Account for existing input amounts throughout coin selection

We'd previously ignore the existing amount transactions were already
attempting to spend when deciding whether we should add more inputs
throughout coin selection. This would result in us attaching more inputs
than necessary to satisfy our target amount. In the case of HTLC
transactions, we'd burn the HTLC amount completely, since the pre-signed
transaction has zero fee (input amount == output amount).

Along the way, we also fix the slight overpayment in anchor
transactions. We now properly account for the fees the transaction
already paid for, simply by pretending the fees are part of the anchor
input amount.

Author: wpaulino

lightning/src/events/bump_transaction.rs

@@ -542,7 +542,7 @@ where
 	fn select_confirmed_utxos_internal(
 		&self, utxos: &[Utxo], claim_id: ClaimId, force_conflicting_utxo_spend: bool,
 		tolerate_high_network_feerates: bool, target_feerate_sat_per_1000_weight: u32,
-		preexisting_tx_weight: u64, target_amount_sat: u64,
+		preexisting_tx_weight: u64, input_amount_sat: u64, target_amount_sat: u64,
 	) -> Result<CoinSelection, ()> {
 		let mut locked_utxos = self.locked_utxos.lock().unwrap();
 		let mut eligible_utxos = utxos.iter().filter_map(|utxo| {
@@ -569,7 +569,7 @@ where
 		}).collect::<Vec<_>>();
 		eligible_utxos.sort_unstable_by_key(|(utxo, _)| utxo.output.value);
 
-		let mut selected_amount = 0;
+		let mut selected_amount = input_amount_sat;
 		let mut total_fees = fee_for_weight(target_feerate_sat_per_1000_weight, preexisting_tx_weight);
 		let mut selected_utxos = Vec::new();
 		for (utxo, fee_to_spend_utxo) in eligible_utxos {
@@ -632,13 +632,14 @@ where
 
 		let preexisting_tx_weight = 2 /* segwit marker & flag */ + total_input_weight +
 			((BASE_TX_SIZE + total_output_size) * WITNESS_SCALE_FACTOR as u64);
+		let input_amount_sat: u64 = must_spend.iter().map(|input| input.previous_utxo.value).sum();
 		let target_amount_sat = must_pay_to.iter().map(|output| output.value).sum();
 		let do_coin_selection = |force_conflicting_utxo_spend: bool, tolerate_high_network_feerates: bool| {
 			log_debug!(self.logger, "Attempting coin selection targeting {} sat/kW (force_conflicting_utxo_spend = {}, tolerate_high_network_feerates = {})",
 				target_feerate_sat_per_1000_weight, force_conflicting_utxo_spend, tolerate_high_network_feerates);
 			self.select_confirmed_utxos_internal(
 				&utxos, claim_id, force_conflicting_utxo_spend, tolerate_high_network_feerates,
-				target_feerate_sat_per_1000_weight, preexisting_tx_weight, target_amount_sat,
+				target_feerate_sat_per_1000_weight, preexisting_tx_weight, input_amount_sat, target_amount_sat,
 			)
 		};
 		do_coin_selection(false, false)
@@ -724,27 +725,20 @@ where
 		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,
-		);
-
-		log_debug!(self.logger, "Peforming coin selection for anchor transaction targeting {} sat/kW",
-			anchor_target_feerate_sat_per_1000_weight);
+		// account. We do so by pretending the commitment tranasction's fee and weight are part of
+		// the anchor input.
+		let mut anchor_utxo = anchor_descriptor.previous_utxo();
+		anchor_utxo.value += commitment_tx_fee_sat;
 		let must_spend = vec![Input {
 			outpoint: anchor_descriptor.outpoint,
-			previous_utxo: anchor_descriptor.previous_utxo(),
+			previous_utxo: anchor_utxo,
 			satisfaction_weight: commitment_tx.weight() as u64 + ANCHOR_INPUT_WITNESS_WEIGHT + EMPTY_SCRIPT_SIG_WEIGHT,
 		}];
+
+		log_debug!(self.logger, "Peforming coin selection for commitment package (commitment and anchor transaction) targeting {} sat/kW",
+			package_target_feerate_sat_per_1000_weight);
 		let coin_selection = self.utxo_source.select_confirmed_utxos(
-			claim_id, must_spend, &[], anchor_target_feerate_sat_per_1000_weight,
+			claim_id, must_spend, &[], package_target_feerate_sat_per_1000_weight,
 		)?;
 
 		let mut anchor_tx = Transaction {