f use a const for htlc affordability count

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -378,6 +378,11 @@ pub const FEE_SPIKE_BUFFER_FEE_INCREASE_MULTIPLE: u64 = 2;
 #[cfg(not(fuzzing))]
 const FEE_SPIKE_BUFFER_FEE_INCREASE_MULTIPLE: u64 = 2;
 
+/// When a channel is opened, we check that the funding amount is enough to pay for relevant
+/// commitment transaction fees, with at least this many HTLCs present on the commitment
+/// transaction (not counting the value of the HTLCs themselves).
+pub(crate) const MIN_AFFORDABLE_HTLC_COUNT: usize = 4;
+
 // TODO: We should refactor this to be an Inbound/OutboundChannel until initial setup handshaking
 // has been completed, and then turn into a Channel to get compiler-time enforcement of things like
 // calling channel_id() before we're set up or things like get_outbound_funding_signed on an
@@ -672,7 +677,7 @@ impl<Signer: Sign> Channel<Signer> {
 		let feerate = fee_estimator.get_est_sat_per_1000_weight(ConfirmationTarget::Normal);
 
 		let value_to_self_msat = channel_value_satoshis * 1000 - push_msat;
-		let commitment_tx_fee = Self::commit_tx_fee_msat(feerate, 4);
+		let commitment_tx_fee = Self::commit_tx_fee_msat(feerate, MIN_AFFORDABLE_HTLC_COUNT);
 		if value_to_self_msat < commitment_tx_fee {
 			return Err(APIError::APIMisuseError{ err: format!("Funding amount ({}) can't even pay fee for initial commitment transaction fee of {}.", value_to_self_msat / 1000, commitment_tx_fee / 1000) });
 		}
@@ -929,10 +934,9 @@ impl<Signer: Sign> Channel<Signer> {
 		}
 
 		// check if the funder's amount for the initial commitment tx is sufficient
-		// for full fee payment. We somewhat arbitrarily pick 4 HTLC outputs here - the funder
-		// should be able to afford at least that many.
+		// for full fee payment plus a few HTLCs to ensure the channel will be useful.
 		let funders_amount_msat = msg.funding_satoshis * 1000 - msg.push_msat;
-		let commitment_tx_fee = Self::commit_tx_fee_msat(msg.feerate_per_kw, 4) / 1000;
+		let commitment_tx_fee = Self::commit_tx_fee_msat(msg.feerate_per_kw, MIN_AFFORDABLE_HTLC_COUNT) / 1000;
 		if funders_amount_msat / 1000 < commitment_tx_fee {
 			return Err(ChannelError::Close(format!("Funding amount ({} sats) can't even pay fee for initial commitment transaction fee of {} sats.", funders_amount_msat / 1000, commitment_tx_fee)));
 		}

lightning/src/ln/functional_tests.rs

@@ -18,7 +18,7 @@ use chain::channelmonitor::{ChannelMonitor, CLTV_CLAIM_BUFFER, LATENCY_GRACE_PER
 use chain::transaction::OutPoint;
 use chain::keysinterface::BaseSign;
 use ln::{PaymentPreimage, PaymentSecret, PaymentHash};
-use ln::channel::{COMMITMENT_TX_BASE_WEIGHT, COMMITMENT_TX_WEIGHT_PER_HTLC};
+use ln::channel::{COMMITMENT_TX_BASE_WEIGHT, COMMITMENT_TX_WEIGHT_PER_HTLC, MIN_AFFORDABLE_HTLC_COUNT};
 use ln::channelmanager::{ChannelManager, ChannelManagerReadArgs, PaymentId, RAACommitmentOrder, PaymentSendFailure, BREAKDOWN_TIMEOUT, MIN_CLTV_EXPIRY_DELTA};
 use ln::channel::{Channel, ChannelError};
 use ln::{chan_utils, onion_utils};
@@ -1398,16 +1398,15 @@ fn test_chan_reserve_violation_outbound_htlc_inbound_chan() {
 	let mut nodes = create_network(2, &node_cfgs, &node_chanmgrs);
 
 	let mut push_amt = 100_000_000;
-	push_amt -= commit_tx_fee_msat(feerate_per_kw, 4);
+	push_amt -= commit_tx_fee_msat(feerate_per_kw, MIN_AFFORDABLE_HTLC_COUNT as u64);
 	push_amt -= Channel::<EnforcingSigner>::get_holder_selected_channel_reserve_satoshis(100_000) * 1000;
 
 	let _ = create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 100_000, push_amt, InitFeatures::known(), InitFeatures::known());
 
 	// Sending exactly enough to hit the reserve amount should be accepted
-	let (_, _, _) = route_payment(&nodes[1], &[&nodes[0]], 1_000_000);
-	let (_, _, _) = route_payment(&nodes[1], &[&nodes[0]], 1_000_000);
-	let (_, _, _) = route_payment(&nodes[1], &[&nodes[0]], 1_000_000);
-	let (_, _, _) = route_payment(&nodes[1], &[&nodes[0]], 1_000_000);
+	for _ in 0..MIN_AFFORDABLE_HTLC_COUNT {
+		let (_, _, _) = route_payment(&nodes[1], &[&nodes[0]], 1_000_000);
+	}
 
 	// However one more HTLC should be significantly over the reserve amount and fail.
 	let (route, our_payment_hash, _, our_payment_secret) = get_route_and_payment_hash!(nodes[1], nodes[0], 1_000_000);
@@ -1429,15 +1428,14 @@ fn test_chan_reserve_violation_inbound_htlc_outbound_channel() {
 	// channel reserve violation (so their balance is channel reserve (1000 sats) + commitment
 	// transaction fee with 0 HTLCs (183 sats)).
 	let mut push_amt = 100_000_000;
-	push_amt -= commit_tx_fee_msat(feerate_per_kw, 4);
+	push_amt -= commit_tx_fee_msat(feerate_per_kw, MIN_AFFORDABLE_HTLC_COUNT as u64);
 	push_amt -= Channel::<EnforcingSigner>::get_holder_selected_channel_reserve_satoshis(100_000) * 1000;
 	let chan = create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 100_000, push_amt, InitFeatures::known(), InitFeatures::known());
 
 	// Send four HTLCs to cover the initial push_msat buffer we're required to include
-	let (_, _, _) = route_payment(&nodes[1], &[&nodes[0]], 1_000_000);
-	let (_, _, _) = route_payment(&nodes[1], &[&nodes[0]], 1_000_000);
-	let (_, _, _) = route_payment(&nodes[1], &[&nodes[0]], 1_000_000);
-	let (_, _, _) = route_payment(&nodes[1], &[&nodes[0]], 1_000_000);
+	for _ in 0..MIN_AFFORDABLE_HTLC_COUNT {
+		let (_, _, _) = route_payment(&nodes[1], &[&nodes[0]], 1_000_000);
+	}
 
 	let (route, payment_hash, _, payment_secret) = get_route_and_payment_hash!(nodes[1], nodes[0], 700_000);
 	// Need to manually create the update_add_htlc message to go around the channel reserve check in send_htlc()
@@ -1449,7 +1447,7 @@ fn test_chan_reserve_violation_inbound_htlc_outbound_channel() {
 	let onion_packet = onion_utils::construct_onion_packet(onion_payloads, onion_keys, [0; 32], &payment_hash);
 	let msg = msgs::UpdateAddHTLC {
 		channel_id: chan.2,
-		htlc_id: 4,
+		htlc_id: MIN_AFFORDABLE_HTLC_COUNT as u64,
 		amount_msat: htlc_msat,
 		payment_hash: payment_hash,
 		cltv_expiry: htlc_cltv,
@@ -1481,7 +1479,7 @@ fn test_chan_reserve_dust_inbound_htlcs_outbound_chan() {
 	// channel reserve violation (so their balance is channel reserve (1000 sats) + commitment
 	// transaction fee with 0 HTLCs (183 sats)).
 	let mut push_amt = 100_000_000;
-	push_amt -= commit_tx_fee_msat(feerate_per_kw, 4);
+	push_amt -= commit_tx_fee_msat(feerate_per_kw, MIN_AFFORDABLE_HTLC_COUNT as u64);
 	push_amt -= Channel::<EnforcingSigner>::get_holder_selected_channel_reserve_satoshis(100_000) * 1000;
 	create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 100000, push_amt, InitFeatures::known(), InitFeatures::known());
 
@@ -1493,10 +1491,9 @@ fn test_chan_reserve_dust_inbound_htlcs_outbound_chan() {
 	let (_, _, _) = route_payment(&nodes[1], &[&nodes[0]], dust_amt);
 
 	// Send four HTLCs to cover the initial push_msat buffer we're required to include
-	let (_, _, _) = route_payment(&nodes[1], &[&nodes[0]], 1_000_000);
-	let (_, _, _) = route_payment(&nodes[1], &[&nodes[0]], 1_000_000);
-	let (_, _, _) = route_payment(&nodes[1], &[&nodes[0]], 1_000_000);
-	let (_, _, _) = route_payment(&nodes[1], &[&nodes[0]], 1_000_000);
+	for _ in 0..MIN_AFFORDABLE_HTLC_COUNT {
+		let (_, _, _) = route_payment(&nodes[1], &[&nodes[0]], 1_000_000);
+	}
 
 	// One more than the dust amt should fail, however.
 	let (route, our_payment_hash, _, our_payment_secret) = get_route_and_payment_hash!(nodes[1], nodes[0], dust_amt + 1);