Add a mandatory --change-address parameter to buy-xmr

Fixes #513.

CHANGELOG.md

@@ -14,6 +14,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Configuration setting for the websocket URL that the ASB connects to in order to receive price ticker updates.
   Can be configured manually by editing the config.toml file directly.
   It is expected that the server behind the url follows the same protocol as the [Kraken websocket api](https://docs.kraken.com/websockets/).
+- A mandatory `--change-address` parameter to the CLI's `buy-xmr` command.
+  The provided address is used to transfer Bitcoin in case of a refund and in case the user transfers more than the specified amount into the swap.
+  For more information see [#513](https://github.com/comit-network/xmr-btc-swap/issues/513).
 
 ### Fixed
 

swap/Cargo.toml

@@ -45,6 +45,7 @@ rust_decimal_macros = "1"
 serde = { version = "1", features = [ "derive" ] }
 serde_cbor = "0.11"
 serde_json = "1"
+serde_with = { version = "1", features = [ "macros" ] }
 sha2 = "0.9"
 sigma_fun = { git = "https://github.com/LLFourn/secp256kfun", default-features = false, features = [ "ed25519", "serde" ] }
 sled = "0.34"

swap/src/bin/asb.rs

@@ -215,7 +215,9 @@ async fn main() -> Result<()> {
                 }
             };
 
-            let psbt = bitcoin_wallet.send_to_address(address, amount).await?;
+            let psbt = bitcoin_wallet
+                .send_to_address(address, amount, None)
+                .await?;
             let signed_tx = bitcoin_wallet.sign_and_finalize(psbt).await?;
 
             bitcoin_wallet.broadcast(signed_tx, "withdraw").await?;

swap/src/bin/swap.rs

@@ -62,6 +62,7 @@ async fn main() -> Result<()> {
             seller,
             bitcoin_electrum_rpc_url,
             bitcoin_target_block,
+            bitcoin_change_address,
             monero_receive_address,
             monero_daemon_address,
             tor_socks5_port,
@@ -130,6 +131,7 @@ async fn main() -> Result<()> {
                 env_config,
                 event_loop_handle,
                 monero_receive_address,
+                bitcoin_change_address,
                 amount,
             );
 

swap/src/bitcoin/lock.rs

@@ -24,6 +24,7 @@ impl TxLock {
         amount: Amount,
         A: PublicKey,
         B: PublicKey,
+        change: bitcoin::Address,
     ) -> Result<Self>
     where
         C: EstimateFeeRate,
@@ -34,7 +35,9 @@ impl TxLock {
             .address(wallet.get_network())
             .expect("can derive address from descriptor");
 
-        let psbt = wallet.send_to_address(address, amount).await?;
+        let psbt = wallet
+            .send_to_address(address, amount, Some(change))
+            .await?;
 
         Ok(Self {
             inner: psbt,
@@ -251,7 +254,11 @@ mod tests {
         wallet: &Wallet<(), bdk::database::MemoryDatabase, StaticFeeRate>,
         amount: Amount,
     ) -> PartiallySignedTransaction {
-        TxLock::new(&wallet, amount, A, B).await.unwrap().into()
+        let change = wallet.new_address().await.unwrap();
+        TxLock::new(&wallet, amount, A, B, change)
+            .await
+            .unwrap()
+            .into()
     }
 
     fn alice_and_bob() -> (PublicKey, PublicKey) {

swap/src/bitcoin/wallet.rs

@@ -309,11 +309,18 @@ where
         &self,
         address: Address,
         amount: Amount,
+        change_override: Option<Address>,
     ) -> Result<PartiallySignedTransaction> {
         if self.network != address.network {
             bail!("Cannot build PSBT because network of given address is {} but wallet is on network {}", address.network, self.network);
         }
 
+        if let Some(change) = change_override.as_ref() {
+            if self.network != change.network {
+                bail!("Cannot build PSBT because network of given address is {} but wallet is on network {}", change.network, self.network);
+            }
+        }
+
         let wallet = self.wallet.lock().await;
         let client = self.client.lock().await;
         let fee_rate = client.estimate_feerate(self.target_block)?;
@@ -340,6 +347,15 @@ where
             _ => bail!("Unexpected transaction layout"),
         }
 
+        if let ([_, change], [_, psbt_output], Some(change_override)) = (
+            &mut psbt.global.unsigned_tx.output.as_mut_slice(),
+            &mut psbt.outputs.as_mut_slice(),
+            change_override,
+        ) {
+            change.script_pubkey = change_override.script_pubkey();
+            psbt_output.bip32_derivation.clear();
+        }
+
         Ok(psbt)
     }
 
@@ -1059,7 +1075,8 @@ mod tests {
         // if the change output is below dust it will be dropped by the BDK
         for amount in above_dust..(balance - (above_dust - 1)) {
             let (A, B) = (PublicKey::random(), PublicKey::random());
-            let txlock = TxLock::new(&wallet, bitcoin::Amount::from_sat(amount), A, B)
+            let change = wallet.new_address().await.unwrap();
+            let txlock = TxLock::new(&wallet, bitcoin::Amount::from_sat(amount), A, B, change)
                 .await
                 .unwrap();
             let txlock_output = txlock.script_pubkey();
@@ -1074,4 +1091,30 @@ mod tests {
             );
         }
     }
+
+    #[tokio::test]
+    async fn can_override_change_address() {
+        let wallet = Wallet::new_funded_default_fees(50_000);
+        let custom_change = "bcrt1q08pfqpsyrt7acllzyjm8q5qsz5capvyahm49rw"
+            .parse::<Address>()
+            .unwrap();
+
+        let psbt = wallet
+            .send_to_address(
+                wallet.new_address().await.unwrap(),
+                Amount::from_sat(10_000),
+                Some(custom_change.clone()),
+            )
+            .await
+            .unwrap();
+        let transaction = wallet.sign_and_finalize(psbt).await.unwrap();
+
+        match transaction.output.as_slice() {
+            [first, change] => {
+                assert_eq!(first.value, 10_000);
+                assert_eq!(change.script_pubkey, custom_change.script_pubkey());
+            }
+            _ => panic!("expected exactly two outputs"),
+        }
+    }
 }