Mind commit tx fee while assembling a route

lightning/src/ln/channel.rs

@@ -2612,17 +2612,49 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 		balance_msat -= outbound_stats.pending_htlcs_value_msat;
 
 		let outbound_capacity_msat = cmp::max(self.value_to_self_msat as i64
-				- outbound_stats.pending_htlcs_value_msat as i64
-				- self.counterparty_selected_channel_reserve_satoshis.unwrap_or(0) as i64 * 1000,
+											  - outbound_stats.pending_htlcs_value_msat as i64
+											  - self.counterparty_selected_channel_reserve_satoshis.unwrap_or(0) as i64 * 1000,
 			0) as u64;
+
+		let mut available_capacity_msat = outbound_capacity_msat;
+
+		if self.is_outbound() {
+			// We should mind channel commit tx fee when computing how much of the available capacity
+			// can be used in the next htlc. Mirrors the logic in send_htlc.
+			//
+			// The fee depends on whether the amount we will be sending is above dust or not,
+			// and the answer will in turn change the amount itself — making it a circular
+			// dependency.
+			// This complicates the computation around dust-values, up to the one-htlc-value.
+			let mut real_dust_limit_success_msat = self.holder_dust_limit_satoshis * 1000;
+			if !self.opt_anchors() {
+				real_dust_limit_success_msat += self.feerate_per_kw as u64 * htlc_success_tx_weight(false);
+			}
+
+			let htlc_above_dust = HTLCCandidate::new(real_dust_limit_success_msat, HTLCInitiator::RemoteOffered);
+			let max_reserved_commit_tx_fee_msat = FEE_SPIKE_BUFFER_FEE_INCREASE_MULTIPLE * self.next_local_commit_tx_fee_msat(htlc_above_dust, None);
+			let htlc_dust = HTLCCandidate::new(real_dust_limit_success_msat - 1000, HTLCInitiator::RemoteOffered);
+			let min_reserved_commit_tx_fee_msat = FEE_SPIKE_BUFFER_FEE_INCREASE_MULTIPLE * self.next_local_commit_tx_fee_msat(htlc_dust, None);
+
+			// We will first subtract the fee as if we were above-dust. Then, if the resulting
+			// value ends up being below dust, we have this fee available again. In that case,
+			// match the value to right-below-dust.
+			available_capacity_msat -= max_reserved_commit_tx_fee_msat;
+			if available_capacity_msat < real_dust_limit_success_msat {
+				let one_htlc_difference_msat = max_reserved_commit_tx_fee_msat - min_reserved_commit_tx_fee_msat;
+				assert!(one_htlc_difference_msat != 0);
+				available_capacity_msat += one_htlc_difference_msat;
+				available_capacity_msat = std::cmp::max(real_dust_limit_success_msat - 1, available_capacity_msat);
+			}
+		}
 		AvailableBalances {
 			inbound_capacity_msat: cmp::max(self.channel_value_satoshis as i64 * 1000
 					- self.value_to_self_msat as i64
 					- self.get_inbound_pending_htlc_stats(None).pending_htlcs_value_msat as i64
 					- self.holder_selected_channel_reserve_satoshis as i64 * 1000,
 				0) as u64,
 			outbound_capacity_msat,
-			next_outbound_htlc_limit_msat: cmp::max(cmp::min(outbound_capacity_msat as i64,
+			next_outbound_htlc_limit_msat: cmp::max(cmp::min(available_capacity_msat as i64,
 					self.counterparty_max_htlc_value_in_flight_msat as i64
 						- outbound_stats.pending_htlcs_value_msat as i64),
 				0) as u64,

lightning/src/ln/functional_test_utils.rs

@@ -1609,6 +1609,19 @@ macro_rules! get_route_and_payment_hash {
 	}}
 }
 
+#[cfg(test)]
+#[macro_export]
+macro_rules! get_route_or_error {
+	($send_node: expr, $recv_node: expr, $recv_value: expr) => {{
+		let payment_params = $crate::routing::router::PaymentParameters::from_node_id($recv_node.node.get_our_node_id(), TEST_FINAL_CLTV)
+			.with_features($recv_node.node.invoice_features());
+		let (payment_preimage, payment_hash, payment_secret) = $crate::get_payment_preimage_hash!($recv_node, Some($recv_value));
+		let route_or_error = $crate::get_route!($send_node, payment_params, $recv_value, TEST_FINAL_CLTV);
+		(route_or_error, payment_hash, payment_preimage, payment_secret)
+	}}
+}
+
+
 #[macro_export]
 #[cfg(any(test, feature = "_bench_unstable", feature = "_test_utils"))]
 macro_rules! expect_payment_claimable {

lightning/src/ln/payment_tests.rs

@@ -20,6 +20,7 @@ use crate::ln::channelmanager::{BREAKDOWN_TIMEOUT, ChannelManager, MPP_TIMEOUT_T
 use crate::ln::features::InvoiceFeatures;
 use crate::ln::msgs;
 use crate::ln::msgs::ChannelMessageHandler;
+use crate::ln::msgs::{LightningError, ErrorAction};
 use crate::ln::outbound_payment::Retry;
 use crate::routing::gossip::{EffectiveCapacity, RoutingFees};
 use crate::routing::router::{get_route, PaymentParameters, Route, RouteHint, RouteHintHop, RouteHop, RouteParameters};
@@ -2752,3 +2753,26 @@ fn test_threaded_payment_retries() {
 		}
 	}
 }
+
+#[test]
+fn test_route_minds_commit_tx_fee() {
+	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, &[None, None]);
+	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+	let amount_sats = 100_000;
+	// Less than 10% should be on our side so that we don't hit the htlc limit of 10%.
+	let our_amount_sats = amount_sats * 9 / 100;
+	create_announced_chan_between_nodes_with_value(&nodes, 0, 1, amount_sats, (amount_sats - our_amount_sats) * 1000);
+
+	let extra_htlc_cost = 1452;
+	let route_err1 = get_route_or_error!(&nodes[0], nodes[1], (our_amount_sats - extra_htlc_cost + 1) * 1000).0;
+
+	if let Err(LightningError{err, action: ErrorAction::IgnoreError}) = route_err1 {
+		assert_eq!(err, "Failed to find a sufficient route to the given destination");
+	} else { panic!(); }
+
+	let (route, _, _, _) = get_route_and_payment_hash!(&nodes[0], nodes[1], (our_amount_sats - extra_htlc_cost) * 1000);
+	assert_eq!(route.paths.len(), 1);
+	// TODO: test other limits, e.g. that our router doesn't ignore counterparty_selected_channel_reserve_satoshis.
+}