f use new change-add utility instead of inline code

Author: TheBlueMatt

lightning/src/chain/keysinterface.rs

@@ -14,8 +14,6 @@
 use bitcoin::blockdata::transaction::{Transaction, TxOut, TxIn, SigHashType};
 use bitcoin::blockdata::script::{Script, Builder};
 use bitcoin::blockdata::opcodes;
-use bitcoin::consensus::Encodable;
-use bitcoin::consensus::encode::VarInt;
 use bitcoin::network::constants::Network;
 use bitcoin::util::bip32::{ExtendedPrivKey, ExtendedPubKey, ChildNumber};
 use bitcoin::util::bip143;
@@ -30,7 +28,7 @@ use bitcoin::secp256k1::key::{SecretKey, PublicKey};
 use bitcoin::secp256k1::{Secp256k1, Signature, Signing};
 use bitcoin::secp256k1;
 
-use util::byte_utils;
+use util::{byte_utils, transaction_utils};
 use util::ser::{Writeable, Writer, Readable};
 
 use chain::transaction::OutPoint;
@@ -891,6 +889,8 @@ impl KeysManager {
 	/// Returns `Err(())` if the output value is greater than the input value minus required fee or
 	/// if a descriptor was duplicated.
 	///
+	/// We do not enforce that outputs meet the dust limit or that any output scripts are standard.
+	///
 	/// May panic if the `SpendableOutputDescriptor`s were not generated by Channels which used
 	/// this KeysManager or one of the `InMemoryChannelKeys` created by this KeysManager.
 	pub fn spend_spendable_outputs<C: Signing>(&self, descriptors: &[SpendableOutputDescriptor], outputs: Vec<TxOut>, change_destination_script: Script, feerate_sat_per_1000_weight: u32, secp_ctx: &Secp256k1<C>) -> Result<Transaction, ()> {
@@ -936,36 +936,13 @@ impl KeysManager {
 			}
 			if input_value > MAX_VALUE_MSAT / 1000 { return Err(()); }
 		}
-		let mut output_value = 0;
-		for output in outputs.iter() {
-			output_value += output.value;
-			if output_value >= input_value { return Err(()); }
-		}
 		let mut spend_tx = Transaction {
 			version: 2,
 			lock_time: 0,
 			input,
 			output: outputs,
 		};
-
-		let mut change_output = TxOut {
-			script_pubkey: change_destination_script,
-			value: 0,
-		};
-		let change_len = change_output.consensus_encode(&mut std::io::sink()).unwrap();
-		let mut weight_with_change: i64 = spend_tx.get_weight() as i64 + 2 + witness_weight as i64 + change_len as i64 * 4;
-		// Include any extra bytes required to push an extra output.
-		weight_with_change += (VarInt(spend_tx.output.len() as u64 + 1).len() - VarInt(spend_tx.output.len() as u64).len()) as i64 * 4;
-		// When calculating weight, add two for the flag bytes
-		let change_value: i64 = (input_value - output_value) as i64 - weight_with_change * feerate_sat_per_1000_weight as i64 / 1000;
-		if change_value > 0 {
-			change_output.value = change_value as u64;
-			spend_tx.output.push(change_output);
-		} else if (input_value - output_value) as i64 - (spend_tx.get_weight() as i64 + 2 + witness_weight as i64) * feerate_sat_per_1000_weight as i64 / 1000 < 0 {
-			return Err(());
-		}
-
-		spend_tx.output[0].value -= (spend_tx.get_weight() + 2 + witness_weight + 3) as u64 * feerate_sat_per_1000_weight as u64 / 1000;
+		transaction_utils::maybe_add_change_output(&mut spend_tx, input_value, witness_weight, feerate_sat_per_1000_weight, change_destination_script)?;
 
 		let mut keys_cache: Option<(InMemoryChannelKeys, (u64, u64))> = None;
 		let mut input_idx = 0;