Add new config setting max_outbound_dusted_htlc_msat

Author: Antoine Riard

lightning/src/ln/channel.rs

@@ -1759,23 +1759,32 @@ impl<Signer: Sign> Channel<Signer> {
 		(self.pending_inbound_htlcs.len() as u32, htlc_inbound_value_msat)
 	}
 
-	/// Returns (outbound_htlc_count, htlc_outbound_value_msat) *including* pending adds in our
-	/// holding cell.
-	fn get_outbound_pending_htlc_stats(&self) -> (u32, u64) {
+	/// Returns (outbound_htlc_count, htlc_outbound_value_msat, outbound_dusted_htlc_value_msat) *including*
+	/// pending adds in our holding cell.
+	fn get_outbound_pending_htlc_stats(&self) -> (u32, u64, u64) {
 		let mut htlc_outbound_value_msat = 0;
+		let mut outbound_dusted_htlc_msat = 0;
+
+		let real_dust_limit_success_sat = (self.feerate_per_kw as u64 * HTLC_SUCCESS_TX_WEIGHT / 1000) + self.counterparty_dust_limit_satoshis;
 		for ref htlc in self.pending_outbound_htlcs.iter() {
 			htlc_outbound_value_msat += htlc.amount_msat;
+			if htlc.amount_msat / 1000 < real_dust_limit_success_sat {
+				outbound_dusted_htlc_msat += htlc.amount_msat + (self.feerate_per_kw as u64 * HTLC_SUCCESS_TX_WEIGHT / 1000) * 1000;
+			}
 		}
 
 		let mut htlc_outbound_count = self.pending_outbound_htlcs.len();
 		for update in self.holding_cell_htlc_updates.iter() {
 			if let &HTLCUpdateAwaitingACK::AddHTLC { ref amount_msat, .. } = update {
 				htlc_outbound_count += 1;
 				htlc_outbound_value_msat += amount_msat;
+				if *amount_msat / 1000 < real_dust_limit_success_sat {
+					outbound_dusted_htlc_msat += amount_msat + (self.feerate_per_kw as u64 * HTLC_SUCCESS_TX_WEIGHT / 1000) * 1000;
+				}
 			}
 		}
 
-		(htlc_outbound_count as u32, htlc_outbound_value_msat)
+		(htlc_outbound_count as u32, htlc_outbound_value_msat, outbound_dusted_htlc_msat)
 	}
 
 	/// Get the available (ie not including pending HTLCs) inbound and outbound balance in msat.
@@ -3432,6 +3441,10 @@ impl<Signer: Sign> Channel<Signer> {
 		cmp::max(self.config.cltv_expiry_delta, MIN_CLTV_EXPIRY_DELTA)
 	}
 
+	pub fn get_max_outbound_dusted_htlc_msat(&self) -> u64 {
+		self.config.max_outbound_dusted_htlc_msat
+	}
+
 	#[cfg(test)]
 	pub fn get_feerate(&self) -> u32 {
 		self.feerate_per_kw
@@ -4075,7 +4088,7 @@ impl<Signer: Sign> Channel<Signer> {
 			return Err(ChannelError::Ignore("Cannot send an HTLC while disconnected from channel counterparty".to_owned()));
 		}
 
-		let (outbound_htlc_count, htlc_outbound_value_msat) = self.get_outbound_pending_htlc_stats();
+		let (outbound_htlc_count, htlc_outbound_value_msat, outbound_dusted_htlc_value_msat) = self.get_outbound_pending_htlc_stats();
 		if outbound_htlc_count + 1 > self.counterparty_max_accepted_htlcs as u32 {
 			return Err(ChannelError::Ignore(format!("Cannot push more than their max accepted HTLCs ({})", self.counterparty_max_accepted_htlcs)));
 		}
@@ -4095,6 +4108,10 @@ impl<Signer: Sign> Channel<Signer> {
 			}
 		}
 
+		if outbound_dusted_htlc_value_msat + amount_msat > self.get_max_outbound_dusted_htlc_msat() {
+			return Err(ChannelError::Ignore(format!("Cannot send value that would put holder dusted balance on counterparty commitment over limit {}", self.get_max_outbound_dusted_htlc_msat())));
+		}
+
 		let pending_value_to_self_msat = self.value_to_self_msat - htlc_outbound_value_msat;
 		if pending_value_to_self_msat < amount_msat {
 			return Err(ChannelError::Ignore(format!("Cannot send value that would overdraw remaining funds. Amount: {}, pending value to self {}", amount_msat, pending_value_to_self_msat)));

lightning/src/util/config.rs

@@ -206,7 +206,18 @@ pub struct ChannelConfig {
 	/// This cannot be changed after a channel has been initialized.
 	///
 	/// Default value: true.
-	pub commit_upfront_shutdown_pubkey: bool
+	pub commit_upfront_shutdown_pubkey: bool,
+	/// Limit on the maximum value of dusted offered HTLCs to the counterparty at any given time
+	/// to limit holder balance exposure to HTLCs. Dusted HTLCs are defined as any HTLC lower
+	/// than counterparty's `dust_limit_satoshis` sumed up with their expexted onchain claim
+	/// cost at minimal mempools congestion.
+	///
+	/// Note, to support reading serialized configs before 0.0.100, the default value will be
+	/// always yielded at deser. A user-selected value must override such value before the
+	/// config is passed to ChannelManager.
+	///
+	/// Default value: 5_000_000 msat.
+	pub max_outbound_dusted_htlc_msat: u64,
 }
 
 impl Default for ChannelConfig {
@@ -218,6 +229,7 @@ impl Default for ChannelConfig {
 			cltv_expiry_delta: 6 * 12, // 6 blocks/hour * 12 hours
 			announced_channel: false,
 			commit_upfront_shutdown_pubkey: true,
+			max_outbound_dusted_htlc_msat: 10_000_000,
 		}
 	}
 }
@@ -228,6 +240,7 @@ impl_writeable_tlv_based!(ChannelConfig, {
 	(4, announced_channel, required),
 	(6, commit_upfront_shutdown_pubkey, required),
 	(8, forwarding_fee_base_msat, required),
+	(default_value, 10_000_000),
 });
 
 /// Top-level config which holds ChannelHandshakeLimits and ChannelConfig.