Account for grind_signatures in splice funding tx

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.

Author: jkczyz

lightning/src/ln/channel.rs

@@ -6547,6 +6547,11 @@ 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;
+			}
 		}
 	}
 
@@ -17427,7 +17432,7 @@ mod tests {
 		// splice initiator
 		assert_eq!(
 			estimate_v2_funding_transaction_fee(&one_input, &[], true, true, 2000),
-			1740,
+			1738,
 		);
 
 		// splice acceptor
@@ -17468,7 +17473,7 @@ mod tests {
 				true,
 				2000,
 			).unwrap(),
-			2284,
+			2282,
 		);
 
 		// negative case, inputs clearly insufficient
@@ -17484,13 +17489,13 @@ mod tests {
 			);
 			assert_eq!(
 				res.err().unwrap(),
-				"Total input amount 100000 is lower than needed for contribution 220000, considering fees of 1740. Need more inputs.",
+				"Total input amount 100000 is lower than needed for contribution 220000, considering fees of 1738. Need more inputs.",
 			);
 		}
 
 		// barely covers
 		{
-			let expected_fee: u64 = 2284;
+			let expected_fee: u64 = 2282;
 			assert_eq!(
 				check_v2_funding_inputs_sufficient(
 					(300_000 - expected_fee - 20) as i64,
@@ -17520,7 +17525,7 @@ mod tests {
 			);
 			assert_eq!(
 				res.err().unwrap(),
-				"Total input amount 300000 is lower than needed for contribution 298032, considering fees of 2513. Need more inputs.",
+				"Total input amount 300000 is lower than needed for contribution 298032, considering fees of 2511. Need more inputs.",
 			);
 		}
 

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)
 			},
 		}
 	}
@@ -2314,6 +2322,11 @@ 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;
+			}
 		}
 	}