Fix off-by-one max witness estimate for P2WPKH StaticPaymentDescriptor

We were not accounting for the extra byte denoting the number of items
in the witness stack.

Author: wpaulino

lightning/src/events/bump_transaction.rs

@@ -26,7 +26,7 @@ use crate::ln::chan_utils::{
 use crate::ln::features::ChannelTypeFeatures;
 use crate::ln::PaymentPreimage;
 use crate::prelude::*;
-use crate::sign::{EcdsaChannelSigner, SignerProvider, WriteableEcdsaChannelSigner};
+use crate::sign::{EcdsaChannelSigner, SignerProvider, WriteableEcdsaChannelSigner, P2WPKH_WITNESS_WEIGHT};
 use crate::sync::Mutex;
 use crate::util::logger::Logger;
 
@@ -384,12 +384,6 @@ pub struct Utxo {
 }
 
 impl Utxo {
-	const P2WPKH_WITNESS_WEIGHT: u64 = 1 /* num stack items */ +
-		1 /* sig length */ +
-		73 /* sig including sighash flag */ +
-		1 /* pubkey length */ +
-		33 /* pubkey */;
-
 	/// Returns a `Utxo` with the `satisfaction_weight` estimate for a legacy P2PKH output.
 	pub fn new_p2pkh(outpoint: OutPoint, value: u64, pubkey_hash: &PubkeyHash) -> Self {
 		let script_sig_size = 1 /* script_sig length */ +
@@ -419,7 +413,7 @@ impl Utxo {
 				value,
 				script_pubkey: Script::new_p2sh(&Script::new_v0_p2wpkh(pubkey_hash).script_hash()),
 			},
-			satisfaction_weight: script_sig_size * WITNESS_SCALE_FACTOR as u64 + Self::P2WPKH_WITNESS_WEIGHT,
+			satisfaction_weight: script_sig_size * WITNESS_SCALE_FACTOR as u64 + P2WPKH_WITNESS_WEIGHT,
 		}
 	}
 
@@ -431,7 +425,7 @@ impl Utxo {
 				value,
 				script_pubkey: Script::new_v0_p2wpkh(pubkey_hash),
 			},
-			satisfaction_weight: EMPTY_SCRIPT_SIG_WEIGHT + Self::P2WPKH_WITNESS_WEIGHT,
+			satisfaction_weight: EMPTY_SCRIPT_SIG_WEIGHT + P2WPKH_WITNESS_WEIGHT,
 		}
 	}
 }

lightning/src/sign/mod.rs

@@ -107,6 +107,12 @@ impl_writeable_tlv_based!(DelayedPaymentOutputDescriptor, {
 	(12, channel_value_satoshis, required),
 });
 
+pub(crate) const P2WPKH_WITNESS_WEIGHT: u64 = 1 /* num stack items */ +
+	1 /* sig length */ +
+	73 /* sig including sighash flag */ +
+	1 /* pubkey length */ +
+	33 /* pubkey */;
+
 /// Information about a spendable output to our "payment key".
 ///
 /// See [`SpendableOutputDescriptor::StaticPaymentOutput`] for more details on how to spend this.
@@ -141,9 +147,7 @@ impl StaticPaymentOutputDescriptor {
 			1 /* num witness items */ + 1 /* sig push */ + 73 /* sig including sighash flag */ +
 				1 /* witness script push */ + witness_script_weight
 		} else {
-			// Calculated as 1 byte legnth + 73 byte signature, 1 byte empty vec push, 1 byte length plus
-			// redeemscript push length.
-			1 + 73 + 34
+			P2WPKH_WITNESS_WEIGHT as usize
 		}
 	}
 }