f use const for witness flag bytes

Author: TheBlueMatt

lightning/src/util/transaction_utils.rs

@@ -36,6 +36,8 @@ pub fn sort_outputs<T, C : Fn(&T, &T) -> Ordering>(outputs: &mut Vec<(TxOut, T)>
 pub(crate) fn maybe_add_change_output(tx: &mut Transaction, input_value: u64, witness_max_weight: usize, feerate_sat_per_1000_weight: u32, change_destination_script: Script) -> Result<usize, ()> {
 	if input_value > MAX_VALUE_MSAT / 1000 { return Err(()); }
 
+	const WITNESS_FLAG_BYTES: i64 = 2;
+
 	let mut output_value = 0;
 	for output in tx.output.iter() {
 		output_value += output.value;
@@ -48,7 +50,7 @@ pub(crate) fn maybe_add_change_output(tx: &mut Transaction, input_value: u64, wi
 		value: 0,
 	};
 	let change_len = change_output.consensus_encode(&mut sink()).unwrap();
-	let mut weight_with_change: i64 = tx.get_weight() as i64 + 2 + witness_max_weight as i64 + change_len as i64 * 4;
+	let mut weight_with_change: i64 = tx.get_weight() as i64 + WITNESS_FLAG_BYTES + witness_max_weight as i64 + change_len as i64 * 4;
 	// Include any extra bytes required to push an extra output.
 	weight_with_change += (VarInt(tx.output.len() as u64 + 1).len() - VarInt(tx.output.len() as u64).len()) as i64 * 4;
 	// When calculating weight, add two for the flag bytes
@@ -60,7 +62,7 @@ pub(crate) fn maybe_add_change_output(tx: &mut Transaction, input_value: u64, wi
 	} else if (input_value - output_value) as i64 - (tx.get_weight() as i64 + 2 + witness_max_weight as i64) * feerate_sat_per_1000_weight as i64 / 1000 < 0 {
 		Err(())
 	} else {
-		Ok(tx.get_weight() + 2 + witness_max_weight)
+		Ok(tx.get_weight() + WITNESS_FLAG_BYTES as usize + witness_max_weight)
 	}
 }