test(wallet): Add test change index for create tx fail

crates/wallet/tests/wallet.rs

@@ -4203,3 +4203,33 @@ fn single_descriptor_wallet_can_create_tx_and_receive_change() {
         "tx change should go to external keychain"
     );
 }
+
+#[test]
+fn create_tx_fail_should_not_increment_change_index() {
+    // check we only mark a change address used if tx creation
+    // succeeds. in other words failure to create a tx for example due
+    // to insufficient funds should not affect the unused status of
+    // a change address.
+    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 change_index = wallet.next_unused_address(KeychainKind::Internal).index;
+    assert_eq!(change_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).index,
+        change_index,
+        "failed tx should not increase change address index"
+    );
+}