Use TxBuilder::get_next_commitment_stats to get AvailableBalances

Also move things around to make the move in the next commit as
straightforward as possible.

We take the conservative route here and include all pending HTLCs,
including those in the holding cell, no matter their state.

Author: tankyleo

lightning/src/ln/channel.rs

@@ -5689,57 +5689,71 @@ where
 	where
 		F::Target: FeeEstimator,
 	{
-		let context = &self;
+		use crate::sign::tx_builder::get_dust_buffer_feerate;
+
 		let holder_channel_constraints = self.get_holder_channel_constraints(funding);
 		let counterparty_channel_constraints = self.get_counterparty_channel_constraints(funding);
 		// Note that we have to handle overflow due to the case mentioned in the docs in general
 		// here.
 
+		let pending_outbound_htlcs = self.pending_outbound_htlcs.iter().map(|htlc| HTLCAmountDirection { amount_msat: htlc.amount_msat, outbound: true });
+		let pending_inbound_htlcs = self.pending_inbound_htlcs.iter().map(|htlc| HTLCAmountDirection { amount_msat: htlc.amount_msat, outbound: false });
+		let holding_cell_htlcs = self.holding_cell_htlc_updates.iter().filter_map(|htlc| {
+			if let &HTLCUpdateAwaitingACK::AddHTLC { amount_msat, .. } = htlc {
+				Some(HTLCAmountDirection { outbound: true, amount_msat })
+			} else {
+				None
+			}
+		});
+
+		let mut pending_htlcs: Vec<HTLCAmountDirection> = Vec::with_capacity(self.pending_outbound_htlcs.len() + self.pending_inbound_htlcs.len() + self.holding_cell_htlc_updates.len());
+		pending_htlcs.extend(pending_outbound_htlcs.chain(pending_inbound_htlcs).chain(holding_cell_htlcs));
+		let pending_htlcs = &pending_htlcs;
+
 		let dust_exposure_limiting_feerate = self.get_dust_exposure_limiting_feerate(
 			&fee_estimator, funding.get_channel_type(),
 		);
-		let htlc_stats = context.get_pending_htlc_stats(funding, None, dust_exposure_limiting_feerate);
+		let max_dust_htlc_exposure_msat = self.get_max_dust_htlc_exposure_msat(dust_exposure_limiting_feerate);
 
-		// Subtract any non-HTLC outputs from the local and remote balances
-		let (local_balance_before_fee_msat, remote_balance_before_fee_msat) = SpecTxBuilder {}.subtract_non_htlc_outputs(
-			funding.is_outbound(),
-			funding.value_to_self_msat.saturating_sub(htlc_stats.pending_outbound_htlcs_value_msat),
-			(funding.get_value_satoshis() * 1000).checked_sub(funding.value_to_self_msat).unwrap().saturating_sub(htlc_stats.pending_inbound_htlcs_value_msat),
-			funding.get_channel_type(),
-		);
+		let is_outbound_from_holder = funding.is_outbound();
+		let channel_value_satoshis = funding.get_value_satoshis();
+		let value_to_holder_msat = funding.get_value_to_self_msat();
+		let feerate_per_kw = self.feerate_per_kw;
+		let channel_type = funding.get_channel_type();
+
+		let fee_spike_buffer_htlc = if channel_type.supports_anchor_zero_fee_commitments() {
+			0
+		} else {
+			1
+		};
 
-		let outbound_capacity_msat = local_balance_before_fee_msat
-				.saturating_sub(
-					holder_channel_constraints.channel_reserve_satoshis * 1000);
+		let local_stats_max_fee = SpecTxBuilder {}.get_next_commitment_stats(true, is_outbound_from_holder, channel_value_satoshis, value_to_holder_msat, pending_htlcs, fee_spike_buffer_htlc + 1, feerate_per_kw, dust_exposure_limiting_feerate, holder_channel_constraints.dust_limit_satoshis, channel_type).unwrap();
+		let local_stats_min_fee = SpecTxBuilder {}.get_next_commitment_stats(true, is_outbound_from_holder, channel_value_satoshis, value_to_holder_msat, pending_htlcs, fee_spike_buffer_htlc, feerate_per_kw, dust_exposure_limiting_feerate, holder_channel_constraints.dust_limit_satoshis, channel_type).unwrap();
+		let remote_stats = SpecTxBuilder {}.get_next_commitment_stats(false, is_outbound_from_holder, channel_value_satoshis, value_to_holder_msat, pending_htlcs, 1, feerate_per_kw, dust_exposure_limiting_feerate, counterparty_channel_constraints.dust_limit_satoshis, channel_type).unwrap();
+
+		let outbound_capacity_msat = local_stats_max_fee.holder_balance_before_fee_msat.saturating_sub(holder_channel_constraints.channel_reserve_satoshis * 1000);
 
 		let mut available_capacity_msat = outbound_capacity_msat;
 		let (real_htlc_success_tx_fee_sat, real_htlc_timeout_tx_fee_sat) = second_stage_tx_fees_sat(
-				funding.get_channel_type(), context.feerate_per_kw,
+			channel_type, feerate_per_kw
 		);
 
-		if funding.is_outbound() {
+		if is_outbound_from_holder {
 			// We should mind channel commit tx fee when computing how much of the available capacity
 			// can be used in the next htlc. Mirrors the logic in send_htlc.
 			//
 			// The fee depends on whether the amount we will be sending is above dust or not,
 			// and the answer will in turn change the amount itself — making it a circular
 			// dependency.
 			// This complicates the computation around dust-values, up to the one-htlc-value.
-			let fee_spike_buffer_htlc = if funding.get_channel_type().supports_anchor_zero_fee_commitments() {
-				None
-			} else {
-				Some(())
-			};
 
 			let real_dust_limit_timeout_sat = real_htlc_timeout_tx_fee_sat + holder_channel_constraints.dust_limit_satoshis;
-			let htlc_above_dust = HTLCCandidate::new(real_dust_limit_timeout_sat * 1000, HTLCInitiator::LocalOffered);
-			let mut max_reserved_commit_tx_fee_msat = context.next_local_commit_tx_fee_msat(&funding, htlc_above_dust, fee_spike_buffer_htlc);
-			let htlc_dust = HTLCCandidate::new(real_dust_limit_timeout_sat * 1000 - 1, HTLCInitiator::LocalOffered);
-			let mut min_reserved_commit_tx_fee_msat = context.next_local_commit_tx_fee_msat(&funding, htlc_dust, fee_spike_buffer_htlc);
+			let mut max_reserved_commit_tx_fee_msat = local_stats_max_fee.commit_tx_fee_sat * 1000;
+			let mut min_reserved_commit_tx_fee_msat = local_stats_min_fee.commit_tx_fee_sat * 1000;
 
-			if !funding.get_channel_type().supports_anchors_zero_fee_htlc_tx() {
-				max_reserved_commit_tx_fee_msat *= FEE_SPIKE_BUFFER_FEE_INCREASE_MULTIPLE;
-				min_reserved_commit_tx_fee_msat *= FEE_SPIKE_BUFFER_FEE_INCREASE_MULTIPLE;
+			if !channel_type.supports_anchors_zero_fee_htlc_tx() {
+				max_reserved_commit_tx_fee_msat *= crate::ln::channel::FEE_SPIKE_BUFFER_FEE_INCREASE_MULTIPLE;
+				min_reserved_commit_tx_fee_msat *= crate::ln::channel::FEE_SPIKE_BUFFER_FEE_INCREASE_MULTIPLE;
 			}
 
 			// We will first subtract the fee as if we were above-dust. Then, if the resulting
@@ -5756,11 +5770,10 @@ where
 			// If the channel is inbound (i.e. counterparty pays the fee), we need to make sure
 			// sending a new HTLC won't reduce their balance below our reserve threshold.
 			let real_dust_limit_success_sat = real_htlc_success_tx_fee_sat + counterparty_channel_constraints.dust_limit_satoshis;
-			let htlc_above_dust = HTLCCandidate::new(real_dust_limit_success_sat * 1000, HTLCInitiator::LocalOffered);
-			let max_reserved_commit_tx_fee_msat = context.next_remote_commit_tx_fee_msat(funding, Some(htlc_above_dust), None);
+			let max_reserved_commit_tx_fee_msat = remote_stats.commit_tx_fee_sat * 1000;
 
 			let holder_selected_chan_reserve_msat = counterparty_channel_constraints.channel_reserve_satoshis * 1000;
-			if remote_balance_before_fee_msat < max_reserved_commit_tx_fee_msat + holder_selected_chan_reserve_msat {
+			if remote_stats.counterparty_balance_before_fee_msat < max_reserved_commit_tx_fee_msat + holder_selected_chan_reserve_msat {
 				// If another HTLC's fee would reduce the remote's balance below the reserve limit
 				// we've selected for them, we can only send dust HTLCs.
 				available_capacity_msat = cmp::min(available_capacity_msat, real_dust_limit_success_sat * 1000 - 1);
@@ -5775,35 +5788,32 @@ where
 		// send above the dust limit (as the router can always overpay to meet the dust limit).
 		let mut remaining_msat_below_dust_exposure_limit = None;
 		let mut dust_exposure_dust_limit_msat = 0;
-		let max_dust_htlc_exposure_msat = context.get_max_dust_htlc_exposure_msat(dust_exposure_limiting_feerate);
 
-		let dust_buffer_feerate = self.get_dust_buffer_feerate(None);
+		let dust_buffer_feerate = get_dust_buffer_feerate(feerate_per_kw);
 		let (buffer_htlc_success_tx_fee_sat, buffer_htlc_timeout_tx_fee_sat) = second_stage_tx_fees_sat(
-			funding.get_channel_type(), dust_buffer_feerate,
+			channel_type, dust_buffer_feerate,
 		);
 		let buffer_dust_limit_success_sat = buffer_htlc_success_tx_fee_sat + counterparty_channel_constraints.dust_limit_satoshis;
 		let buffer_dust_limit_timeout_sat = buffer_htlc_timeout_tx_fee_sat + holder_channel_constraints.dust_limit_satoshis;
 
-		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, buffer_dust_limit_success_sat * 1000);
-			}
+		if remote_stats.extra_accepted_htlc_dust_exposure_msat > 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, buffer_dust_limit_success_sat * 1000);
 		}
 
-		if htlc_stats.on_counterparty_tx_dust_exposure_msat.saturating_add(buffer_dust_limit_success_sat * 1000) > max_dust_htlc_exposure_msat.saturating_add(1) {
+		if remote_stats.dust_exposure_msat.saturating_add(buffer_dust_limit_success_sat * 1000) > max_dust_htlc_exposure_msat.saturating_add(1) {
 			// Note that we don't use the `counterparty_tx_dust_exposure` (with
 			// `htlc_dust_exposure_msat`) here as it only applies to non-dust HTLCs.
 			remaining_msat_below_dust_exposure_limit =
-				Some(max_dust_htlc_exposure_msat.saturating_sub(htlc_stats.on_counterparty_tx_dust_exposure_msat));
+				Some(max_dust_htlc_exposure_msat.saturating_sub(remote_stats.dust_exposure_msat));
 			dust_exposure_dust_limit_msat = cmp::max(dust_exposure_dust_limit_msat, buffer_dust_limit_success_sat * 1000);
 		}
 
-		if htlc_stats.on_holder_tx_dust_exposure_msat as i64 + buffer_dust_limit_timeout_sat as i64 * 1000 - 1 > max_dust_htlc_exposure_msat.try_into().unwrap_or(i64::max_value()) {
+		if local_stats_max_fee.dust_exposure_msat as i64 + buffer_dust_limit_timeout_sat as i64 * 1000 - 1 > max_dust_htlc_exposure_msat.try_into().unwrap_or(i64::max_value()) {
 			remaining_msat_below_dust_exposure_limit = Some(cmp::min(
 				remaining_msat_below_dust_exposure_limit.unwrap_or(u64::max_value()),
-				max_dust_htlc_exposure_msat.saturating_sub(htlc_stats.on_holder_tx_dust_exposure_msat)));
+				max_dust_htlc_exposure_msat.saturating_sub(local_stats_max_fee.dust_exposure_msat)));
 			dust_exposure_dust_limit_msat = cmp::max(dust_exposure_dust_limit_msat, buffer_dust_limit_timeout_sat * 1000);
 		}
 
@@ -5816,15 +5826,15 @@ where
 		}
 
 		available_capacity_msat = cmp::min(available_capacity_msat,
-			counterparty_channel_constraints.max_htlc_value_in_flight_msat - htlc_stats.pending_outbound_htlcs_value_msat);
+			counterparty_channel_constraints.max_htlc_value_in_flight_msat - pending_htlcs.iter().filter(|htlc| htlc.outbound).map(|htlc| htlc.amount_msat).sum::<u64>());
 
-		if htlc_stats.pending_outbound_htlcs + 1 > counterparty_channel_constraints.max_accepted_htlcs as usize {
+		if pending_htlcs.iter().filter(|htlc| htlc.outbound).count() + 1 > counterparty_channel_constraints.max_accepted_htlcs as usize {
 			available_capacity_msat = 0;
 		}
 
 		#[allow(deprecated)] // TODO: Remove once balance_msat is removed.
 		AvailableBalances {
-			inbound_capacity_msat: remote_balance_before_fee_msat.saturating_sub(counterparty_channel_constraints.channel_reserve_satoshis * 1000),
+			inbound_capacity_msat: remote_stats.counterparty_balance_before_fee_msat.saturating_sub(counterparty_channel_constraints.channel_reserve_satoshis * 1000),
 			outbound_capacity_msat,
 			next_outbound_htlc_limit_msat: available_capacity_msat,
 			next_outbound_htlc_minimum_msat,

lightning/src/sign/tx_builder.rs

@@ -144,7 +144,7 @@ fn subtract_addl_outputs(
 	}
 }
 
-fn get_dust_buffer_feerate(feerate_per_kw: u32) -> u32 {
+pub(crate) fn get_dust_buffer_feerate(feerate_per_kw: u32) -> u32 {
 	// When calculating our exposure to dust HTLCs, we assume that the channel feerate
 	// may, at any point, increase by at least 10 sat/vB (i.e 2530 sat/kWU) or 25%,
 	// whichever is higher. This ensures that we aren't suddenly exposed to significantly