Use multiplier in dust exposure threshold calculation

This commit makes use of the added config knob to calculate the dust
exposure threshold based on the current fee rate. This also updates
tests to ensure it works as intended.

lightning/src/ln/channel.rs

@@ -4966,7 +4966,10 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 	}
 
 	pub fn get_max_dust_htlc_exposure_msat(&self) -> u64 {
-		self.config.options.max_dust_htlc_exposure_msat
+		match (self.config.options.max_dust_htlc_exposure_multiplier_thousandths, self.high_priority_feerate_per_kw) {
+			(Some(multiplier_thousandths), Some(feerate_per_kw)) => feerate_per_kw as u64 * multiplier_thousandths,
+			_ => self.config.options.max_dust_htlc_exposure_msat,
+		}
 	}
 
 	/// Sets the channel's high priority feerate per kw.

lightning/src/ln/functional_test_utils.rs

@@ -2528,7 +2528,12 @@ pub fn test_default_channel_config() -> UserConfig {
 	default_config.channel_handshake_config.our_htlc_minimum_msat = 1000;
 	// When most of our tests were written, we didn't have the notion of a `max_dust_htlc_exposure_msat`,
 	// It now defaults to 5_000_000 msat; to avoid interfering with tests we bump it to 50_000_000 msat.
+	// This is less relevant now as it's overriden by max_dust_htlc_exposure_multiplier_thousandths
+	// by default.
 	default_config.channel_config.max_dust_htlc_exposure_msat = 50_000_000;
+	// Similarly to the previous threshold, this was added after most of our tests were written, so
+	// we bump the default value to avoid interference.
+	default_config.channel_config.max_dust_htlc_exposure_multiplier_thousandths = Some(50_000_000 / 253);
 	default_config
 }
 

lightning/src/ln/functional_tests.rs

@@ -13,7 +13,7 @@
 
 use crate::chain;
 use crate::chain::{ChannelMonitorUpdateStatus, Confirm, Listen, Watch};
-use crate::chain::chaininterface::LowerBoundedFeeEstimator;
+use crate::chain::chaininterface::{LowerBoundedFeeEstimator, FeeEstimator, ConfirmationTarget};
 use crate::chain::channelmonitor;
 use crate::chain::channelmonitor::{CLTV_CLAIM_BUFFER, LATENCY_GRACE_PERIOD_BLOCKS, ANTI_REORG_DELAY};
 use crate::chain::transaction::OutPoint;
@@ -9565,7 +9565,7 @@ enum ExposureEvent {
 	AtUpdateFeeOutbound,
 }
 
-fn do_test_max_dust_htlc_exposure(dust_outbound_balance: bool, exposure_breach_event: ExposureEvent, on_holder_tx: bool) {
+fn do_test_max_dust_htlc_exposure(dust_outbound_balance: bool, exposure_breach_event: ExposureEvent, on_holder_tx: bool, multiplier_dust_limit: bool) {
 	// Test that we properly reject dust HTLC violating our `max_dust_htlc_exposure_msat`
 	// policy.
 	//
@@ -9581,6 +9581,9 @@ fn do_test_max_dust_htlc_exposure(dust_outbound_balance: bool, exposure_breach_e
 	let chanmon_cfgs = create_chanmon_cfgs(2);
 	let mut config = test_default_channel_config();
 	config.channel_config.max_dust_htlc_exposure_msat = 5_000_000; // default setting value
+	// Default test fee estimator rate is 253 sat/kw, so we set the multiplier to 5_000_000 / 253
+	// to get roughly the same initial value as the default setting.
+	config.channel_config.max_dust_htlc_exposure_multiplier_thousandths = if multiplier_dust_limit { Some(5_000_000 / 253) } else { None };
 	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
 	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[Some(config), None]);
 	let mut nodes = create_network(2, &node_cfgs, &node_chanmgrs);
@@ -9630,14 +9633,21 @@ fn do_test_max_dust_htlc_exposure(dust_outbound_balance: bool, exposure_breach_e
 		let chan = chan_lock.channel_by_id.get(&channel_id).unwrap();
 		chan.get_dust_buffer_feerate(None) as u64
 	};
+	let max_dust_htlc_exposure_msat = if multiplier_dust_limit {
+		nodes[0].fee_estimator.get_est_sat_per_1000_weight(ConfirmationTarget::HighPriority) as u64
+		* config.channel_config.max_dust_htlc_exposure_multiplier_thousandths.unwrap()
+	} else {
+		config.channel_config.max_dust_htlc_exposure_msat
+	};
+
 	let dust_outbound_htlc_on_holder_tx_msat: u64 = (dust_buffer_feerate * htlc_timeout_tx_weight(opt_anchors) / 1000 + open_channel.dust_limit_satoshis - 1) * 1000;
-	let dust_outbound_htlc_on_holder_tx: u64 = config.channel_config.max_dust_htlc_exposure_msat / dust_outbound_htlc_on_holder_tx_msat;
+	let dust_outbound_htlc_on_holder_tx: u64 = max_dust_htlc_exposure_msat / dust_outbound_htlc_on_holder_tx_msat;
 
 	let dust_inbound_htlc_on_holder_tx_msat: u64 = (dust_buffer_feerate * htlc_success_tx_weight(opt_anchors) / 1000 + open_channel.dust_limit_satoshis - 1) * 1000;
-	let dust_inbound_htlc_on_holder_tx: u64 = config.channel_config.max_dust_htlc_exposure_msat / dust_inbound_htlc_on_holder_tx_msat;
+	let dust_inbound_htlc_on_holder_tx: u64 = max_dust_htlc_exposure_msat / dust_inbound_htlc_on_holder_tx_msat;
 
 	let dust_htlc_on_counterparty_tx: u64 = 4;
-	let dust_htlc_on_counterparty_tx_msat: u64 = config.channel_config.max_dust_htlc_exposure_msat / dust_htlc_on_counterparty_tx;
+	let dust_htlc_on_counterparty_tx_msat: u64 = max_dust_htlc_exposure_msat / dust_htlc_on_counterparty_tx;
 
 	if on_holder_tx {
 		if dust_outbound_balance {
@@ -9689,7 +9699,7 @@ fn do_test_max_dust_htlc_exposure(dust_outbound_balance: bool, exposure_breach_e
 				), true, APIError::ChannelUnavailable { .. }, {});
 		}
 	} else if exposure_breach_event == ExposureEvent::AtHTLCReception {
-		let (route, payment_hash, _, payment_secret) = get_route_and_payment_hash!(nodes[1], nodes[0], if on_holder_tx { dust_inbound_htlc_on_holder_tx_msat } else { dust_htlc_on_counterparty_tx_msat + 1 });
+		let (route, payment_hash, _, payment_secret) = get_route_and_payment_hash!(nodes[1], nodes[0], if on_holder_tx { dust_inbound_htlc_on_holder_tx_msat } else { dust_htlc_on_counterparty_tx_msat + 4 });
 		nodes[1].node.send_payment_with_route(&route, payment_hash,
 			RecipientOnionFields::secret_only(payment_secret), PaymentId(payment_hash.0)).unwrap();
 		check_added_monitors!(nodes[1], 1);
@@ -9702,18 +9712,24 @@ fn do_test_max_dust_htlc_exposure(dust_outbound_balance: bool, exposure_breach_e
 			// Outbound dust balance: 6399 sats
 			let dust_inbound_overflow = dust_inbound_htlc_on_holder_tx_msat * (dust_inbound_htlc_on_holder_tx + 1);
 			let dust_outbound_overflow = dust_outbound_htlc_on_holder_tx_msat * dust_outbound_htlc_on_holder_tx + dust_inbound_htlc_on_holder_tx_msat;
-			nodes[0].logger.assert_log("lightning::ln::channel".to_string(), format!("Cannot accept value that would put our exposure to dust HTLCs at {} over the limit {} on holder commitment tx", if dust_outbound_balance { dust_outbound_overflow } else { dust_inbound_overflow }, config.channel_config.max_dust_htlc_exposure_msat), 1);
+			nodes[0].logger.assert_log("lightning::ln::channel".to_string(), format!("Cannot accept value that would put our exposure to dust HTLCs at {} over the limit {} on holder commitment tx", if dust_outbound_balance { dust_outbound_overflow } else { dust_inbound_overflow }, max_dust_htlc_exposure_msat), 1);
 		} else {
 			// Outbound dust balance: 5200 sats
 			nodes[0].logger.assert_log("lightning::ln::channel".to_string(),
 				format!("Cannot accept value that would put our exposure to dust HTLCs at {} over the limit {} on counterparty commitment tx",
-					dust_htlc_on_counterparty_tx_msat * (dust_htlc_on_counterparty_tx - 1) + dust_htlc_on_counterparty_tx_msat + 1,
-					config.channel_config.max_dust_htlc_exposure_msat), 1);
+					dust_htlc_on_counterparty_tx_msat * (dust_htlc_on_counterparty_tx - 1) + dust_htlc_on_counterparty_tx_msat + 4,
+					max_dust_htlc_exposure_msat), 1);
 		}
 	} else if exposure_breach_event == ExposureEvent::AtUpdateFeeOutbound {
 		route.paths[0].hops.last_mut().unwrap().fee_msat = 2_500_000;
-		nodes[0].node.send_payment_with_route(&route, payment_hash,
-			RecipientOnionFields::secret_only(payment_secret), PaymentId(payment_hash.0)).unwrap();
+		// For the multiplier dust exposure limit, since it scales with feerate,
+		// we need to add a lot of HTLCs that will become dust at the new feerate
+		// to cross the threshold.
+		for _ in 0..20 {
+			let (_, payment_hash, payment_secret) = get_payment_preimage_hash(&nodes[1], Some(1_000), None);
+			nodes[0].node.send_payment_with_route(&route, payment_hash,
+				RecipientOnionFields::secret_only(payment_secret), PaymentId(payment_hash.0)).unwrap();
+		}
 		{
 			let mut feerate_lock = chanmon_cfgs[0].fee_estimator.sat_per_kw.lock().unwrap();
 			*feerate_lock = *feerate_lock * 10;
@@ -9728,20 +9744,25 @@ fn do_test_max_dust_htlc_exposure(dust_outbound_balance: bool, exposure_breach_e
 	added_monitors.clear();
 }
 
+fn do_test_max_dust_htlc_exposure_by_threshold_type(multiplier_dust_limit: bool) {
+	do_test_max_dust_htlc_exposure(true, ExposureEvent::AtHTLCForward, true, multiplier_dust_limit);
+	do_test_max_dust_htlc_exposure(false, ExposureEvent::AtHTLCForward, true, multiplier_dust_limit);
+	do_test_max_dust_htlc_exposure(false, ExposureEvent::AtHTLCReception, true, multiplier_dust_limit);
+	do_test_max_dust_htlc_exposure(false, ExposureEvent::AtHTLCReception, false, multiplier_dust_limit);
+	do_test_max_dust_htlc_exposure(true, ExposureEvent::AtHTLCForward, false, multiplier_dust_limit);
+	do_test_max_dust_htlc_exposure(true, ExposureEvent::AtHTLCReception, false, multiplier_dust_limit);
+	do_test_max_dust_htlc_exposure(true, ExposureEvent::AtHTLCReception, true, multiplier_dust_limit);
+	do_test_max_dust_htlc_exposure(false, ExposureEvent::AtHTLCForward, false, multiplier_dust_limit);
+	do_test_max_dust_htlc_exposure(true, ExposureEvent::AtUpdateFeeOutbound, true, multiplier_dust_limit);
+	do_test_max_dust_htlc_exposure(true, ExposureEvent::AtUpdateFeeOutbound, false, multiplier_dust_limit);
+	do_test_max_dust_htlc_exposure(false, ExposureEvent::AtUpdateFeeOutbound, false, multiplier_dust_limit);
+	do_test_max_dust_htlc_exposure(false, ExposureEvent::AtUpdateFeeOutbound, true, multiplier_dust_limit);
+}
+
 #[test]
 fn test_max_dust_htlc_exposure() {
-	do_test_max_dust_htlc_exposure(true, ExposureEvent::AtHTLCForward, true);
-	do_test_max_dust_htlc_exposure(false, ExposureEvent::AtHTLCForward, true);
-	do_test_max_dust_htlc_exposure(false, ExposureEvent::AtHTLCReception, true);
-	do_test_max_dust_htlc_exposure(false, ExposureEvent::AtHTLCReception, false);
-	do_test_max_dust_htlc_exposure(true, ExposureEvent::AtHTLCForward, false);
-	do_test_max_dust_htlc_exposure(true, ExposureEvent::AtHTLCReception, false);
-	do_test_max_dust_htlc_exposure(true, ExposureEvent::AtHTLCReception, true);
-	do_test_max_dust_htlc_exposure(false, ExposureEvent::AtHTLCForward, false);
-	do_test_max_dust_htlc_exposure(true, ExposureEvent::AtUpdateFeeOutbound, true);
-	do_test_max_dust_htlc_exposure(true, ExposureEvent::AtUpdateFeeOutbound, false);
-	do_test_max_dust_htlc_exposure(false, ExposureEvent::AtUpdateFeeOutbound, false);
-	do_test_max_dust_htlc_exposure(false, ExposureEvent::AtUpdateFeeOutbound, true);
+	do_test_max_dust_htlc_exposure_by_threshold_type(false);
+	do_test_max_dust_htlc_exposure_by_threshold_type(true);
 }
 
 #[test]

lightning/src/ln/onion_route_tests.rs

@@ -671,6 +671,7 @@ fn do_test_onion_failure_stale_channel_update(announced_channel: bool) {
 	config.channel_handshake_config.announced_channel = announced_channel;
 	config.channel_handshake_limits.force_announced_channel_preference = false;
 	config.accept_forwards_to_priv_channels = !announced_channel;
+	config.channel_config.max_dust_htlc_exposure_multiplier_thousandths = None;
 	let chanmon_cfgs = create_chanmon_cfgs(3);
 	let persister;
 	let chain_monitor;
@@ -1371,11 +1372,20 @@ fn test_phantom_failure_too_low_recv_amt() {
 
 #[test]
 fn test_phantom_dust_exposure_failure() {
+	do_test_phantom_dust_exposure_failure(false);
+	do_test_phantom_dust_exposure_failure(true);
+}
+
+fn do_test_phantom_dust_exposure_failure(multiplier_dust_limit: bool) {
 	// Set the max dust exposure to the dust limit.
 	let max_dust_exposure = 546;
 	let mut receiver_config = UserConfig::default();
 	receiver_config.channel_config.max_dust_htlc_exposure_msat = max_dust_exposure;
 	receiver_config.channel_handshake_config.announced_channel = true;
+	// Default test fee estimator rate is 253, so to set the max dust exposure to the dust limit,
+	// we need to set the multiplier to 2.
+	receiver_config.channel_config.max_dust_htlc_exposure_multiplier_thousandths =
+		if multiplier_dust_limit { Some(2) } else { None };
 
 	let chanmon_cfgs = create_chanmon_cfgs(2);
 	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);

lightning/src/ln/priv_short_conf_tests.rs

@@ -141,10 +141,12 @@ fn do_test_1_conf_open(connect_style: ConnectStyle) {
 	alice_config.channel_handshake_config.minimum_depth = 1;
 	alice_config.channel_handshake_config.announced_channel = true;
 	alice_config.channel_handshake_limits.force_announced_channel_preference = false;
+	alice_config.channel_config.max_dust_htlc_exposure_multiplier_thousandths = None;
 	let mut bob_config = UserConfig::default();
 	bob_config.channel_handshake_config.minimum_depth = 1;
 	bob_config.channel_handshake_config.announced_channel = true;
 	bob_config.channel_handshake_limits.force_announced_channel_preference = false;
+	bob_config.channel_config.max_dust_htlc_exposure_multiplier_thousandths = None;
 	let chanmon_cfgs = create_chanmon_cfgs(2);
 	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
 	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[Some(alice_config), Some(bob_config)]);