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.

Author: wpaulino

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));