
@@ -773,6 +778,16 @@ where
                        // never underestimate.
                        assert!(expected_signed_tx_weight >= signed_tx_weight &&
                                expected_signed_tx_weight - (expected_signed_tx_weight / 100) <= signed_tx_weight);
+
+                       let expected_package_fee = fee_for_weight(package_target_feerate_sat_per_1000_weight,
+                               signed_tx_weight + commitment_tx.weight() as u64);
+                       let package_fee = total_input_amount -
+                               anchor_tx.output.iter().map(|output| output.value).sum::<u64>();
+                       // Our fee should be within a 5% error margin of the expected fee based on the
+                       // feerate and transaction weight and we should never pay less than required.
+                       let fee_error_margin = expected_package_fee * 5 / 100;
+                       assert!(package_fee >= expected_package_fee &&
+                               package_fee - fee_error_margin <= expected_package_fee);
                }
 
                log_info!(self.logger, "Broadcasting anchor transaction {} to bump channel close with txid {}",
@@ -812,16 +827,24 @@ where
 
                log_debug!(self.logger, "Peforming coin selection for HTLC transaction targeting {} sat/kW",
                        target_feerate_sat_per_1000_weight);
+
                #[cfg(debug_assertions)]
                let must_spend_satisfaction_weight =
                        must_spend.iter().map(|input| input.satisfaction_weight).sum::<u64>();
+               #[cfg(debug_assertions)]
+               let must_spend_amount = must_spend.iter().map(|input| input.previous_utxo.value).sum::<u64>();
+
                let coin_selection = self.utxo_source.select_confirmed_utxos(
                        claim_id, must_spend, &htlc_tx.output, target_feerate_sat_per_1000_weight,
                )?;
+
+               #[cfg(debug_assertions)]
+               let total_satisfaction_weight = must_spend_satisfaction_weight +
+                       coin_selection.confirmed_utxos.iter().map(|utxo| utxo.satisfaction_weight).sum::<u64>();
                #[cfg(debug_assertions)]
-               let total_satisfaction_weight =
-                       coin_selection.confirmed_utxos.iter().map(|utxo| utxo.satisfaction_weight).sum::<u64>() +
-                               must_spend_satisfaction_weight;
+               let total_input_amount = must_spend_amount +
+                       coin_selection.confirmed_utxos.iter().map(|utxo| utxo.output.value).sum::<u64>();
+
                self.process_coin_selection(&mut htlc_tx, coin_selection);
 
                #[cfg(debug_assertions)]
@@ -846,6 +869,15 @@ where
                        // never underestimate.
                        assert!(expected_signed_tx_weight >= signed_tx_weight &&
                                expected_signed_tx_weight - (expected_signed_tx_weight / 100) <= signed_tx_weight);
+
+                       let expected_signed_tx_fee = fee_for_weight(target_feerate_sat_per_1000_weight, signed_tx_weight);
+                       let signed_tx_fee = total_input_amount -
+                               htlc_tx.output.iter().map(|output| output.value).sum::<u64>();
+                       // Our fee should be within a 5% error margin of the expected fee based on the
+                       // feerate and transaction weight and we should never pay less than required.
+                       let fee_error_margin = expected_signed_tx_fee * 5 / 100;
+                       assert!(signed_tx_fee >= expected_signed_tx_fee &&
+                               signed_tx_fee - fee_error_margin <= expected_signed_tx_fee);
                }
 
                log_info!(self.logger, "Broadcasting {}", log_tx!(htlc_tx));