ln: return Option for dust_exposure_limiting_feerate

This fee rate is currently used in two scenarios:
- To count any fees above what we consider to be a sane estimate towards
  our dust exposure.
- To get a maximum dust exposure (when using
  MaxDustHTLCExposure::FeeEstimator strategy).

When we have zero fee commitments:
- Commitments are zero fee, so we don't need to count fees towards dust
  exposure.
- The amount of dust we have is not dependent on fees, as everything is
  zero fee.
- We still want to limit our total dust exposure.

This commit updates get_dust_exposure_limiting_feerate to allow a None
value to prepare for support for zero fee commitments. This clearly
allows us to indicate when we don't care about fee rates for dust
considerations.

In get_max_dust_htlc_exposure_msat, we simply hardcode a value of
1 sat/vbyte if a feerate dependent strategy is being used.

Author: carlaKC

lightning/src/ln/channel.rs

@@ -3833,17 +3833,21 @@ where
 
 	fn get_dust_exposure_limiting_feerate<F: Deref>(
 		&self, fee_estimator: &LowerBoundedFeeEstimator<F>,
-	) -> u32
+	) -> Option<u32>
 	where
 		F::Target: FeeEstimator,
 	{
-		fee_estimator.bounded_sat_per_1000_weight(ConfirmationTarget::MaximumFeeEstimate)
+		Some(fee_estimator.bounded_sat_per_1000_weight(ConfirmationTarget::MaximumFeeEstimate))
 	}
 
-	pub fn get_max_dust_htlc_exposure_msat(&self, limiting_feerate_sat_per_kw: u32) -> u64 {
+	/// Returns the maximum configured dust exposure.
+	///
+	/// Uses a default of 1 sat/vbyte if `limiting_feerate_sat_per_kw` is `None` and the dust
+	/// exposure policy depends on fee rate.
+	pub fn get_max_dust_htlc_exposure_msat(&self, limiting_feerate_sat_per_kw: Option<u32>) -> u64 {
 		match self.config.options.max_dust_htlc_exposure {
 			MaxDustHTLCExposure::FeeRateMultiplier(multiplier) => {
-				(limiting_feerate_sat_per_kw as u64).saturating_mul(multiplier)
+				(limiting_feerate_sat_per_kw.unwrap_or(250) as u64).saturating_mul(multiplier)
 			},
 			MaxDustHTLCExposure::FixedLimitMsat(limit) => limit,
 		}
@@ -4220,7 +4224,7 @@ where
 	#[rustfmt::skip]
 	fn can_accept_incoming_htlc<L: Deref>(
 		&self, funding: &FundingScope, msg: &msgs::UpdateAddHTLC,
-		dust_exposure_limiting_feerate: u32, logger: &L,
+		dust_exposure_limiting_feerate: Option<u32>, logger: &L,
 	) -> Result<(), LocalHTLCFailureReason>
 	where
 		L::Target: Logger,
@@ -4583,7 +4587,7 @@ where
 	#[rustfmt::skip]
 	fn get_pending_htlc_stats(
 		&self, funding: &FundingScope, outbound_feerate_update: Option<u32>,
-		dust_exposure_limiting_feerate: u32,
+		dust_exposure_limiting_feerate: Option<u32>,
 	) -> HTLCStats {
 		let context = self;
 		let uses_0_htlc_fee_anchors = funding.get_channel_type().supports_anchors_zero_fee_htlc_tx();
@@ -4662,7 +4666,11 @@ where
 		let excess_feerate_opt = outbound_feerate_update
 			.or(self.pending_update_fee.map(|(fee, _)| fee))
 			.unwrap_or(self.feerate_per_kw)
-			.checked_sub(dust_exposure_limiting_feerate);
+			.checked_sub(dust_exposure_limiting_feerate.unwrap_or(0));
+
+		if funding.get_channel_type().supports_anchor_zero_fee_commitments() {
+			debug_assert!(dust_exposure_limiting_feerate.is_none());
+		}
 		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, funding.get_channel_type()) * 1000;

lightning/src/ln/functional_tests.rs

@@ -9953,7 +9953,7 @@ fn do_test_max_dust_htlc_exposure(
 		let chan = chan_lock.channel_by_id.get(&channel_id).unwrap();
 		(
 			chan.context().get_dust_buffer_feerate(None) as u64,
-			chan.context().get_max_dust_htlc_exposure_msat(253),
+			chan.context().get_max_dust_htlc_exposure_msat(Some(253)),
 		)
 	};
 	assert_eq!(dust_buffer_feerate, expected_dust_buffer_feerate as u64);

lightning/src/util/config.rs

@@ -438,12 +438,20 @@ pub enum MaxDustHTLCExposure {
 	/// on HTLC outputs means your channel may be subject to more dust exposure in the event of
 	/// increases in fee rate.
 	///
+	/// Note that because zero-commitment-fee anchor channels do not allow for feerate updates (and
+	/// thus never experience dust exposure changes due to feerate shifts, resulting in no
+	/// force-closes due to dust exposure limits), such channels will calculate their maximum
+	/// dust exposure using a constant feerate of 250 sat/KW when using this variant.
+	///
 	/// # Backwards Compatibility
 	/// This variant only became available in LDK 0.0.116, so if you downgrade to a prior version
 	/// by default this will be set to a [`Self::FixedLimitMsat`] of 5,000,000 msat.
 	///
 	/// [`FeeEstimator`]: crate::chain::chaininterface::FeeEstimator
 	/// [`ConfirmationTarget::MaximumFeeEstimate`]: crate::chain::chaininterface::ConfirmationTarget::MaximumFeeEstimate
+	//
+	// TODO: link ChannelHandshakeConfig::negotiate_anchor_zero_fee_commitment in zero fee
+	// commitment doc once field is no longer behind cfg[test] flag.
 	FeeRateMultiplier(u64),
 }