Account for grind_signatures in splice funding tx

jkczyz · TheBlueMatt · commit 714777567be2 · 2025-11-12T13:02:43.000Z
When estimating the splice funding transaction fees, adjust for grind_signatures. Since LDK supplies one of the signatures, only adjust by 1 WU even though spending the shared input requires two signatures. Backport of 4698026
diff --git a/lightning/src/ln/chan_utils.rs b/lightning/src/ln/chan_utils.rs
@@ -127,6 +127,9 @@ const MULTISIG_SCRIPT_SIZE: u64 = 1 + // OP_2
 /// Unlike in the [spec], 72 WU is used for the max signature size since 73 WU signatures are
 /// non-standard.
 ///
+/// Note: If you have the `grind_signatures` feature enabled, this will be at least 1 byte
+/// shorter.
+///
 /// [spec]: https://github.com/lightning/bolts/blob/master/03-transactions.md#expected-weight-of-the-commitment-transaction
 pub const FUNDING_TRANSACTION_WITNESS_WEIGHT: u64 = 1 + // number_of_witness_elements
 	1 + // nil_len
diff --git a/lightning/src/ln/channel.rs b/lightning/src/ln/channel.rs
@@ -6561,6 +6561,11 @@ fn estimate_v2_funding_transaction_fee(
 				.saturating_add(BASE_INPUT_WEIGHT)
 				.saturating_add(EMPTY_SCRIPT_SIG_WEIGHT)
 				.saturating_add(FUNDING_TRANSACTION_WITNESS_WEIGHT);
+			#[cfg(feature = "grind_signatures")]
+			{
+				// Guarantees a low R signature
+				weight -= 1;
+			}
 		}
 	}
 
@@ -17413,19 +17418,19 @@ mod tests {
 		// 2 inputs, initiator, 2000 sat/kw feerate
 		assert_eq!(
 			estimate_v2_funding_transaction_fee(&two_inputs, &[], true, false, 2000),
-			1516,
+			if cfg!(feature = "grind_signatures") { 1512 } else { 1516 },
 		);
 
 		// higher feerate
 		assert_eq!(
 			estimate_v2_funding_transaction_fee(&two_inputs, &[], true, false, 3000),
-			2274,
+			if cfg!(feature = "grind_signatures") { 2268 } else { 2274 },
 		);
 
 		// only 1 input
 		assert_eq!(
 			estimate_v2_funding_transaction_fee(&one_input, &[], true, false, 2000),
-			972,
+			if cfg!(feature = "grind_signatures") { 970 } else { 972 },
 		);
 
 		// 0 inputs
@@ -17443,13 +17448,13 @@ mod tests {
 		// splice initiator
 		assert_eq!(
 			estimate_v2_funding_transaction_fee(&one_input, &[], true, true, 2000),
-			1740,
+			if cfg!(feature = "grind_signatures") { 1736 } else { 1740 },
 		);
 
 		// splice acceptor
 		assert_eq!(
 			estimate_v2_funding_transaction_fee(&one_input, &[], false, true, 2000),
-			544,
+			if cfg!(feature = "grind_signatures") { 542 } else { 544 },
 		);
 	}
 
@@ -17473,40 +17478,46 @@ mod tests {
 		use crate::ln::channel::check_v2_funding_inputs_sufficient;
 
 		// positive case, inputs well over intended contribution
-		assert_eq!(
-			check_v2_funding_inputs_sufficient(
-				220_000,
-				&[
-					funding_input_sats(200_000),
-					funding_input_sats(100_000),
-				],
-				true,
-				true,
-				2000,
-			).unwrap(),
-			2284,
-		);
+		{
+			let expected_fee = if cfg!(feature = "grind_signatures") { 2278 } else { 2284 };
+			assert_eq!(
+				check_v2_funding_inputs_sufficient(
+					220_000,
+					&[
+						funding_input_sats(200_000),
+						funding_input_sats(100_000),
+					],
+					true,
+					true,
+					2000,
+				).unwrap(),
+				expected_fee,
+			);
+		}
 
 		// negative case, inputs clearly insufficient
 		{
-			let res = check_v2_funding_inputs_sufficient(
-				220_000,
-				&[
-					funding_input_sats(100_000),
-				],
-				true,
-				true,
-				2000,
-			);
+			let expected_fee = if cfg!(feature = "grind_signatures") { 1736 } else { 1740 };
 			assert_eq!(
-				res.err().unwrap(),
-				"Total input amount 100000 is lower than needed for contribution 220000, considering fees of 1740. Need more inputs.",
+				check_v2_funding_inputs_sufficient(
+					220_000,
+					&[
+						funding_input_sats(100_000),
+					],
+					true,
+					true,
+					2000,
+				),
+				Err(format!(
+					"Total input amount 100000 is lower than needed for contribution 220000, considering fees of {}. Need more inputs.",
+					expected_fee,
+				)),
 			);
 		}
 
 		// barely covers
 		{
-			let expected_fee: u64 = 2284;
+			let expected_fee = if cfg!(feature = "grind_signatures") { 2278 } else { 2284 };
 			assert_eq!(
 				check_v2_funding_inputs_sufficient(
 					(300_000 - expected_fee - 20) as i64,
@@ -17524,25 +17535,28 @@ mod tests {
 
 		// higher fee rate, does not cover
 		{
-			let res = check_v2_funding_inputs_sufficient(
-				298032,
-				&[
-					funding_input_sats(200_000),
-					funding_input_sats(100_000),
-				],
-				true,
-				true,
-				2200,
-			);
+			let expected_fee = if cfg!(feature = "grind_signatures") { 2506 } else { 2513 };
 			assert_eq!(
-				res.err().unwrap(),
-				"Total input amount 300000 is lower than needed for contribution 298032, considering fees of 2513. Need more inputs.",
+				check_v2_funding_inputs_sufficient(
+					298032,
+					&[
+						funding_input_sats(200_000),
+						funding_input_sats(100_000),
+					],
+					true,
+					true,
+					2200,
+				),
+				Err(format!(
+					"Total input amount 300000 is lower than needed for contribution 298032, considering fees of {}. Need more inputs.",
+					expected_fee
+				)),
 			);
 		}
 
-		// barely covers, less fees (no extra weight, no init)
+		// barely covers, less fees (no extra weight, not initiator)
 		{
-			let expected_fee: u64 = 1088;
+			let expected_fee = if cfg!(feature = "grind_signatures") { 1084 } else { 1088 };
 			assert_eq!(
 				check_v2_funding_inputs_sufficient(
 					(300_000 - expected_fee - 20) as i64,
diff --git a/lightning/src/ln/funding.rs b/lightning/src/ln/funding.rs
@@ -143,7 +143,13 @@ impl FundingTxInput {
 	/// [`TxIn::sequence`]: bitcoin::TxIn::sequence
 	/// [`set_sequence`]: Self::set_sequence
 	pub fn new_p2wpkh(prevtx: Transaction, vout: u32) -> Result<Self, ()> {
-		let witness_weight = Weight::from_wu(P2WPKH_WITNESS_WEIGHT);
+		let witness_weight = Weight::from_wu(P2WPKH_WITNESS_WEIGHT)
+			- if cfg!(feature = "grind_signatures") {
+				// Guarantees a low R signature
+				Weight::from_wu(1)
+			} else {
+				Weight::ZERO
+			};
 		FundingTxInput::new(prevtx, vout, witness_weight, Script::is_p2wpkh)
 	}
 
diff --git a/lightning/src/ln/interactivetxs.rs b/lightning/src/ln/interactivetxs.rs
@@ -1706,7 +1706,15 @@ impl InputOwned {
 			InputOwned::Single(single) => single.satisfaction_weight,
 			// TODO(taproot): Needs to consider different weights based on channel type
 			InputOwned::Shared(_) => {
-				Weight::from_wu(EMPTY_SCRIPT_SIG_WEIGHT + FUNDING_TRANSACTION_WITNESS_WEIGHT)
+				let mut weight = 0;
+				weight += EMPTY_SCRIPT_SIG_WEIGHT + FUNDING_TRANSACTION_WITNESS_WEIGHT;
+				#[cfg(feature = "grind_signatures")]
+				{
+					// Guarantees a low R signature
+					weight -= 1;
+				}
+
+				Weight::from_wu(weight)
 			},
 		}
 	}
@@ -2312,6 +2320,11 @@ pub(super) fn calculate_change_output_value(
 			weight = weight.saturating_add(BASE_INPUT_WEIGHT);
 			weight = weight.saturating_add(EMPTY_SCRIPT_SIG_WEIGHT);
 			weight = weight.saturating_add(FUNDING_TRANSACTION_WITNESS_WEIGHT);
+			#[cfg(feature = "grind_signatures")]
+			{
+				// Guarantees a low R signature
+				weight -= 1;
+			}
 		}
 	}
 
@@ -3357,7 +3370,11 @@ mod tests {
 
 		let total_inputs: Amount = input_prevouts.iter().map(|o| o.value).sum();
 		let total_outputs: Amount = outputs.iter().map(|o| o.value).sum();
-		let fees = Amount::from_sat(1740);
+		let fees = if cfg!(feature = "grind_signatures") {
+			Amount::from_sat(1734)
+		} else {
+			Amount::from_sat(1740)
+		};
 		let common_fees = Amount::from_sat(234);
 
 		// There is leftover for change
diff --git a/lightning/src/sign/mod.rs b/lightning/src/sign/mod.rs
@@ -117,7 +117,8 @@ pub struct DelayedPaymentOutputDescriptor {
 
 impl DelayedPaymentOutputDescriptor {
 	/// The maximum length a well-formed witness spending one of these should have.
-	/// Note: If you have the grind_signatures feature enabled, this will be at least 1 byte
+	///
+	/// Note: If you have the `grind_signatures` feature enabled, this will be at least 1 byte
 	/// shorter.
 	pub const MAX_WITNESS_LENGTH: u64 = (1 /* witness items */
 		+ 1 /* sig push */
@@ -198,7 +199,8 @@ impl StaticPaymentOutputDescriptor {
 	}
 
 	/// The maximum length a well-formed witness spending one of these should have.
-	/// Note: If you have the grind_signatures feature enabled, this will be at least 1 byte
+	///
+	/// Note: If you have the `grind_signatures` feature enabled, this will be at least 1 byte
 	/// shorter.
 	pub fn max_witness_length(&self) -> u64 {
 		if self.needs_csv_1_for_spend() {
diff --git a/lightning/src/util/test_utils.rs b/lightning/src/util/test_utils.rs
@@ -2220,10 +2220,16 @@ impl TestWalletSource {
 					utxo.output.value,
 					EcdsaSighashType::All,
 				)?;
+				#[cfg(not(feature = "grind_signatures"))]
 				let signature = self.secp.sign_ecdsa(
 					&secp256k1::Message::from_digest(sighash.to_byte_array()),
 					&self.secret_key,
 				);
+				#[cfg(feature = "grind_signatures")]
+				let signature = self.secp.sign_ecdsa_low_r(
+					&secp256k1::Message::from_digest(sighash.to_byte_array()),
+					&self.secret_key,
+				);
 				let bitcoin_sig =
 					bitcoin::ecdsa::Signature { signature, sighash_type: EcdsaSighashType::All };
 				tx.input[i].witness =
