test(wallet): Add test_create_tx_increment_change_index

crates/wallet/tests/wallet.rs

@@ -1392,6 +1392,55 @@ fn test_create_tx_global_xpubs_with_origin() {
     assert_eq!(psbt.xpub.get(&key), Some(&(fingerprint, path)));
 }
 
+#[test]
+fn test_create_tx_increment_change_index() {
+    use coin_selection::Error;
+    let (desc, change_desc) = get_test_tr_single_sig_xprv_with_change_desc();
+    let (mut wallet, _) = get_funded_wallet_with_change(desc, change_desc);
+    let addr = wallet.next_unused_address(KeychainKind::External);
+    let internal_addr0 = wallet.next_unused_address(KeychainKind::Internal);
+    assert_eq!(internal_addr0.index, 0);
+
+    // try to send an amount that results in `InsufficientFunds` error
+    let amount = wallet.balance().total();
+    let mut builder = wallet.build_tx();
+    builder.add_recipient(addr.script_pubkey(), amount);
+    let err = builder.finish().unwrap_err();
+    assert!(matches!(
+        err,
+        CreateTxError::CoinSelection(Error::InsufficientFunds { .. }),
+    ));
+    assert_eq!(
+        wallet.next_unused_address(KeychainKind::Internal),
+        internal_addr0,
+        "create tx fail should not mark change address used",
+    );
+
+    // send all (no change) should not increment index
+    let fee = Amount::from_sat(150);
+    let mut builder = wallet.build_tx();
+    builder.add_recipient(addr.script_pubkey(), amount - fee);
+    let psbt = builder.finish().unwrap();
+    assert_eq!(psbt.unsigned_tx.output.len(), 1);
+    assert_eq!(
+        wallet.next_unused_address(KeychainKind::Internal),
+        internal_addr0,
+        "no change output should not mark change address used",
+    );
+
+    // create tx with change should increment index
+    let mut builder = wallet.build_tx();
+    builder.add_recipient(addr.script_pubkey(), amount / 2);
+    let psbt = builder.finish().unwrap();
+    assert_eq!(psbt.unsigned_tx.output.len(), 2);
+    let internal_addr1 = wallet.next_unused_address(KeychainKind::Internal);
+    assert_eq!(
+        internal_addr1.index, 1,
+        "internally derived drain output should mark change address used"
+    );
+    assert!(!wallet.mark_used(KeychainKind::Internal, 0))
+}
+
 #[test]
 fn test_add_foreign_utxo() {
     let (mut wallet1, _) = get_funded_wallet_wpkh();