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

Fixes #513.

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"),
+        }
+    }
 }