Enforce max_balance_dust_htlc_msat at HTLC reception/forward

At `update_add_htlc()`/`send_htlc()`, we verify that the inbound/
outbound dust or the sum of both, on either sides of the link isn't
above new config setting `max_balance_dust_htlc_msat`.

A dust HTLC is hence defined as a trimmed-to-dust one, i.e including
the fee cost to publish its claiming transaction.

Author: Antoine Riard

lightning/src/ln/channel.rs

@@ -2061,6 +2061,20 @@ impl<Signer: Sign> Channel<Signer> {
 			}
 		}
 
+		if msg.amount_msat / 1000 < (self.feerate_per_kw as u64 * HTLC_TIMEOUT_TX_WEIGHT / 1000) + self.counterparty_dust_limit_satoshis {
+			if on_counterparty_tx_dust_inbound + on_counterparty_tx_dust_outbound + msg.amount_msat > self.get_max_balance_dust_htlc_msat() {
+				log_info!(logger, "Cannot accept value that would put holder dusted balance {} on counterparty commitment over limit {}", on_counterparty_tx_dust_inbound + on_counterparty_tx_dust_outbound + msg.amount_msat, self.get_max_balance_dust_htlc_msat());
+				pending_forward_status = create_pending_htlc_status(self, pending_forward_status, 0x1000|7);
+			}
+		}
+
+		if msg.amount_msat / 1000 < (self.feerate_per_kw as u64 * HTLC_SUCCESS_TX_WEIGHT / 1000) + self.holder_dust_limit_satoshis {
+			if on_holder_tx_dust_inbound + on_holder_tx_dust_outbound + msg.amount_msat > self.get_max_balance_dust_htlc_msat() {
+				log_info!(logger, "Cannot accept value that would put holder dusted balance {} on holder commitment over limit {}", on_holder_tx_dust_inbound + on_holder_tx_dust_outbound + msg.amount_msat, self.get_max_balance_dust_htlc_msat());
+				pending_forward_status = create_pending_htlc_status(self, pending_forward_status, 0x1000|7);
+			}
+		}
+
 		let pending_value_to_self_msat =
 			self.value_to_self_msat + htlc_inbound_value_msat - removed_outbound_total_msat;
 		let pending_remote_value_msat =
@@ -3461,6 +3475,10 @@ impl<Signer: Sign> Channel<Signer> {
 		cmp::max(self.config.cltv_expiry_delta, MIN_CLTV_EXPIRY_DELTA)
 	}
 
+	pub fn get_max_balance_dust_htlc_msat(&self) -> u64 {
+		self.config.max_balance_dust_htlc_msat
+	}
+
 	#[cfg(test)]
 	pub fn get_feerate(&self) -> u32 {
 		self.feerate_per_kw
@@ -4125,6 +4143,18 @@ impl<Signer: Sign> Channel<Signer> {
 			}
 		}
 
+		if amount_msat / 1000 < (self.feerate_per_kw as u64 * HTLC_SUCCESS_TX_WEIGHT / 1000) + self.counterparty_dust_limit_satoshis {
+			if on_counterparty_tx_dust_inbound + on_counterparty_tx_dust_outbound + amount_msat > self.get_max_balance_dust_htlc_msat() {
+				return Err(ChannelError::Ignore(format!("Cannot send value that would put holder dusted balance {} on counterparty commitment over limit {}", on_counterparty_tx_dust_inbound + on_counterparty_tx_dust_outbound + amount_msat, self.get_max_balance_dust_htlc_msat())));
+			}
+		}
+
+		if amount_msat / 1000 < (self.feerate_per_kw as u64 * HTLC_TIMEOUT_TX_WEIGHT / 1000) + self.holder_dust_limit_satoshis {
+			if on_holder_tx_dust_inbound + on_holder_tx_dust_outbound + amount_msat > self.get_max_balance_dust_htlc_msat() {
+				return Err(ChannelError::Ignore(format!("Cannot send value that would put holder dusted balance {} on holder commitment over limit {}", on_holder_tx_dust_inbound + on_holder_tx_dust_outbound + amount_msat, self.get_max_balance_dust_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/ln/functional_test_utils.rs

@@ -1354,6 +1354,9 @@ pub fn test_default_channel_config() -> UserConfig {
 	// When most of our tests were written, the default HTLC minimum was fixed at 1000.
 	// It now defaults to 1, so we simply set it to the expected value here.
 	default_config.own_channel_config.our_htlc_minimum_msat = 1000;
+	// When most of our tests were written, we didn't have notion of a `max_balance_dust_htlc_msat`,
+	// It now defaults to 5_000_000 msat, to avoid interferring with tests we bump it to 50_000_000 msat.
+	default_config.channel_options.max_balance_dust_htlc_msat = 50_000_000;
 	default_config
 }