feat(wallet): add method replace_tx for TxBuilder

- Add method `TxBuilder::previous_fee` for getting the previous
feerate of the replaced tx

crates/wallet/src/wallet/mod.rs

@@ -889,6 +889,21 @@ impl Wallet {
             .next()
     }
 
+    /// Get a local output if the txout referenced by `outpoint` exists on chain and can
+    /// be found in the inner tx graph.
+    fn get_output(&self, outpoint: OutPoint) -> Option<LocalOutput> {
+        let ((keychain, index), _) = self.indexed_graph.index.txout(outpoint)?;
+        self.indexed_graph
+            .graph()
+            .filter_chain_txouts(
+                &self.chain,
+                self.chain.tip().block_id(),
+                core::iter::once(((), outpoint)),
+            )
+            .map(|(_, full_txo)| new_local_utxo(keychain, index, full_txo))
+            .next()
+    }
+
     /// Inserts a [`TxOut`] at [`OutPoint`] into the wallet's transaction graph.
     ///
     /// This is used for providing a previous output's value so that we can use [`calculate_fee`]
@@ -1537,7 +1552,9 @@ impl Wallet {
     ///
     /// Returns an error if the transaction is already confirmed or doesn't explicitly signal
     /// *replace by fee* (RBF). If the transaction can be fee bumped then it returns a [`TxBuilder`]
-    /// pre-populated with the inputs and outputs of the original transaction.
+    /// pre-populated with the inputs and outputs of the original transaction. If you just
+    /// want to build a transaction that conflicts with the tx of the given `txid`, consider
+    /// using [`TxBuilder::replace_tx`].
     ///
     /// ## Example
     ///
@@ -2570,7 +2587,7 @@ macro_rules! floating_rate {
 /// Macro for getting a wallet for use in a doctest
 macro_rules! doctest_wallet {
     () => {{
-        use $crate::bitcoin::{BlockHash, Transaction, absolute, TxOut, Network, hashes::Hash};
+        use $crate::bitcoin::{transaction, Amount, BlockHash, Transaction, absolute, TxOut, Network, hashes::Hash};
         use $crate::chain::{ConfirmationBlockTime, BlockId, TxGraph, tx_graph};
         use $crate::{Update, KeychainKind, Wallet};
         use $crate::test_utils::*;

crates/wallet/src/wallet/tx_builder.rs

@@ -274,25 +274,29 @@ impl<'a, Cs> TxBuilder<'a, Cs> {
     /// These have priority over the "unspendable" utxos, meaning that if a utxo is present both in
     /// the "utxos" and the "unspendable" list, it will be spent.
     pub fn add_utxos(&mut self, outpoints: &[OutPoint]) -> Result<&mut Self, AddUtxoError> {
-        {
-            let wallet = &mut self.wallet;
-            let utxos = outpoints
-                .iter()
-                .map(|outpoint| {
-                    wallet
-                        .get_utxo(*outpoint)
-                        .ok_or(AddUtxoError::UnknownUtxo(*outpoint))
-                })
-                .collect::<Result<Vec<_>, _>>()?;
-
-            for utxo in utxos {
-                let descriptor = wallet.public_descriptor(utxo.keychain);
-                let satisfaction_weight = descriptor.max_weight_to_satisfy().unwrap();
-                self.params.utxos.push(WeightedUtxo {
-                    satisfaction_weight,
-                    utxo: Utxo::Local(utxo),
-                });
-            }
+        let wallet = &mut self.wallet;
+        let utxos = outpoints
+            .iter()
+            .map(|outpoint| {
+                wallet
+                    .get_utxo(*outpoint)
+                    .or_else(|| {
+                        // allow selecting a spent output if we're bumping fee
+                        self.params
+                            .bumping_fee
+                            .and_then(|_| wallet.get_output(*outpoint))
+                    })
+                    .ok_or(AddUtxoError::UnknownUtxo(*outpoint))
+            })
+            .collect::<Result<Vec<_>, _>>()?;
+
+        for utxo in utxos {
+            let descriptor = wallet.public_descriptor(utxo.keychain);
+            let satisfaction_weight = descriptor.max_weight_to_satisfy().unwrap();
+            self.params.utxos.push(WeightedUtxo {
+                satisfaction_weight,
+                utxo: Utxo::Local(utxo),
+            });
         }
 
         Ok(self)
@@ -306,6 +310,106 @@ impl<'a, Cs> TxBuilder<'a, Cs> {
         self.add_utxos(&[outpoint])
     }
 
+    /// Replace an unconfirmed transaction.
+    ///
+    /// This method attempts to create a replacement for the transaction with `txid` by
+    /// looking for the largest input that is owned by this wallet and adding it to the
+    /// list of UTXOs to spend.
+    ///
+    /// # Note
+    ///
+    /// Aside from reusing one of the inputs, the method makes no assumptions about the
+    /// structure of the replacement, so if you need to reuse the original recipient(s)
+    /// and/or change address, you should add them manually before [`finish`] is called.
+    ///
+    /// # Example
+    ///
+    /// Create a replacement for an unconfirmed wallet transaction
+    ///
+    /// ```rust,no_run
+    /// # let mut wallet = bdk_wallet::doctest_wallet!();
+    /// let wallet_txs = wallet.transactions().collect::<Vec<_>>();
+    /// let tx = wallet_txs.first().expect("must have wallet tx");
+    ///
+    /// if !tx.chain_position.is_confirmed() {
+    ///     let txid = tx.tx_node.txid;
+    ///     let mut builder = wallet.build_tx();
+    ///     builder.replace_tx(txid).expect("should replace");
+    ///
+    ///     // Continue building tx...
+    ///
+    ///     let psbt = builder.finish()?;
+    /// }
+    /// # Ok::<_, anyhow::Error>(())
+    /// ```
+    ///
+    /// # Errors
+    ///
+    /// - If the original transaction is not found in the tx graph
+    /// - If the orginal transaction is confirmed
+    /// - If none of the inputs are owned by this wallet
+    ///
+    /// [`finish`]: TxBuilder::finish
+    pub fn replace_tx(&mut self, txid: Txid) -> Result<&mut Self, ReplaceTxError> {
+        let tx = self
+            .wallet
+            .indexed_graph
+            .graph()
+            .get_tx(txid)
+            .ok_or(ReplaceTxError::MissingTransaction)?;
+        if self
+            .wallet
+            .transactions()
+            .find(|c| c.tx_node.txid == txid)
+            .map(|c| c.chain_position.is_confirmed())
+            .unwrap_or(false)
+        {
+            return Err(ReplaceTxError::TransactionConfirmed);
+        }
+        let outpoint = tx
+            .input
+            .iter()
+            .filter_map(|txin| {
+                let prev_tx = self
+                    .wallet
+                    .indexed_graph
+                    .graph()
+                    .get_tx(txin.previous_output.txid)?;
+                let txout = &prev_tx.output[txin.previous_output.vout as usize];
+                if self.wallet.is_mine(txout.script_pubkey.clone()) {
+                    Some((txin.previous_output, txout.value))
+                } else {
+                    None
+                }
+            })
+            .max_by_key(|(_, value)| *value)
+            .map(|(op, _)| op)
+            .ok_or(ReplaceTxError::NonReplaceable)?;
+
+        // add previous fee
+        if let Ok(absolute) = self.wallet.calculate_fee(&tx) {
+            let rate = absolute / tx.weight();
+            let previous_fee = PreviousFee { absolute, rate };
+            self.params.bumping_fee = Some(previous_fee);
+        }
+
+        self.add_utxo(outpoint).map_err(|e| match e {
+            AddUtxoError::UnknownUtxo(op) => ReplaceTxError::MissingOutput(op),
+        })?;
+
+        // do not try to spend the outputs of the tx being replaced
+        self.params
+            .unspendable
+            .extend((0..tx.output.len()).map(|vout| OutPoint::new(txid, vout as u32)));
+
+        Ok(self)
+    }
+
+    /// Get the previous feerate, i.e. the feerate of the tx being fee-bumped, if any.
+    pub fn previous_fee(&self) -> Option<FeeRate> {
+        self.params.bumping_fee.map(|p| p.rate)
+    }
+
     /// Add a foreign UTXO i.e. a UTXO not owned by this wallet.
     ///
     /// At a minimum to add a foreign UTXO we need:
@@ -697,6 +801,35 @@ impl fmt::Display for AddUtxoError {
 #[cfg(feature = "std")]
 impl std::error::Error for AddUtxoError {}
 
+/// Error returned by [`TxBuilder::replace_tx`].
+#[derive(Debug)]
+pub enum ReplaceTxError {
+    /// Unable to find a locally owned output
+    MissingOutput(OutPoint),
+    /// Transaction was not found in tx graph
+    MissingTransaction,
+    /// Transaction can't be replaced by this wallet
+    NonReplaceable,
+    /// Transaction is already confirmed
+    TransactionConfirmed,
+}
+
+impl fmt::Display for ReplaceTxError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::MissingOutput(op) => {
+                write!(f, "could not find wallet output for outpoint {}", op)
+            }
+            Self::MissingTransaction => write!(f, "transaction not found in tx graph"),
+            Self::NonReplaceable => write!(f, "no replaceable input found"),
+            Self::TransactionConfirmed => write!(f, "cannot replace a confirmed tx"),
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for ReplaceTxError {}
+
 #[derive(Debug)]
 /// Error returned from [`TxBuilder::add_foreign_utxo`].
 pub enum AddForeignUtxoError {

crates/wallet/tests/wallet.rs

@@ -4280,3 +4280,49 @@ fn test_wallet_transactions_relevant() {
     assert!(full_tx_count_before < full_tx_count_after);
     assert!(canonical_tx_count_before < canonical_tx_count_after);
 }
+
+#[test]
+fn replace_tx_allows_selecting_spent_inputs() {
+    let (mut wallet, txid_0) = get_funded_wallet_wpkh();
+    let outpoint_1 = OutPoint::new(txid_0, 0);
+
+    // receive output 2
+    let outpoint_2 = receive_output_in_latest_block(&mut wallet, 49_000);
+    assert_eq!(wallet.list_unspent().count(), 2);
+    assert_eq!(wallet.balance().total().to_sat(), 99_000);
+
+    // create tx1: 2-in/1-out sending all to `recip`
+    let recip = ScriptBuf::from_hex("0014446906a6560d8ad760db3156706e72e171f3a2aa").unwrap();
+    let mut builder = wallet.build_tx();
+    builder.add_recipient(recip.clone(), Amount::from_sat(98_800));
+    let psbt = builder.finish().unwrap();
+    let tx1 = psbt.unsigned_tx;
+    let txid1 = tx1.compute_txid();
+    insert_tx(&mut wallet, tx1);
+    assert!(wallet.list_unspent().next().is_none());
+
+    // now replace tx1 with a new transaction
+    let mut builder = wallet.build_tx();
+    builder.replace_tx(txid1).expect("should replace input");
+    let prev_feerate = builder.previous_fee().unwrap();
+    builder.add_recipient(recip, Amount::from_sat(98_500));
+    builder.fee_rate(FeeRate::from_sat_per_kwu(
+        prev_feerate.to_sat_per_kwu() + 250,
+    ));
+
+    // Because outpoint 2 was spent in tx1, by default it won't be available for selection,
+    // but we can add it manually, with the caveat that the builder is in a bump-fee
+    // context.
+    builder.add_utxo(outpoint_2).expect("should add output");
+    let psbt = builder.finish().unwrap();
+
+    let tx2 = psbt.unsigned_tx;
+    assert!(tx2
+        .input
+        .iter()
+        .any(|txin| txin.previous_output == outpoint_1));
+    assert!(tx2
+        .input
+        .iter()
+        .any(|txin| txin.previous_output == outpoint_2));
+}