Add max dust exposure multiplier config knob

With fee rates rising dramatically in mid-April 2023, thresholds for
what is considered dust have risen, often exceeding our previous dust
exposure threshold of 5k sats. This causes all payments and HTLC
forwards between 5k sats and new dust thresholds to fail.

This commit adds a config knob that will allow us to implement setting
our max dust exposure limit to the product of a multiplier and the
high priority feerate.

Author: alecchendev

lightning/src/util/config.rs

@@ -397,6 +397,21 @@ pub struct ChannelConfig {
 	/// [`Normal`]: crate::chain::chaininterface::ConfirmationTarget::Normal
 	/// [`Background`]: crate::chain::chaininterface::ConfirmationTarget::Background
 	pub force_close_avoidance_max_fee_satoshis: u64,
+	/// Similar to [`Self::max_dust_htlc_exposure_msat`], but instead of setting a fixed maximum,
+	/// this sets a multiplier on the estimated high priority feerate (sats/KW, as obtained from
+	/// [`FeeEstimator`]) to determine the maximum allowed dust exposure. If this field is set to
+	/// `Some(value)`, then the maximum dust exposure in sats is calculated as:
+	/// `high_priority_feerate_per_kw * value / 1000`.
+	///
+	/// This allows the maximum dust exposure to automatically scale with fee rate changes. With
+	/// a fixed maximum, if the feerate increases significantly, then without a manually increase
+	/// to this maximum, the channel may be unable to send/receive HTLCs between the maximum dust
+	/// exposure and the new minimum value for HTLCs to be economically viable to claim.
+	///
+	/// Default value: `Some(5000)`.
+	///
+	/// [`FeeEstimator`]: crate::chain::chaininterface::FeeEstimator
+	pub max_dust_htlc_exposure_multiplier_thousandths: Option<u64>,
 }
 
 impl ChannelConfig {
@@ -417,6 +432,9 @@ impl ChannelConfig {
 		if let Some(force_close_avoidance_max_fee_satoshis) = update.force_close_avoidance_max_fee_satoshis {
 			self.force_close_avoidance_max_fee_satoshis = force_close_avoidance_max_fee_satoshis;
 		}
+		if let Some(max_dust_htlc_exposure_multiplier_thousandths) = update.max_dust_htlc_exposure_multiplier_thousandths {
+			self.max_dust_htlc_exposure_multiplier_thousandths = max_dust_htlc_exposure_multiplier_thousandths;
+		}
 	}
 }
 
@@ -429,6 +447,7 @@ impl Default for ChannelConfig {
 			cltv_expiry_delta: 6 * 12, // 6 blocks/hour * 12 hours
 			max_dust_htlc_exposure_msat: 5_000_000,
 			force_close_avoidance_max_fee_satoshis: 1000,
+			max_dust_htlc_exposure_multiplier_thousandths: Some(5000),
 		}
 	}
 }
@@ -442,6 +461,7 @@ impl_writeable_tlv_based!(ChannelConfig, {
 	// LegacyChannelConfig. To make sure that serialization is not compatible with this one, we use
 	// the next required type of 10, which if seen by the old serialization will always fail.
 	(10, force_close_avoidance_max_fee_satoshis, required),
+	(12, max_dust_htlc_exposure_multiplier_thousandths, required)
 });
 
 /// A parallel struct to [`ChannelConfig`] to define partial updates.
@@ -452,6 +472,7 @@ pub struct ChannelConfigUpdate {
 	pub cltv_expiry_delta: Option<u16>,
 	pub max_dust_htlc_exposure_msat: Option<u64>,
 	pub force_close_avoidance_max_fee_satoshis: Option<u64>,
+	pub max_dust_htlc_exposure_multiplier_thousandths: Option<Option<u64>>,
 }
 
 impl Default for ChannelConfigUpdate {
@@ -462,6 +483,7 @@ impl Default for ChannelConfigUpdate {
 			cltv_expiry_delta: None,
 			max_dust_htlc_exposure_msat: None,
 			force_close_avoidance_max_fee_satoshis: None,
+			max_dust_htlc_exposure_multiplier_thousandths: None,
 		}
 	}
 }
@@ -474,6 +496,7 @@ impl From<ChannelConfig> for ChannelConfigUpdate {
 			cltv_expiry_delta: Some(config.cltv_expiry_delta),
 			max_dust_htlc_exposure_msat: Some(config.max_dust_htlc_exposure_msat),
 			force_close_avoidance_max_fee_satoshis: Some(config.force_close_avoidance_max_fee_satoshis),
+			max_dust_htlc_exposure_multiplier_thousandths: Some(config.max_dust_htlc_exposure_multiplier_thousandths),
 		}
 	}
 }
@@ -513,6 +536,7 @@ impl crate::util::ser::Writeable for LegacyChannelConfig {
 			(4, self.announced_channel, required),
 			(6, self.commit_upfront_shutdown_pubkey, required),
 			(8, self.options.forwarding_fee_base_msat, required),
+			(10, self.options.max_dust_htlc_exposure_multiplier_thousandths, option),
 		});
 		Ok(())
 	}
@@ -527,6 +551,7 @@ impl crate::util::ser::Readable for LegacyChannelConfig {
 		let mut announced_channel = false;
 		let mut commit_upfront_shutdown_pubkey = false;
 		let mut forwarding_fee_base_msat = 0;
+		let mut max_dust_htlc_exposure_multiplier_thousandths = None;
 		read_tlv_fields!(reader, {
 			(0, forwarding_fee_proportional_millionths, required),
 			(1, max_dust_htlc_exposure_msat, (default_value, 5_000_000u64)),
@@ -535,6 +560,7 @@ impl crate::util::ser::Readable for LegacyChannelConfig {
 			(4, announced_channel, required),
 			(6, commit_upfront_shutdown_pubkey, required),
 			(8, forwarding_fee_base_msat, required),
+			(10, max_dust_htlc_exposure_multiplier_thousandths, option),
 		});
 		Ok(Self {
 			options: ChannelConfig {
@@ -543,6 +569,7 @@ impl crate::util::ser::Readable for LegacyChannelConfig {
 				cltv_expiry_delta,
 				force_close_avoidance_max_fee_satoshis,
 				forwarding_fee_base_msat,
+				max_dust_htlc_exposure_multiplier_thousandths,
 			},
 			announced_channel,
 			commit_upfront_shutdown_pubkey,