f: Do not round HTLCStats::on_counterparty_tx_dust_exposure

All the other dust calculations in `get_pending_htlc_stats` are done
with msat precision, so use msat precision when calculating the excess
fees of the counterparty transaction.

Author: tankyleo

lightning/src/ln/chan_utils.rs

@@ -189,13 +189,23 @@ pub(crate) fn commitment_tx_base_weight(channel_type_features: &ChannelTypeFeatu
 
 /// Get the fee cost of a commitment tx with a given number of HTLC outputs.
 /// Note that num_htlcs should not include dust HTLCs.
+/// If no rounding and msat precision is required, use `commit_tx_fee_msat`.
 pub(crate) fn commit_tx_fee_sat(feerate_per_kw: u32, num_htlcs: usize, channel_type_features: &ChannelTypeFeatures) -> u64 {
 	feerate_per_kw as u64 *
 		(commitment_tx_base_weight(channel_type_features) +
 			num_htlcs as u64 * COMMITMENT_TX_WEIGHT_PER_HTLC)
 		/ 1000
 }
 
+/// Get the fee cost of a commitment tx with a given number of HTLC outputs.
+/// Note that num_htlcs should not include dust HTLCs.
+/// This version of the function does not round, and returns the number in msat precision. See also `commit_tx_fee_sat`.
+pub(crate) fn commit_tx_fee_msat(feerate_per_kw: u32, num_htlcs: usize, channel_type_features: &ChannelTypeFeatures) -> u64 {
+	feerate_per_kw as u64 *
+		(commitment_tx_base_weight(channel_type_features) +
+			num_htlcs as u64 * COMMITMENT_TX_WEIGHT_PER_HTLC)
+}
+
 pub(crate) fn per_outbound_htlc_counterparty_commit_tx_fee_msat(feerate_per_kw: u32, channel_type_features: &ChannelTypeFeatures) -> u64 {
 	// We avoid rounding here to make the returned value consistent with the slope (commit_tx_fee_sat/num_htlcs)
 	let commitment_tx_fee_msat = COMMITMENT_TX_WEIGHT_PER_HTLC * feerate_per_kw as u64;

lightning/src/ln/channel.rs

@@ -46,7 +46,7 @@ use crate::ln::chan_utils::{
 	HolderCommitmentTransaction, ChannelTransactionParameters,
 	CounterpartyChannelTransactionParameters, MAX_HTLCS,
 	get_commitment_transaction_number_obscure_factor,
-	ClosingTransaction, commit_tx_fee_sat, per_outbound_htlc_counterparty_commit_tx_fee_msat,
+	ClosingTransaction, commit_tx_fee_sat, per_outbound_htlc_counterparty_commit_tx_fee_msat, commit_tx_fee_msat,
 };
 use crate::ln::chan_utils;
 use crate::ln::onion_utils::HTLCFailReason;
@@ -3665,7 +3665,7 @@ impl<SP: Deref> ChannelContext<SP> where SP::Target: SignerProvider {
 			let on_counterparty_tx_nondust_htlcs =
 				on_counterparty_tx_accepted_nondust_htlcs + on_counterparty_tx_offered_nondust_htlcs;
 			on_counterparty_tx_dust_exposure_msat +=
-				commit_tx_fee_sat(excess_feerate, on_counterparty_tx_nondust_htlcs, &self.channel_type) * 1000;
+				commit_tx_fee_msat(excess_feerate, on_counterparty_tx_nondust_htlcs, &self.channel_type);
 			if !self.channel_type.supports_anchors_zero_fee_htlc_tx() {
 				on_counterparty_tx_dust_exposure_msat +=
 					on_counterparty_tx_accepted_nondust_htlcs as u64 * htlc_success_tx_weight(&self.channel_type)

lightning/src/ln/functional_tests.rs

@@ -10300,7 +10300,7 @@ fn do_test_max_dust_htlc_exposure(dust_outbound_balance: bool, exposure_breach_e
 	} else { 0 };
 	let initial_feerate = if apply_excess_fee { 253 * 2 } else { 253 };
 	let expected_dust_buffer_feerate = initial_feerate + 2530;
-	let mut commitment_tx_cost = commit_tx_fee_msat(initial_feerate - 253, nondust_htlc_count_in_limit, &ChannelTypeFeatures::empty());
+	let mut commitment_tx_cost = chan_utils::commit_tx_fee_msat(initial_feerate - 253, nondust_htlc_count_in_limit as usize, &ChannelTypeFeatures::empty());
 	commitment_tx_cost +=
 		if on_holder_tx {
 			htlc_success_tx_weight(&ChannelTypeFeatures::empty())
@@ -10646,14 +10646,14 @@ fn do_test_nondust_htlc_fees_dust_exposure_delta(features: ChannelTypeFeatures)
 	}
 
 	// Set `expected_dust_exposure_msat` to match the calculation in `FundedChannel::can_accept_incoming_htlc`
-	// only_static_remote_key: 500_000 + 22 * (724 + 172) / 1000 * 1000 + 22 * 663 = 533_586
-	// anchors_zero_htlc_fee: 500_000 + 22 * (1_124 + 172) / 1000 * 1000 = 528_000
-	let mut expected_dust_exposure_msat = BASE_DUST_EXPOSURE_MSAT + EXCESS_FEERATE * (commitment_tx_base_weight(&features) + COMMITMENT_TX_WEIGHT_PER_HTLC) / 1000 * 1000;
+	// only_static_remote_key: 500_000 + 22 * (724 + 172) + 22 * 663 = 534_298
+	// anchors_zero_htlc_fee: 500_000 + 22 * (1_124 + 172) = 528_512
+	let mut expected_dust_exposure_msat = BASE_DUST_EXPOSURE_MSAT + EXCESS_FEERATE * (commitment_tx_base_weight(&features) + COMMITMENT_TX_WEIGHT_PER_HTLC);
 	if features == ChannelTypeFeatures::only_static_remote_key() {
 		expected_dust_exposure_msat += EXCESS_FEERATE * htlc_timeout_tx_weight(&features);
-		assert_eq!(expected_dust_exposure_msat, 533_586);
+		assert_eq!(expected_dust_exposure_msat, 534_298);
 	} else {
-		assert_eq!(expected_dust_exposure_msat, 528_000);
+		assert_eq!(expected_dust_exposure_msat, 528_512);
 	}
 
 	let mut default_config = test_default_channel_config();
@@ -10749,15 +10749,13 @@ fn do_test_nondust_htlc_fees_dust_exposure_delta(features: ChannelTypeFeatures)
 	assert_eq!(nodes[0].node.list_channels()[0].pending_outbound_htlcs.len(), DUST_HTLC_COUNT);
 	assert_eq!(nodes[1].node.list_channels()[0].pending_inbound_htlcs.len(), DUST_HTLC_COUNT);
 
-	// Set `expected_dust_exposure_msat` to match the calculation in `ChannelContext::get_available_balances`
-	// only_static_remote_key: 500_000 + 22 * 724 / 1000 * 1000 + 22 * 172 + 22 * 703 = 534_250
-	// anchors_zero_htlc_fee: 500_000 + 22 * 1124 / 1000 * 1000 + 22 * 172 = 527_784
-	let mut expected_dust_exposure_msat = BASE_DUST_EXPOSURE_MSAT + EXCESS_FEERATE * commitment_tx_base_weight(&features) / 1000 * 1000 + EXCESS_FEERATE * COMMITMENT_TX_WEIGHT_PER_HTLC;
+	// The `expected_dust_exposure_msat` for the outbound htlc changes in the non-anchor case, as the htlc success and timeout transactions have different weights
+	// only_static_remote_key: 500_000 + 22 * 724 + 22 * 172 + 22 * 703 = 535_178
 	if features == ChannelTypeFeatures::only_static_remote_key() {
-		expected_dust_exposure_msat += EXCESS_FEERATE * htlc_success_tx_weight(&features);
-		assert_eq!(expected_dust_exposure_msat, 534_250);
+		expected_dust_exposure_msat = BASE_DUST_EXPOSURE_MSAT + EXCESS_FEERATE * commitment_tx_base_weight(&features) + EXCESS_FEERATE * COMMITMENT_TX_WEIGHT_PER_HTLC + EXCESS_FEERATE * htlc_success_tx_weight(&features);
+		assert_eq!(expected_dust_exposure_msat, 535_178);
 	} else {
-		assert_eq!(expected_dust_exposure_msat, 527_784);
+		assert_eq!(expected_dust_exposure_msat, 528_512);
 	}
 
 	// Set node 1's max dust htlc exposure to 1msat below `expected_dust_exposure_msat`