For the candidate outbound htlc, sum weights, then sum fees

Previously, we calculated the fee of the commitment transaction with n
htlcs, and the fee due to the candidate htlc, rounded the two fees to
the lower satoshi, and then summed the fees. This is not equal to how
fees of commitment transactions are calculated, which is to add up the
total weight of the (n+1) htlc commitment transaction, convert to fee,
then round to the lower satoshi.

This commit corrects this delta by running the full fee calculation
twice, once for the n htlc, and once for the (n+1) htlc counterparty
commitment transactions.

Author: tankyleo

lightning/src/ln/chan_utils.rs

@@ -196,15 +196,16 @@ pub(crate) fn commit_tx_fee_sat(feerate_per_kw: u32, num_htlcs: usize, channel_t
 		/ 1000
 }
 
-pub(crate) fn per_outbound_htlc_counterparty_commit_tx_fee_msat(feerate_per_kw: u32, channel_type_features: &ChannelTypeFeatures) -> u64 {
-	// Note that we need to divide before multiplying to round properly,
-	// since the lowest denomination of bitcoin on-chain is the satoshi.
-	let commitment_tx_fee = COMMITMENT_TX_WEIGHT_PER_HTLC * feerate_per_kw as u64 / 1000 * 1000;
-	if channel_type_features.supports_anchors_zero_fee_htlc_tx() {
-		commitment_tx_fee + htlc_success_tx_weight(channel_type_features) * feerate_per_kw as u64 / 1000
+pub(crate) fn commit_and_htlc_tx_fees_sat(feerate_per_kw: u32, num_accepted_htlcs: usize, num_offered_htlcs: usize, channel_type_features: &ChannelTypeFeatures) -> u64 {
+	let num_htlcs = num_accepted_htlcs + num_offered_htlcs;
+	let commit_tx_fees_sat = commit_tx_fee_sat(feerate_per_kw, num_htlcs, channel_type_features);
+	let htlc_tx_fees_sat = if !channel_type_features.supports_anchors_zero_fee_htlc_tx() {
+		num_accepted_htlcs as u64 * htlc_success_tx_weight(channel_type_features) * feerate_per_kw as u64 / 1000
+	  + num_offered_htlcs as u64 * htlc_timeout_tx_weight(channel_type_features) * feerate_per_kw as u64 / 1000
 	} else {
-		commitment_tx_fee
-	}
+		0
+	};
+	commit_tx_fees_sat + htlc_tx_fees_sat
 }
 
 // Various functions for key derivation and transaction creation for use within channels. Primarily

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,
 };
 use crate::ln::chan_utils;
 use crate::ln::onion_utils::HTLCFailReason;
@@ -834,6 +834,10 @@ struct HTLCStats {
 	pending_inbound_htlcs_value_msat: u64,
 	pending_outbound_htlcs_value_msat: u64,
 	on_counterparty_tx_dust_exposure_msat: u64,
+	// If the counterparty sets a feerate on the channel in excess of our dust_exposure_limiting_feerate,
+	// this will be set to the dust exposure that would result from us adding an additional nondust outbound
+	// htlc on the counterparty's commitment transaction.
+	extra_nondust_htlc_on_counterparty_tx_dust_exposure_msat: Option<u64>,
 	on_holder_tx_dust_exposure_msat: u64,
 	outbound_holding_cell_msat: u64,
 	on_holder_tx_outbound_holding_cell_htlcs_count: u32, // dust HTLCs *non*-included
@@ -3705,27 +3709,21 @@ impl<SP: Deref> ChannelContext<SP> where SP::Target: SignerProvider {
 			.or(self.pending_update_fee.map(|(fee, _)| fee))
 			.unwrap_or(self.feerate_per_kw)
 			.checked_sub(dust_exposure_limiting_feerate);
-		if let Some(excess_feerate) = excess_feerate_opt {
-			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;
-			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)
-					* excess_feerate as u64 / 1000;
-				on_counterparty_tx_dust_exposure_msat +=
-					on_counterparty_tx_offered_nondust_htlcs as u64 * htlc_timeout_tx_weight(&self.channel_type)
-					* excess_feerate as u64 / 1000;
-			}
-		}
+		let extra_nondust_htlc_on_counterparty_tx_dust_exposure_msat = excess_feerate_opt.map(|excess_feerate| {
+			let extra_htlc_dust_exposure = on_counterparty_tx_dust_exposure_msat
+				+ chan_utils::commit_and_htlc_tx_fees_sat(excess_feerate, on_counterparty_tx_accepted_nondust_htlcs + 1, on_counterparty_tx_offered_nondust_htlcs, &self.channel_type) * 1000;
+			on_counterparty_tx_dust_exposure_msat
+				+= chan_utils::commit_and_htlc_tx_fees_sat(excess_feerate, on_counterparty_tx_accepted_nondust_htlcs, on_counterparty_tx_offered_nondust_htlcs, &self.channel_type) * 1000;
+			extra_htlc_dust_exposure
+		});
 
 		HTLCStats {
 			pending_inbound_htlcs: self.pending_inbound_htlcs.len(),
 			pending_outbound_htlcs,
 			pending_inbound_htlcs_value_msat,
 			pending_outbound_htlcs_value_msat,
 			on_counterparty_tx_dust_exposure_msat,
+			extra_nondust_htlc_on_counterparty_tx_dust_exposure_msat,
 			on_holder_tx_dust_exposure_msat,
 			outbound_holding_cell_msat,
 			on_holder_tx_outbound_holding_cell_htlcs_count,
@@ -3930,13 +3928,8 @@ impl<SP: Deref> ChannelContext<SP> where SP::Target: SignerProvider {
 			 context.holder_dust_limit_satoshis       + dust_buffer_feerate * htlc_timeout_tx_weight(context.get_channel_type()) / 1000)
 		};
 
-		let excess_feerate_opt = self.feerate_per_kw.checked_sub(dust_exposure_limiting_feerate);
-		if let Some(excess_feerate) = excess_feerate_opt {
-			let htlc_dust_exposure_msat =
-				per_outbound_htlc_counterparty_commit_tx_fee_msat(excess_feerate, &context.channel_type);
-			let nondust_htlc_counterparty_tx_dust_exposure =
-				htlc_stats.on_counterparty_tx_dust_exposure_msat.saturating_add(htlc_dust_exposure_msat);
-			if nondust_htlc_counterparty_tx_dust_exposure > max_dust_htlc_exposure_msat {
+		if let Some(extra_htlc_dust_exposure) = htlc_stats.extra_nondust_htlc_on_counterparty_tx_dust_exposure_msat {
+			if extra_htlc_dust_exposure > max_dust_htlc_exposure_msat {
 				// If adding an extra HTLC would put us over the dust limit in total fees, we cannot
 				// send any non-dust HTLCs.
 				available_capacity_msat = cmp::min(available_capacity_msat, htlc_success_dust_limit * 1000);