Fix rounding bug when checking counterparty fees

Instead of calculating the fee for the entire weight contributed by the
counterparty, each portion of the weight was calculated individually
before summed. That caused a rounding error that is avoided by summing
the weights first.

Author: jkczyz

lightning/src/ln/interactivetxs.rs

@@ -915,20 +915,18 @@ impl NegotiationContext {
 	fn check_counterparty_fees(
 		&self, counterparty_fees_contributed: u64,
 	) -> Result<(), AbortReason> {
-		let counterparty_weight_contributed = self
+		let mut counterparty_weight_contributed = self
 			.remote_inputs_weight()
 			.to_wu()
 			.saturating_add(self.remote_outputs_weight().to_wu());
-		let mut required_counterparty_contribution_fee =
-			fee_for_weight(self.feerate_sat_per_kw, counterparty_weight_contributed);
 		if !self.holder_is_initiator {
 			// if is the non-initiator:
 			// 	- the initiator's fees do not cover the common fields (version, segwit marker + flag,
 			// 		input count, output count, locktime)
-			let tx_common_fields_fee =
-				fee_for_weight(self.feerate_sat_per_kw, TX_COMMON_FIELDS_WEIGHT);
-			required_counterparty_contribution_fee += tx_common_fields_fee;
+			counterparty_weight_contributed += TX_COMMON_FIELDS_WEIGHT;
 		}
+		let required_counterparty_contribution_fee =
+			fee_for_weight(self.feerate_sat_per_kw, counterparty_weight_contributed);
 		if counterparty_fees_contributed < required_counterparty_contribution_fee {
 			return Err(AbortReason::InsufficientFees);
 		}
@@ -2432,16 +2430,12 @@ mod tests {
 			outputs_b: vec![],
 			expect_error: Some((AbortReason::InsufficientFees, ErrorCulprit::NodeA)),
 		});
-		let p2wpkh_fee = fee_for_weight(TEST_FEERATE_SATS_PER_KW, P2WPKH_INPUT_WEIGHT_LOWER_BOUND);
-		let outputs_fee = fee_for_weight(
-			TEST_FEERATE_SATS_PER_KW,
-			get_output_weight(&generate_p2wsh_script_pubkey()).to_wu(),
-		);
-		let tx_common_fields_fee =
-			fee_for_weight(TEST_FEERATE_SATS_PER_KW, TX_COMMON_FIELDS_WEIGHT);
-
-		let amount_adjusted_with_p2wpkh_fee =
-			1_000_000 - p2wpkh_fee - outputs_fee - tx_common_fields_fee + 1;
+		let outputs_weight = get_output_weight(&generate_p2wsh_script_pubkey()).to_wu();
+		let amount_adjusted_with_p2wpkh_fee = 1_000_000
+			- fee_for_weight(
+				TEST_FEERATE_SATS_PER_KW,
+				P2WPKH_INPUT_WEIGHT_LOWER_BOUND + TX_COMMON_FIELDS_WEIGHT + outputs_weight,
+			);
 		do_test_interactive_tx_constructor(TestSession {
 			description: "Single contribution, with P2WPKH input, insufficient fees",
 			inputs_a: generate_inputs(&[TestOutput::P2WPKH(1_000_000)]),
@@ -2473,9 +2467,11 @@ mod tests {
 			outputs_b: vec![],
 			expect_error: None,
 		});
-		let p2wsh_fee = fee_for_weight(TEST_FEERATE_SATS_PER_KW, P2WSH_INPUT_WEIGHT_LOWER_BOUND);
-		let amount_adjusted_with_p2wsh_fee =
-			1_000_000 - p2wsh_fee - outputs_fee - tx_common_fields_fee + 1;
+		let amount_adjusted_with_p2wsh_fee = 1_000_000
+			- fee_for_weight(
+				TEST_FEERATE_SATS_PER_KW,
+				P2WSH_INPUT_WEIGHT_LOWER_BOUND + TX_COMMON_FIELDS_WEIGHT + outputs_weight,
+			);
 		do_test_interactive_tx_constructor(TestSession {
 			description: "Single contribution, with P2WSH input, insufficient fees",
 			inputs_a: generate_inputs(&[TestOutput::P2WSH(1_000_000)]),
@@ -2507,9 +2503,11 @@ mod tests {
 			outputs_b: vec![],
 			expect_error: None,
 		});
-		let p2tr_fee = fee_for_weight(TEST_FEERATE_SATS_PER_KW, P2TR_INPUT_WEIGHT_LOWER_BOUND);
-		let amount_adjusted_with_p2tr_fee =
-			1_000_000 - p2tr_fee - outputs_fee - tx_common_fields_fee;
+		let amount_adjusted_with_p2tr_fee = 1_000_000
+			- fee_for_weight(
+				TEST_FEERATE_SATS_PER_KW,
+				P2TR_INPUT_WEIGHT_LOWER_BOUND + TX_COMMON_FIELDS_WEIGHT + outputs_weight,
+			);
 		do_test_interactive_tx_constructor(TestSession {
 			description: "Single contribution, with P2TR input, insufficient fees",
 			inputs_a: generate_inputs(&[TestOutput::P2TR(1_000_000)]),