Struct TxBuilder

pub struct TxBuilder<'a, Cs> { /* private fields */ }

A transaction builder

A TxBuilder is created by calling build_tx or build_fee_bump on a wallet. After assigning it, you set options on it until finally calling finish to consume the builder and generate the transaction.

Each option setting method on TxBuilder takes and returns &mut self so you can chain calls as in the following example:

// chaining
let psbt1 = {
    let mut builder = wallet.build_tx();
    builder
        .ordering(TxOrdering::Untouched)
        .add_recipient(addr1.script_pubkey(), Amount::from_sat(50_000))
        .add_recipient(addr2.script_pubkey(), Amount::from_sat(50_000));
    builder.finish()?
};

// non-chaining
let psbt2 = {
    let mut builder = wallet.build_tx();
    builder.ordering(TxOrdering::Untouched);
    for addr in &[addr1, addr2] {
        builder.add_recipient(addr.script_pubkey(), Amount::from_sat(50_000));
    }
    builder.finish()?
};

assert_eq!(psbt1.unsigned_tx.output[..2], psbt2.unsigned_tx.output[..2]);

At the moment coin_selection is an exception to the rule as it consumes self. This means it is usually best to call coin_selection on the return value of build_tx before assigning it.

For further examples see this module’s documentation;

Implementations

impl<'a, Cs> TxBuilder<'a, Cs>

pub fn fee_rate(&mut self, fee_rate: FeeRate) -> &mut Self

Set a custom fee rate.

This method sets the mining fee paid by the transaction as a rate on its size. This means that the total fee paid is equal to fee_rate times the size of the transaction. Default is 1 sat/vB in accordance with Bitcoin Core’s default relay policy.

Note that this is really a minimum feerate – it’s possible to overshoot it slightly since adding a change output to drain the remaining excess might not be viable.

pub fn fee_absolute(&mut self, fee_amount: Amount) -> &mut Self

Set an absolute fee The fee_absolute method refers to the absolute transaction fee in [Amount]. If anyone sets both the fee_absolute method and the fee_rate method, the FeePolicy enum will be set by whichever method was called last, as the [FeeRate] and FeeAmount are mutually exclusive.

Note that this is really a minimum absolute fee – it’s possible to overshoot it slightly since adding a change output to drain the remaining excess might not be viable.

pub fn policy_path( &mut self, policy_path: BTreeMap<String, Vec<usize>>, keychain: KeychainKind, ) -> &mut Self

Set the policy path to use while creating the transaction for a given keychain.

This method accepts a map where the key is the policy node id (see Policy::id) and the value is the list of the indexes of the items that are intended to be satisfied from the policy node (see SatisfiableItem::Thresh::items).

Example

An example of when the policy path is needed is the following descriptor: wsh(thresh(2,pk(A),sj:and_v(v:pk(B),n:older(6)),snj:and_v(v:pk(C),after(630000)))), derived from the miniscript policy thresh(2,pk(A),and(pk(B),older(6)),and(pk(C),after(630000))). It declares three descriptor fragments, and at the top level it uses thresh() to ensure that at least two of them are satisfied. The individual fragments are:

1. pk(A)
2. and(pk(B),older(6))
3. and(pk(C),after(630000))

When those conditions are combined in pairs, it’s clear that the transaction needs to be created differently depending on how the user intends to satisfy the policy afterwards:

• If fragments 1 and 2 are used, the transaction will need to use a specific n_sequence in order to spend an OP_CSV branch.
• If fragments 1 and 3 are used, the transaction will need to use a specific locktime in order to spend an OP_CLTV branch.
• If fragments 2 and 3 are used, the transaction will need both.

When the spending policy is represented as a tree (see Wallet::policies), every node is assigned a unique identifier that can be used in the policy path to specify which of the node’s children the user intends to satisfy: for instance, assuming the thresh() root node of this example has an id of aabbccdd, the policy path map would look like:

{ "aabbccdd" => [0, 1] }

where the key is the node’s id, and the value is a list of the children that should be used, in no particular order.

If a particularly complex descriptor has multiple ambiguous thresholds in its structure, multiple entries can be added to the map, one for each node that requires an explicit path.

Address::from_str("2N4eQYCbKUHCCTUjBJeHcJp9ok6J2GZsTDt")
    .unwrap()
    .assume_checked();
let mut path = BTreeMap::new();
path.insert("aabbccdd".to_string(), vec![0, 1]);

let builder = wallet
    .build_tx()
    .add_recipient(to_address.script_pubkey(), Amount::from_sat(50_000))
    .policy_path(path, KeychainKind::External);

pub fn add_utxos( &mut self, outpoints: &[OutPoint], ) -> Result<&mut Self, AddUtxoError>

Add the list of outpoints to the internal list of UTXOs that must be spent.

If an error occurs while adding any of the UTXOs then none of them are added and the error is returned.

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_utxo( &mut self, outpoint: OutPoint, ) -> Result<&mut Self, AddUtxoError>

Add a utxo to the internal list of utxos that must be spent

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_foreign_utxo( &mut self, outpoint: OutPoint, psbt_input: Input, satisfaction_weight: Weight, ) -> Result<&mut Self, AddForeignUtxoError>

Add a foreign UTXO i.e. a UTXO not owned by this wallet.

At a minimum to add a foreign UTXO we need:

1. outpoint: To add it to the raw transaction.
2. psbt_input: To know the value.
3. satisfaction_weight: To know how much weight/vbytes the input will add to the transaction for fee calculation.

There are several security concerns about adding foreign UTXOs that application developers should consider. First, how do you know the value of the input is correct? If a non_witness_utxo is provided in the psbt_input then this method implicitly verifies the value by checking it against the transaction. If only a witness_utxo is provided then this method doesn’t verify the value but just takes it as a given – it is up to you to check that whoever sent you the input_psbt was not lying!

Secondly, you must somehow provide satisfaction_weight of the input. Depending on your application it may be important that this be known precisely. If not, a malicious counterparty may fool you into putting in a value that is too low, giving the transaction a lower than expected feerate. They could also fool you into putting a value that is too high causing you to pay a fee that is too high. The party who is broadcasting the transaction can of course check the real input weight matches the expected weight prior to broadcasting.

To guarantee the max_weight_to_satisfy is correct, you can require the party providing the psbt_input provide a miniscript descriptor for the input so you can check it against the script_pubkey and then ask it for the max_weight_to_satisfy.

This is an EXPERIMENTAL feature, API and other major changes are expected.

In order to use Wallet::calculate_fee or Wallet::calculate_fee_rate for a transaction created with foreign UTXO(s) you must manually insert the corresponding TxOut(s) into the tx graph using the Wallet::insert_txout function.

Errors

This method returns errors in the following circumstances:

1. The psbt_input does not contain a witness_utxo or non_witness_utxo.
2. The data in non_witness_utxo does not match what is in outpoint.

Note unless you set only_witness_utxo any non-taproot psbt_input you pass to this method must have non_witness_utxo set otherwise you will get an error when finish is called.

pub fn add_foreign_utxo_with_sequence( &mut self, outpoint: OutPoint, psbt_input: Input, satisfaction_weight: Weight, sequence: Sequence, ) -> Result<&mut Self, AddForeignUtxoError>

Same as add_foreign_utxo but allows to set the nSequence value.

pub fn manually_selected_only(&mut self) -> &mut Self

Only spend utxos added by add_utxo.

The wallet will not add additional utxos to the transaction even if they are needed to make the transaction valid.

pub fn unspendable(&mut self, unspendable: Vec<OutPoint>) -> &mut Self

Replace the internal list of unspendable utxos with a new list

It’s important to note that the “must-be-spent” utxos added with TxBuilder::add_utxo have priority over these. See the docs of the two linked methods for more details.

pub fn add_unspendable(&mut self, unspendable: OutPoint) -> &mut Self

Add a utxo to the internal list of unspendable utxos

It’s important to note that the “must-be-spent” utxos added with TxBuilder::add_utxo have priority over this. See the docs of the two linked methods for more details.

pub fn sighash(&mut self, sighash: PsbtSighashType) -> &mut Self

Sign with a specific sig hash

Use this option very carefully

pub fn ordering(&mut self, ordering: TxOrdering) -> &mut Self

Choose the ordering for inputs and outputs of the transaction

pub fn nlocktime(&mut self, locktime: LockTime) -> &mut Self

Use a specific nLockTime while creating the transaction

This can cause conflicts if the wallet’s descriptors contain an “after” (OP_CLTV) operator.

pub fn version(&mut self, version: i32) -> &mut Self

Build a transaction with a specific version

The version should always be greater than 0 and greater than 1 if the wallet’s descriptors contain an “older” (OP_CSV) operator.

pub fn do_not_spend_change(&mut self) -> &mut Self

Do not spend change outputs

This effectively adds all the change outputs to the “unspendable” list. See TxBuilder::unspendable. This method assumes the presence of an internal keychain, otherwise it has no effect.

pub fn only_spend_change(&mut self) -> &mut Self

Only spend change outputs

This effectively adds all the non-change outputs to the “unspendable” list. See TxBuilder::unspendable. This method assumes the presence of an internal keychain, otherwise it has no effect.

pub fn change_policy(&mut self, change_policy: ChangeSpendPolicy) -> &mut Self

Set a specific ChangeSpendPolicy. See TxBuilder::do_not_spend_change and TxBuilder::only_spend_change for some shortcuts. This method assumes the presence of an internal keychain, otherwise it has no effect.

pub fn only_witness_utxo(&mut self) -> &mut Self

Only Fill-in the psbt::Input::witness_utxo field when spending from SegWit descriptors.

This reduces the size of the PSBT, but some signers might reject them due to the lack of the non_witness_utxo.

pub fn include_output_redeem_witness_script(&mut self) -> &mut Self

Fill-in the psbt::Output::redeem_script and psbt::Output::witness_script fields.

This is useful for signers which always require it, like ColdCard hardware wallets.

pub fn add_global_xpubs(&mut self) -> &mut Self

Fill-in the PSBT_GLOBAL_XPUB field with the extended keys contained in both the external and internal descriptors

This is useful for offline signers that take part to a multisig. Some hardware wallets like BitBox and ColdCard are known to require this.

pub fn drain_wallet(&mut self) -> &mut Self

Spend all the available inputs. This respects filters like TxBuilder::unspendable and the change policy.

pub fn coin_selection<P: CoinSelectionAlgorithm>( self, coin_selection: P, ) -> TxBuilder<'a, P>

Choose the coin selection algorithm

Overrides the CoinSelectionAlgorithm.

Note that this function consumes the builder and returns it so it is usually best to put this as the first call on the builder.

pub fn set_exact_sequence(&mut self, n_sequence: Sequence) -> &mut Self

Set an exact nSequence value

This can cause conflicts if the wallet’s descriptors contain an “older” (OP_CSV) operator and the given nsequence is lower than the CSV value.

pub fn current_height(&mut self, height: u32) -> &mut Self

Set the current blockchain height.

This will be used to:

1. Set the nLockTime for preventing fee sniping. Note: This will be ignored if you manually specify a nlocktime using TxBuilder::nlocktime.
2. Decide whether coinbase outputs are mature or not. If the coinbase outputs are not mature at current_height, we ignore them in the coin selection. If you want to create a transaction that spends immature coinbase inputs, manually add them using TxBuilder::add_utxos.

In both cases, if you don’t provide a current height, we use the last sync height.

pub fn allow_dust(&mut self, allow_dust: bool) -> &mut Self

Set whether or not the dust limit is checked.

Note: by avoiding a dust limit check you may end up with a transaction that is non-standard.

pub fn set_recipients( &mut self, recipients: Vec<(ScriptBuf, Amount)>, ) -> &mut Self

Replace the recipients already added with a new list

pub fn add_recipient( &mut self, script_pubkey: ScriptBuf, amount: Amount, ) -> &mut Self

Add a recipient to the internal list

pub fn add_data<T: AsRef<PushBytes>>(&mut self, data: &T) -> &mut Self

Add data as an output, using OP_RETURN

pub fn drain_to(&mut self, script_pubkey: ScriptBuf) -> &mut Self

Sets the address to drain excess coins to.

Usually, when there are excess coins they are sent to a change address generated by the wallet. This option replaces the usual change address with an arbitrary script_pubkey of your choosing. Just as with a change output, if the drain output is not needed (the excess coins are too small) it will not be included in the resulting transaction. The only difference is that it is valid to use drain_to without setting any ordinary recipients with add_recipient (but it is perfectly fine to add recipients as well).

If you choose not to set any recipients, you should provide the utxos that the transaction should spend via add_utxos.

Example

drain_to is very useful for draining all the coins in a wallet with drain_wallet to a single address.

Address::from_str("2N4eQYCbKUHCCTUjBJeHcJp9ok6J2GZsTDt")
    .unwrap()
    .assume_checked();
let mut tx_builder = wallet.build_tx();

tx_builder
    // Spend all outputs in this wallet.
    .drain_wallet()
    // Send the excess (which is all the coins minus the fee) to this address.
    .drain_to(to_address.script_pubkey())
    .fee_rate(FeeRate::from_sat_per_vb(5).expect("valid feerate"));
let psbt = tx_builder.finish()?;

impl<Cs: CoinSelectionAlgorithm> TxBuilder<'_, Cs>

pub fn finish(self) -> Result<Psbt, CreateTxError>

Finish building the transaction.

Uses the thread-local random number generator (rng).

Returns a new [Psbt] per BIP174.

WARNING: To avoid change address reuse you must persist the changes resulting from one or more calls to this method before closing the wallet. See Wallet::reveal_next_address.

pub fn finish_with_aux_rand( self, rng: &mut impl RngCore, ) -> Result<Psbt, CreateTxError>

Finish building the transaction.

Uses a provided random number generator (rng).

Returns a new [Psbt] per BIP174.

WARNING: To avoid change address reuse you must persist the changes resulting from one or more calls to this method before closing the wallet. See Wallet::reveal_next_address.

Trait Implementations

impl<'a, Cs: Debug> Debug for TxBuilder<'a, Cs>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.