add slope test

Author: tankyleo

lightning/src/ln/chan_utils.rs

@@ -1893,7 +1893,7 @@ pub fn get_commitment_transaction_number_obscure_factor(
 mod tests {
 	use super::{CounterpartyCommitmentSecrets, ChannelPublicKeys};
 	use crate::chain;
-	use crate::ln::chan_utils::{get_htlc_redeemscript, get_to_countersignatory_with_anchors_redeemscript, CommitmentTransaction, TxCreationKeys, ChannelTransactionParameters, CounterpartyChannelTransactionParameters, HTLCOutputInCommitment};
+	use crate::ln::chan_utils::{get_htlc_redeemscript, get_to_countersignatory_with_anchors_redeemscript, CommitmentTransaction, TxCreationKeys, ChannelTransactionParameters, CounterpartyChannelTransactionParameters, HTLCOutputInCommitment, commit_tx_fee_sat, htlc_success_tx_weight, per_outbound_htlc_counterparty_commit_tx_fee_msat};
 	use bitcoin::secp256k1::{PublicKey, SecretKey, Secp256k1};
 	use crate::util::test_utils;
 	use crate::sign::{ChannelSigner, SignerProvider};
@@ -2430,4 +2430,25 @@ mod tests {
 			assert!(monitor.provide_secret(281474976710648, secrets.last().unwrap().clone()).is_err());
 		}
 	}
+
+	#[test]
+	fn test_commit_tx_fee_sat_num_htlcs_slope_per_outbound_htlc_counterparty_commit_tx_fee_msat() {
+		// Test that the slope (commit_tx_fee_sat/num_htlcs) is consistent with per_outbound_htlc_counterparty_commit_tx_fee_msat
+		// We previously rounded down the per output fee in `per_outbound_htlc_counterparty_commit_tx_fee_msat` causing the discrepancy with (commit_tx_fee_sat/num_htlcs),
+		// this test guards against this behavior
+
+		let feerate = 267; // ~250sat/vb, some feerate that causes a big delta if the per output fee is rounded down, here 267 * 172 / 1000 = 45.924
+		let num_htlcs = 966; // BOLT #2 maximum number of htlcs on a commit tx
+
+		let features = ChannelTypeFeatures::anchors_zero_htlc_fee_and_dependencies();
+		let got = commit_tx_fee_sat(feerate, 0, &features)
+			+ per_outbound_htlc_counterparty_commit_tx_fee_msat(feerate, &features) * num_htlcs / 1000;
+		let want = commit_tx_fee_sat(feerate, num_htlcs as usize, &features);
+		assert_eq!(got, want);
+
+		let features = ChannelTypeFeatures::only_static_remote_key();
+		let got = commit_tx_fee_sat(feerate, 0, &features) + num_htlcs * per_outbound_htlc_counterparty_commit_tx_fee_msat(feerate, &features) / 1000;
+		let want = commit_tx_fee_sat(feerate, num_htlcs as usize, &features) + num_htlcs * feerate as u64 * htlc_success_tx_weight(&features) / 1000;
+		assert_eq!(got, want);
+	}
 }