Merge #1812: fix(wallet): use map_keychain in Wallet::build_fee_bump

515309e test(wallet): test `build_fee_bump` with single descriptor wallet (valued mammal)
53d16e1 fix(wallet): use `map_keychain` in `Wallet::build_fee_bump` (valued mammal)

Pull request description:

  Previously we failed to remove the change output if the wallet has no internal keychain which caused tx building to fail at the new higher feerate. Fix this by mapping the internal keychain to the de-facto change keychain so that the drain output can be recalculated.

  fixes #1807

  ### Changelog notice

  Fixed

  - wallet: Fixed an issue preventing `build_fee_bump` from re-targeting the drain value for some wallets

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### Bugfixes:

  * [x] I've added tests to reproduce the issue which are now passing
  * [x] I'm linking the issue being fixed by this PR

ACKs for top commit:
  notmandatory:
    ACK 515309e
  evanlinjin:
    ACK 515309e

Tree-SHA512: 53266c4b0f4a2ebc1fd47cd1c9afb4499fb3c46d75e8bd121a98644cfe879cebf5fa26fab0e6af706452aea79a959660149582dc8df9e53ada6582e62ea5be19

Author: ValuedMammal

crates/wallet/src/wallet/mod.rs

@@ -1676,7 +1676,7 @@ impl Wallet {
         if tx.output.len() > 1 {
             let mut change_index = None;
             for (index, txout) in tx.output.iter().enumerate() {
-                let change_keychain = KeychainKind::Internal;
+                let change_keychain = self.map_keychain(KeychainKind::Internal);
                 match txout_index.index_of_spk(txout.script_pubkey.clone()) {
                     Some((keychain, _)) if *keychain == change_keychain => {
                         change_index = Some(index)

crates/wallet/src/wallet/tx_builder.rs

@@ -1052,4 +1052,50 @@ mod test {
         assert_eq!(filtered.len(), 1);
         assert_eq!(filtered[0].keychain, KeychainKind::Internal);
     }
+
+    #[test]
+    fn test_build_fee_bump_remove_change_output_single_desc() {
+        use crate::test_utils::*;
+        use bdk_chain::BlockId;
+        use bitcoin::{hashes::Hash, BlockHash, Network};
+
+        let mut wallet = Wallet::create_single(get_test_tr_single_sig())
+            .network(Network::Regtest)
+            .create_wallet_no_persist()
+            .unwrap();
+
+        insert_checkpoint(
+            &mut wallet,
+            BlockId {
+                height: 1,
+                hash: BlockHash::all_zeros(),
+            },
+        );
+
+        receive_output_in_latest_block(&mut wallet, Amount::ONE_BTC.to_sat());
+
+        // tx1 sending 15k sat to a recipient
+        let recip = ScriptBuf::from_hex(
+            "5120e8f5c4dc2f5d6a7595e7b108cb063da9c7550312da1e22875d78b9db62b59cd5",
+        )
+        .unwrap();
+        let mut builder = wallet.build_tx();
+        builder.add_recipient(recip.clone(), Amount::from_sat(15_000));
+        builder.fee_absolute(Amount::from_sat(1_000));
+        let psbt = builder.finish().unwrap();
+
+        let tx = psbt.extract_tx().unwrap();
+        let txid = tx.compute_txid();
+        let feerate = wallet.calculate_fee_rate(&tx).unwrap().to_sat_per_kwu();
+        insert_tx(&mut wallet, tx);
+
+        // build fee bump
+        let mut builder = wallet.build_fee_bump(txid).unwrap();
+        assert_eq!(
+            builder.params.recipients,
+            vec![(recip, Amount::from_sat(15_000))]
+        );
+        builder.fee_rate(FeeRate::from_sat_per_kwu(feerate + 250));
+        let _ = builder.finish().unwrap();
+    }
 }