Test the consistency of functions used to calculate the fees of htlcs

Specifically, we check that the slope `commit_tx_fee_sat` / `num_htlcs`
is consistent with `per_outbound_htlc_counterparty_commit_tx_fee_msat`.

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 +2430,35 @@ 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.
+		// `got`: floor(feerate * commit_tx_base_weight) + floor(num_htlcs * feerate * htlc_output_weight + num_htlcs * feerate * htlc_tx_weight)
+		// `want`: floor(feerate * commit_tx_base_weight + num_htlcs * feerate * htlc_output_weight) + floor(num_htlcs * feerate * htlc_tx_weight)
+		// At most, the difference between the two calculations is 1 satoshi, depending on how the num_htlcs * feerate * htlc_output_weight term
+		// adds with the two others.
+		//
+		// In the case of anchor channels, set `htlc_tx_weight` above to zero; the fees are exogenous.
+
+		let num_htlcs = 966; // BOLT #2 maximum number of htlcs on a commit tx
+
+		let features = ChannelTypeFeatures::only_static_remote_key();
+		// Tests the range 1sat/vb - 250sat/vb
+		for feerate in 253..62500 {
+			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;
+			let diff = u64::abs_diff(got, want);
+			assert!(diff <= 1, "assert failed for feerate: {}, absolute difference was: {}", feerate, diff);
+		}
+
+		let features = ChannelTypeFeatures::anchors_zero_htlc_fee_and_dependencies();
+		// Tests the range 1sat/vb - 250sat/vb
+		for feerate in 253..62500 {
+			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);
+			let diff = u64::checked_sub(want, got).expect("got should not be greater than want");
+			assert!(diff <= 1, "assert failed for feerate: {}, checked difference was: {}", feerate, diff);
+		}
+	}
 }