[rust-lightning] / lightning / src / events / bump_transaction.rs

// This file is Copyright its original authors, visible in version control
// history.
//
// This file is licensed under the Apache License, Version 2.0 <LICENSE-APACHE
// or http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your option.
// You may not use this file except in accordance with one or both of these
// licenses.

//! Utilities for bumping transactions originating from [`Event`]s.
//!
//! [`Event`]: crate::events::Event

use alloc::collections::BTreeMap;
use core::ops::Deref;

use crate::chain::chaininterface::{BroadcasterInterface, fee_for_weight};
use crate::chain::ClaimId;
use crate::io_extras::sink;
use crate::ln::channel::ANCHOR_OUTPUT_VALUE_SATOSHI;
use crate::ln::chan_utils;
use crate::ln::chan_utils::{
        ANCHOR_INPUT_WITNESS_WEIGHT, HTLC_SUCCESS_INPUT_ANCHOR_WITNESS_WEIGHT,
        HTLC_TIMEOUT_INPUT_ANCHOR_WITNESS_WEIGHT, HTLCOutputInCommitment
};
use crate::prelude::*;
use crate::sign::{
        ChannelDerivationParameters, HTLCDescriptor, SignerProvider, P2WPKH_WITNESS_WEIGHT
};
use crate::sign::ecdsa::{EcdsaChannelSigner, WriteableEcdsaChannelSigner};
use crate::sync::Mutex;
use crate::util::logger::Logger;

use bitcoin::{OutPoint, PubkeyHash, Sequence, ScriptBuf, Transaction, TxIn, TxOut, Witness, WPubkeyHash};
use bitcoin::blockdata::constants::WITNESS_SCALE_FACTOR;
use bitcoin::blockdata::locktime::absolute::LockTime;
use bitcoin::consensus::Encodable;
use bitcoin::secp256k1;
use bitcoin::secp256k1::Secp256k1;
use bitcoin::secp256k1::ecdsa::Signature;

const EMPTY_SCRIPT_SIG_WEIGHT: u64 = 1 /* empty script_sig */ * WITNESS_SCALE_FACTOR as u64;

const BASE_INPUT_SIZE: u64 = 32 /* txid */ + 4 /* vout */ + 4 /* sequence */;

const BASE_INPUT_WEIGHT: u64 = BASE_INPUT_SIZE * WITNESS_SCALE_FACTOR as u64;

/// A descriptor used to sign for a commitment transaction's anchor output.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct AnchorDescriptor {
        /// The parameters required to derive the signer for the anchor input.
        pub channel_derivation_parameters: ChannelDerivationParameters,
        /// The transaction input's outpoint corresponding to the commitment transaction's anchor
        /// output.
        pub outpoint: OutPoint,
}

impl AnchorDescriptor {
        /// Returns the UTXO to be spent by the anchor input, which can be obtained via
        /// [`Self::unsigned_tx_input`].
        pub fn previous_utxo(&self) -> TxOut {
                TxOut {
                        script_pubkey: self.witness_script().to_v0_p2wsh(),
                        value: ANCHOR_OUTPUT_VALUE_SATOSHI,
                }
        }

        /// Returns the unsigned transaction input spending the anchor output in the commitment
        /// transaction.
        pub fn unsigned_tx_input(&self) -> TxIn {
                TxIn {
                        previous_output: self.outpoint.clone(),
                        script_sig: ScriptBuf::new(),
                        sequence: Sequence::ENABLE_RBF_NO_LOCKTIME,
                        witness: Witness::new(),
                }
        }

        /// Returns the witness script of the anchor output in the commitment transaction.
        pub fn witness_script(&self) -> ScriptBuf {
                let channel_params = self.channel_derivation_parameters.transaction_parameters.as_holder_broadcastable();
                chan_utils::get_anchor_redeemscript(&channel_params.broadcaster_pubkeys().funding_pubkey)
        }

        /// Returns the fully signed witness required to spend the anchor output in the commitment
        /// transaction.
        pub fn tx_input_witness(&self, signature: &Signature) -> Witness {
                let channel_params = self.channel_derivation_parameters.transaction_parameters.as_holder_broadcastable();
                chan_utils::build_anchor_input_witness(&channel_params.broadcaster_pubkeys().funding_pubkey, signature)
        }

        /// Derives the channel signer required to sign the anchor input.
        pub fn derive_channel_signer<S: WriteableEcdsaChannelSigner, SP: Deref>(&self, signer_provider: &SP) -> S
        where
                SP::Target: SignerProvider<EcdsaSigner= S>
        {
                let mut signer = signer_provider.derive_channel_signer(
                        self.channel_derivation_parameters.value_satoshis,
                        self.channel_derivation_parameters.keys_id,
                );
                signer.provide_channel_parameters(&self.channel_derivation_parameters.transaction_parameters);
                signer
        }
}

/// Represents the different types of transactions, originating from LDK, to be bumped.
#[derive(Clone, Debug, PartialEq, Eq)]
pub enum BumpTransactionEvent {
        /// Indicates that a channel featuring anchor outputs is to be closed by broadcasting the local
        /// commitment transaction. Since commitment transactions have a static feerate pre-agreed upon,
        /// they may need additional fees to be attached through a child transaction using the popular
        /// [Child-Pays-For-Parent](https://bitcoinops.org/en/topics/cpfp) fee bumping technique. This
        /// child transaction must include the anchor input described within `anchor_descriptor` along
        /// with additional inputs to meet the target feerate. Failure to meet the target feerate
        /// decreases the confirmation odds of the transaction package (which includes the commitment
        /// and child anchor transactions), possibly resulting in a loss of funds. Once the transaction
        /// is constructed, it must be fully signed for and broadcast by the consumer of the event
        /// along with the `commitment_tx` enclosed. Note that the `commitment_tx` must always be
        /// broadcast first, as the child anchor transaction depends on it.
        ///
        /// The consumer should be able to sign for any of the additional inputs included within the
        /// child anchor transaction. To sign its anchor input, an [`EcdsaChannelSigner`] should be
        /// re-derived through [`AnchorDescriptor::derive_channel_signer`]. The anchor input signature
        /// can be computed with [`EcdsaChannelSigner::sign_holder_anchor_input`], which can then be
        /// provided to [`build_anchor_input_witness`] along with the `funding_pubkey` to obtain the
        /// full witness required to spend.
        ///
        /// It is possible to receive more than one instance of this event if a valid child anchor
        /// transaction is never broadcast or is but not with a sufficient fee to be mined. Care should
        /// be taken by the consumer of the event to ensure any future iterations of the child anchor
        /// transaction adhere to the [Replace-By-Fee
        /// rules](https://github.com/bitcoin/bitcoin/blob/master/doc/policy/mempool-replacements.md)
        /// for fee bumps to be accepted into the mempool, and eventually the chain. As the frequency of
        /// these events is not user-controlled, users may ignore/drop the event if they are no longer
        /// able to commit external confirmed funds to the child anchor transaction.
        ///
        /// The set of `pending_htlcs` on the commitment transaction to be broadcast can be inspected to
        /// determine whether a significant portion of the channel's funds are allocated to HTLCs,
        /// enabling users to make their own decisions regarding the importance of the commitment
        /// transaction's confirmation. Note that this is not required, but simply exists as an option
        /// for users to override LDK's behavior. On commitments with no HTLCs (indicated by those with
        /// an empty `pending_htlcs`), confirmation of the commitment transaction can be considered to
        /// be not urgent.
        ///
        /// [`EcdsaChannelSigner`]: crate::sign::ecdsa::EcdsaChannelSigner
        /// [`EcdsaChannelSigner::sign_holder_anchor_input`]: crate::sign::ecdsa::EcdsaChannelSigner::sign_holder_anchor_input
        /// [`build_anchor_input_witness`]: crate::ln::chan_utils::build_anchor_input_witness
        ChannelClose {
                /// The unique identifier for the claim of the anchor output in the commitment transaction.
                ///
                /// The identifier must map to the set of external UTXOs assigned to the claim, such that
                /// they can be reused when a new claim with the same identifier needs to be made, resulting
                /// in a fee-bumping attempt.
                claim_id: ClaimId,
                /// The target feerate that the transaction package, which consists of the commitment
                /// transaction and the to-be-crafted child anchor transaction, must meet.
                package_target_feerate_sat_per_1000_weight: u32,
                /// The channel's commitment transaction to bump the fee of. This transaction should be
                /// broadcast along with the anchor transaction constructed as a result of consuming this
                /// event.
                commitment_tx: Transaction,
                /// The absolute fee in satoshis of the commitment transaction. This can be used along the
                /// with weight of the commitment transaction to determine its feerate.
                commitment_tx_fee_satoshis: u64,
                /// The descriptor to sign the anchor input of the anchor transaction constructed as a
                /// result of consuming this event.
                anchor_descriptor: AnchorDescriptor,
                /// The set of pending HTLCs on the commitment transaction that need to be resolved once the
                /// commitment transaction confirms.
                pending_htlcs: Vec<HTLCOutputInCommitment>,
        },
        /// Indicates that a channel featuring anchor outputs has unilaterally closed on-chain by a
        /// holder commitment transaction and its HTLC(s) need to be resolved on-chain. With the
        /// zero-HTLC-transaction-fee variant of anchor outputs, the pre-signed HTLC
        /// transactions have a zero fee, thus requiring additional inputs and/or outputs to be attached
        /// for a timely confirmation within the chain. These additional inputs and/or outputs must be
        /// appended to the resulting HTLC transaction to meet the target feerate. Failure to meet the
        /// target feerate decreases the confirmation odds of the transaction, possibly resulting in a
        /// loss of funds. Once the transaction meets the target feerate, it must be signed for and
        /// broadcast by the consumer of the event.
        ///
        /// The consumer should be able to sign for any of the non-HTLC inputs added to the resulting
        /// HTLC transaction. To sign HTLC inputs, an [`EcdsaChannelSigner`] should be re-derived
        /// through [`HTLCDescriptor::derive_channel_signer`]. Each HTLC input's signature can be
        /// computed with [`EcdsaChannelSigner::sign_holder_htlc_transaction`], which can then be
        /// provided to [`HTLCDescriptor::tx_input_witness`] to obtain the fully signed witness required
        /// to spend.
        ///
        /// It is possible to receive more than one instance of this event if a valid HTLC transaction
        /// is never broadcast or is but not with a sufficient fee to be mined. Care should be taken by
        /// the consumer of the event to ensure any future iterations of the HTLC transaction adhere to
        /// the [Replace-By-Fee
        /// rules](https://github.com/bitcoin/bitcoin/blob/master/doc/policy/mempool-replacements.md)
        /// for fee bumps to be accepted into the mempool, and eventually the chain. As the frequency of
        /// these events is not user-controlled, users may ignore/drop the event if either they are no
        /// longer able to commit external confirmed funds to the HTLC transaction or the fee committed
        /// to the HTLC transaction is greater in value than the HTLCs being claimed.
        ///
        /// [`EcdsaChannelSigner`]: crate::sign::ecdsa::EcdsaChannelSigner
        /// [`EcdsaChannelSigner::sign_holder_htlc_transaction`]: crate::sign::ecdsa::EcdsaChannelSigner::sign_holder_htlc_transaction
        HTLCResolution {
                /// The unique identifier for the claim of the HTLCs in the confirmed commitment
                /// transaction.
                ///
                /// The identifier must map to the set of external UTXOs assigned to the claim, such that
                /// they can be reused when a new claim with the same identifier needs to be made, resulting
                /// in a fee-bumping attempt.
                claim_id: ClaimId,
                /// The target feerate that the resulting HTLC transaction must meet.
                target_feerate_sat_per_1000_weight: u32,
                /// The set of pending HTLCs on the confirmed commitment that need to be claimed, preferably
                /// by the same transaction.
                htlc_descriptors: Vec<HTLCDescriptor>,
                /// The locktime required for the resulting HTLC transaction.
                tx_lock_time: LockTime,
        },
}

/// An input that must be included in a transaction when performing coin selection through
/// [`CoinSelectionSource::select_confirmed_utxos`]. It is guaranteed to be a SegWit input, so it
/// must have an empty [`TxIn::script_sig`] when spent.
#[derive(Clone, Debug, Hash, PartialOrd, Ord, PartialEq, Eq)]
pub struct Input {
        /// The unique identifier of the input.
        pub outpoint: OutPoint,
        /// The UTXO being spent by the input.
        pub previous_utxo: TxOut,
        /// The upper-bound weight consumed by the input's full [`TxIn::script_sig`] and
        /// [`TxIn::witness`], each with their lengths included, required to satisfy the output's
        /// script.
        pub satisfaction_weight: u64,
}

/// An unspent transaction output that is available to spend resulting from a successful
/// [`CoinSelection`] attempt.
#[derive(Clone, Debug, Hash, PartialOrd, Ord, PartialEq, Eq)]
pub struct Utxo {
        /// The unique identifier of the output.
        pub outpoint: OutPoint,
        /// The output to spend.
        pub output: TxOut,
        /// The upper-bound weight consumed by the input's full [`TxIn::script_sig`] and [`TxIn::witness`], each
        /// with their lengths included, required to satisfy the output's script. The weight consumed by
        /// the input's `script_sig` must account for [`WITNESS_SCALE_FACTOR`].
        pub satisfaction_weight: u64,
}

impl Utxo {
        /// Returns a `Utxo` with the `satisfaction_weight` estimate for a legacy P2PKH output.
        pub fn new_p2pkh(outpoint: OutPoint, value: u64, pubkey_hash: &PubkeyHash) -> Self {
                let script_sig_size = 1 /* script_sig length */ +
                        1 /* OP_PUSH73 */ +
                        73 /* sig including sighash flag */ +
                        1 /* OP_PUSH33 */ +
                        33 /* pubkey */;
                Self {
                        outpoint,
                        output: TxOut {
                                value,
                                script_pubkey: ScriptBuf::new_p2pkh(pubkey_hash),
                        },
                        satisfaction_weight: script_sig_size * WITNESS_SCALE_FACTOR as u64 + 1 /* empty witness */,
                }
        }

        /// Returns a `Utxo` with the `satisfaction_weight` estimate for a P2WPKH nested in P2SH output.
        pub fn new_nested_p2wpkh(outpoint: OutPoint, value: u64, pubkey_hash: &WPubkeyHash) -> Self {
                let script_sig_size = 1 /* script_sig length */ +
                        1 /* OP_0 */ +
                        1 /* OP_PUSH20 */ +
                        20 /* pubkey_hash */;
                Self {
                        outpoint,
                        output: TxOut {
                                value,
                                script_pubkey: ScriptBuf::new_p2sh(&ScriptBuf::new_v0_p2wpkh(pubkey_hash).script_hash()),
                        },
                        satisfaction_weight: script_sig_size * WITNESS_SCALE_FACTOR as u64 + P2WPKH_WITNESS_WEIGHT,
                }
        }

        /// Returns a `Utxo` with the `satisfaction_weight` estimate for a SegWit v0 P2WPKH output.
        pub fn new_v0_p2wpkh(outpoint: OutPoint, value: u64, pubkey_hash: &WPubkeyHash) -> Self {
                Self {
                        outpoint,
                        output: TxOut {
                                value,
                                script_pubkey: ScriptBuf::new_v0_p2wpkh(pubkey_hash),
                        },
                        satisfaction_weight: EMPTY_SCRIPT_SIG_WEIGHT + P2WPKH_WITNESS_WEIGHT,
                }
        }
}

/// The result of a successful coin selection attempt for a transaction requiring additional UTXOs
/// to cover its fees.
#[derive(Clone, Debug)]
pub struct CoinSelection {
        /// The set of UTXOs (with at least 1 confirmation) to spend and use within a transaction
        /// requiring additional fees.
        pub confirmed_utxos: Vec<Utxo>,
        /// An additional output tracking whether any change remained after coin selection. This output
        /// should always have a value above dust for its given `script_pubkey`. It should not be
        /// spent until the transaction it belongs to confirms to ensure mempool descendant limits are
        /// not met. This implies no other party should be able to spend it except us.
        pub change_output: Option<TxOut>,
}

/// An abstraction over a bitcoin wallet that can perform coin selection over a set of UTXOs and can
/// sign for them. The coin selection method aims to mimic Bitcoin Core's `fundrawtransaction` RPC,
/// which most wallets should be able to satisfy. Otherwise, consider implementing [`WalletSource`],
/// which can provide a default implementation of this trait when used with [`Wallet`].
pub trait CoinSelectionSource {
        /// Performs coin selection of a set of UTXOs, with at least 1 confirmation each, that are
        /// available to spend. Implementations are free to pick their coin selection algorithm of
        /// choice, as long as the following requirements are met:
        ///
        /// 1. `must_spend` contains a set of [`Input`]s that must be included in the transaction
        ///    throughout coin selection, but must not be returned as part of the result.
        /// 2. `must_pay_to` contains a set of [`TxOut`]s that must be included in the transaction
        ///    throughout coin selection. In some cases, like when funding an anchor transaction, this
        ///    set is empty. Implementations should ensure they handle this correctly on their end,
        ///    e.g., Bitcoin Core's `fundrawtransaction` RPC requires at least one output to be
        ///    provided, in which case a zero-value empty OP_RETURN output can be used instead.
        /// 3. Enough inputs must be selected/contributed for the resulting transaction (including the
        ///    inputs and outputs noted above) to meet `target_feerate_sat_per_1000_weight`.
        ///
        /// Implementations must take note that [`Input::satisfaction_weight`] only tracks the weight of
        /// the input's `script_sig` and `witness`. Some wallets, like Bitcoin Core's, may require
        /// providing the full input weight. Failing to do so may lead to underestimating fee bumps and
        /// delaying block inclusion.
        ///
        /// The `claim_id` must map to the set of external UTXOs assigned to the claim, such that they
        /// can be re-used within new fee-bumped iterations of the original claiming transaction,
        /// ensuring that claims don't double spend each other. If a specific `claim_id` has never had a
        /// transaction associated with it, and all of the available UTXOs have already been assigned to
        /// other claims, implementations must be willing to double spend their UTXOs. The choice of
        /// which UTXOs to double spend is left to the implementation, but it must strive to keep the
        /// set of other claims being double spent to a minimum.
        fn select_confirmed_utxos(
                &self, claim_id: ClaimId, must_spend: Vec<Input>, must_pay_to: &[TxOut],
                target_feerate_sat_per_1000_weight: u32,
        ) -> Result<CoinSelection, ()>;
        /// Signs and provides the full witness for all inputs within the transaction known to the
        /// trait (i.e., any provided via [`CoinSelectionSource::select_confirmed_utxos`]).
        fn sign_tx(&self, tx: Transaction) -> Result<Transaction, ()>;
}

/// An alternative to [`CoinSelectionSource`] that can be implemented and used along [`Wallet`] to
/// provide a default implementation to [`CoinSelectionSource`].
pub trait WalletSource {
        /// Returns all UTXOs, with at least 1 confirmation each, that are available to spend.
        fn list_confirmed_utxos(&self) -> Result<Vec<Utxo>, ()>;
        /// Returns a script to use for change above dust resulting from a successful coin selection
        /// attempt.
        fn get_change_script(&self) -> Result<ScriptBuf, ()>;
        /// Signs and provides the full [`TxIn::script_sig`] and [`TxIn::witness`] for all inputs within
        /// the transaction known to the wallet (i.e., any provided via
        /// [`WalletSource::list_confirmed_utxos`]).
        fn sign_tx(&self, tx: Transaction) -> Result<Transaction, ()>;
}

/// A wrapper over [`WalletSource`] that implements [`CoinSelection`] by preferring UTXOs that would
/// avoid conflicting double spends. If not enough UTXOs are available to do so, conflicting double
/// spends may happen.
pub struct Wallet<W: Deref, L: Deref>
where
        W::Target: WalletSource,
        L::Target: Logger
{
        source: W,
        logger: L,
        // TODO: Do we care about cleaning this up once the UTXOs have a confirmed spend? We can do so
        // by checking whether any UTXOs that exist in the map are no longer returned in
        // `list_confirmed_utxos`.
        locked_utxos: Mutex<HashMap<OutPoint, ClaimId>>,
}

impl<W: Deref, L: Deref> Wallet<W, L>
where
        W::Target: WalletSource,
        L::Target: Logger
{
        /// Returns a new instance backed by the given [`WalletSource`] that serves as an implementation
        /// of [`CoinSelectionSource`].
        pub fn new(source: W, logger: L) -> Self {
                Self { source, logger, locked_utxos: Mutex::new(HashMap::new()) }
        }

        /// Performs coin selection on the set of UTXOs obtained from
        /// [`WalletSource::list_confirmed_utxos`]. Its algorithm can be described as "smallest
        /// above-dust-after-spend first", with a slight twist: we may skip UTXOs that are above dust at
        /// the target feerate after having spent them in a separate claim transaction if
        /// `force_conflicting_utxo_spend` is unset to avoid producing conflicting transactions. If
        /// `tolerate_high_network_feerates` is set, we'll attempt to spend UTXOs that contribute at
        /// least 1 satoshi at the current feerate, otherwise, we'll only attempt to spend those which
        /// contribute at least twice their fee.
        fn select_confirmed_utxos_internal(
                &self, utxos: &[Utxo], claim_id: ClaimId, force_conflicting_utxo_spend: bool,
                tolerate_high_network_feerates: bool, target_feerate_sat_per_1000_weight: u32,
                preexisting_tx_weight: u64, input_amount_sat: u64, target_amount_sat: u64,
        ) -> Result<CoinSelection, ()> {
                let mut locked_utxos = self.locked_utxos.lock().unwrap();
                let mut eligible_utxos = utxos.iter().filter_map(|utxo| {
                        if let Some(utxo_claim_id) = locked_utxos.get(&utxo.outpoint) {
                                if *utxo_claim_id != claim_id && !force_conflicting_utxo_spend {
                                        log_trace!(self.logger, "Skipping UTXO {} to prevent conflicting spend", utxo.outpoint);
                                        return None;
                                }
                        }
                        let fee_to_spend_utxo = fee_for_weight(
                                target_feerate_sat_per_1000_weight, BASE_INPUT_WEIGHT + utxo.satisfaction_weight,
                        );
                        let should_spend = if tolerate_high_network_feerates {
                                utxo.output.value > fee_to_spend_utxo
                        } else {
                                utxo.output.value >= fee_to_spend_utxo * 2
                        };
                        if should_spend {
                                Some((utxo, fee_to_spend_utxo))
                        } else {
                                log_trace!(self.logger, "Skipping UTXO {} due to dust proximity after spend", utxo.outpoint);
                                None
                        }
                }).collect::<Vec<_>>();
                eligible_utxos.sort_unstable_by_key(|(utxo, _)| utxo.output.value);

                let mut selected_amount = input_amount_sat;
                let mut total_fees = fee_for_weight(target_feerate_sat_per_1000_weight, preexisting_tx_weight);
                let mut selected_utxos = Vec::new();
                for (utxo, fee_to_spend_utxo) in eligible_utxos {
                        if selected_amount >= target_amount_sat + total_fees {
                                break;
                        }
                        selected_amount += utxo.output.value;
                        total_fees += fee_to_spend_utxo;
                        selected_utxos.push(utxo.clone());
                }
                if selected_amount < target_amount_sat + total_fees {
                        log_debug!(self.logger, "Insufficient funds to meet target feerate {} sat/kW",
                                target_feerate_sat_per_1000_weight);
                        return Err(());
                }
                for utxo in &selected_utxos {
                        locked_utxos.insert(utxo.outpoint, claim_id);
                }
                core::mem::drop(locked_utxos);

                let remaining_amount = selected_amount - target_amount_sat - total_fees;
                let change_script = self.source.get_change_script()?;
                let change_output_fee = fee_for_weight(
                        target_feerate_sat_per_1000_weight,
                        (8 /* value */ + change_script.consensus_encode(&mut sink()).unwrap() as u64) *
                                WITNESS_SCALE_FACTOR as u64,
                );
                let change_output_amount = remaining_amount.saturating_sub(change_output_fee);
                let change_output = if change_output_amount < change_script.dust_value().to_sat() {
                        log_debug!(self.logger, "Coin selection attempt did not yield change output");
                        None
                } else {
                        Some(TxOut { script_pubkey: change_script, value: change_output_amount })
                };

                Ok(CoinSelection {
                        confirmed_utxos: selected_utxos,
                        change_output,
                })
        }
}

impl<W: Deref, L: Deref> CoinSelectionSource for Wallet<W, L>
where
        W::Target: WalletSource,
        L::Target: Logger
{
        fn select_confirmed_utxos(
                &self, claim_id: ClaimId, must_spend: Vec<Input>, must_pay_to: &[TxOut],
                target_feerate_sat_per_1000_weight: u32,
        ) -> Result<CoinSelection, ()> {
                let utxos = self.source.list_confirmed_utxos()?;
                // TODO: Use fee estimation utils when we upgrade to bitcoin v0.30.0.
                const BASE_TX_SIZE: u64 = 4 /* version */ + 1 /* input count */ + 1 /* output count */ + 4 /* locktime */;
                let total_output_size: u64 = must_pay_to.iter().map(|output|
                        8 /* value */ + 1 /* script len */ + output.script_pubkey.len() as u64
                ).sum();
                let total_satisfaction_weight: u64 = must_spend.iter().map(|input| input.satisfaction_weight).sum();
                let total_input_weight = (BASE_INPUT_WEIGHT * must_spend.len() as u64) + total_satisfaction_weight;

                let preexisting_tx_weight = 2 /* segwit marker & flag */ + total_input_weight +
                        ((BASE_TX_SIZE + total_output_size) * WITNESS_SCALE_FACTOR as u64);
                let input_amount_sat: u64 = must_spend.iter().map(|input| input.previous_utxo.value).sum();
                let target_amount_sat = must_pay_to.iter().map(|output| output.value).sum();
                let do_coin_selection = |force_conflicting_utxo_spend: bool, tolerate_high_network_feerates: bool| {
                        log_debug!(self.logger, "Attempting coin selection targeting {} sat/kW (force_conflicting_utxo_spend = {}, tolerate_high_network_feerates = {})",
                                target_feerate_sat_per_1000_weight, force_conflicting_utxo_spend, tolerate_high_network_feerates);
                        self.select_confirmed_utxos_internal(
                                &utxos, claim_id, force_conflicting_utxo_spend, tolerate_high_network_feerates,
                                target_feerate_sat_per_1000_weight, preexisting_tx_weight, input_amount_sat, target_amount_sat,
                        )
                };
                do_coin_selection(false, false)
                        .or_else(|_| do_coin_selection(false, true))
                        .or_else(|_| do_coin_selection(true, false))
                        .or_else(|_| do_coin_selection(true, true))
        }

        fn sign_tx(&self, tx: Transaction) -> Result<Transaction, ()> {
                self.source.sign_tx(tx)
        }
}

/// A handler for [`Event::BumpTransaction`] events that sources confirmed UTXOs from a
/// [`CoinSelectionSource`] to fee bump transactions via Child-Pays-For-Parent (CPFP) or
/// Replace-By-Fee (RBF).
///
/// [`Event::BumpTransaction`]: crate::events::Event::BumpTransaction
pub struct BumpTransactionEventHandler<B: Deref, C: Deref, SP: Deref, L: Deref>
where
        B::Target: BroadcasterInterface,
        C::Target: CoinSelectionSource,
        SP::Target: SignerProvider,
        L::Target: Logger,
{
        broadcaster: B,
        utxo_source: C,
        signer_provider: SP,
        logger: L,
        secp: Secp256k1<secp256k1::All>,
}

impl<B: Deref, C: Deref, SP: Deref, L: Deref> BumpTransactionEventHandler<B, C, SP, L>
where
        B::Target: BroadcasterInterface,
        C::Target: CoinSelectionSource,
        SP::Target: SignerProvider,
        L::Target: Logger,
{
        /// Returns a new instance capable of handling [`Event::BumpTransaction`] events.
        ///
        /// [`Event::BumpTransaction`]: crate::events::Event::BumpTransaction
        pub fn new(broadcaster: B, utxo_source: C, signer_provider: SP, logger: L) -> Self {
                Self {
                        broadcaster,
                        utxo_source,
                        signer_provider,
                        logger,
                        secp: Secp256k1::new(),
                }
        }

        /// Updates a transaction with the result of a successful coin selection attempt.
        fn process_coin_selection(&self, tx: &mut Transaction, mut coin_selection: CoinSelection) {
                for utxo in coin_selection.confirmed_utxos.drain(..) {
                        tx.input.push(TxIn {
                                previous_output: utxo.outpoint,
                                script_sig: ScriptBuf::new(),
                                sequence: Sequence::ZERO,
                                witness: Witness::new(),
                        });
                }
                if let Some(change_output) = coin_selection.change_output.take() {
                        tx.output.push(change_output);
                } else if tx.output.is_empty() {
                        // We weren't provided a change output, likely because the input set was a perfect
                        // match, but we still need to have at least one output in the transaction for it to be
                        // considered standard. We choose to go with an empty OP_RETURN as it is the cheapest
                        // way to include a dummy output.
                        log_debug!(self.logger, "Including dummy OP_RETURN output since an output is needed and a change output was not provided");
                        tx.output.push(TxOut {
                                value: 0,
                                script_pubkey: ScriptBuf::new_op_return(&[]),
                        });
                }
        }

        /// Handles a [`BumpTransactionEvent::ChannelClose`] event variant by producing a fully-signed
        /// transaction spending an anchor output of the commitment transaction to bump its fee and
        /// broadcasts them to the network as a package.
        fn handle_channel_close(
                &self, claim_id: ClaimId, package_target_feerate_sat_per_1000_weight: u32,
                commitment_tx: &Transaction, commitment_tx_fee_sat: u64, anchor_descriptor: &AnchorDescriptor,
        ) -> Result<(), ()> {
                // Our commitment transaction already has fees allocated to it, so we should take them into
                // account. We do so by pretending the commitment tranasction's fee and weight are part of
                // the anchor input.
                let mut anchor_utxo = anchor_descriptor.previous_utxo();
                anchor_utxo.value += commitment_tx_fee_sat;
                let must_spend = vec![Input {
                        outpoint: anchor_descriptor.outpoint,
                        previous_utxo: anchor_utxo,
                        satisfaction_weight: commitment_tx.weight().to_wu() + ANCHOR_INPUT_WITNESS_WEIGHT + EMPTY_SCRIPT_SIG_WEIGHT,
                }];
                #[cfg(debug_assertions)]
                let must_spend_amount = must_spend.iter().map(|input| input.previous_utxo.value).sum::<u64>();

                log_debug!(self.logger, "Peforming coin selection for commitment package (commitment and anchor transaction) targeting {} sat/kW",
                        package_target_feerate_sat_per_1000_weight);
                let coin_selection = self.utxo_source.select_confirmed_utxos(
                        claim_id, must_spend, &[], package_target_feerate_sat_per_1000_weight,
                )?;

                let mut anchor_tx = Transaction {
                        version: 2,
                        lock_time: LockTime::ZERO, // TODO: Use next best height.
                        input: vec![anchor_descriptor.unsigned_tx_input()],
                        output: vec![],
                };

                #[cfg(debug_assertions)]
                let total_satisfaction_weight = ANCHOR_INPUT_WITNESS_WEIGHT + EMPTY_SCRIPT_SIG_WEIGHT +
                        coin_selection.confirmed_utxos.iter().map(|utxo| utxo.satisfaction_weight).sum::<u64>();
                #[cfg(debug_assertions)]
                let total_input_amount = must_spend_amount +
                        coin_selection.confirmed_utxos.iter().map(|utxo| utxo.output.value).sum::<u64>();

                self.process_coin_selection(&mut anchor_tx, coin_selection);
                let anchor_txid = anchor_tx.txid();

                debug_assert_eq!(anchor_tx.output.len(), 1);
                #[cfg(debug_assertions)]
                let unsigned_tx_weight = anchor_tx.weight().to_wu() - (anchor_tx.input.len() as u64 * EMPTY_SCRIPT_SIG_WEIGHT);

                log_debug!(self.logger, "Signing anchor transaction {}", anchor_txid);
                anchor_tx = self.utxo_source.sign_tx(anchor_tx)?;

                let signer = anchor_descriptor.derive_channel_signer(&self.signer_provider);
                let anchor_sig = signer.sign_holder_anchor_input(&anchor_tx, 0, &self.secp)?;
                anchor_tx.input[0].witness = anchor_descriptor.tx_input_witness(&anchor_sig);

                #[cfg(debug_assertions)] {
                        let signed_tx_weight = anchor_tx.weight().to_wu();
                        let expected_signed_tx_weight = unsigned_tx_weight + total_satisfaction_weight;
                        // Our estimate should be within a 1% error margin of the actual weight and we should
                        // never underestimate.
                        assert!(expected_signed_tx_weight >= signed_tx_weight &&
                                expected_signed_tx_weight - (expected_signed_tx_weight / 100) <= signed_tx_weight);

                        let expected_package_fee = fee_for_weight(package_target_feerate_sat_per_1000_weight,
                                signed_tx_weight + commitment_tx.weight().to_wu());
                        let package_fee = total_input_amount -
                                anchor_tx.output.iter().map(|output| output.value).sum::<u64>();
                        // Our fee should be within a 5% error margin of the expected fee based on the
                        // feerate and transaction weight and we should never pay less than required.
                        let fee_error_margin = expected_package_fee * 5 / 100;
                        assert!(package_fee >= expected_package_fee &&
                                package_fee - fee_error_margin <= expected_package_fee);
                }

                log_info!(self.logger, "Broadcasting anchor transaction {} to bump channel close with txid {}",
                        anchor_txid, commitment_tx.txid());
                self.broadcaster.broadcast_transactions(&[&commitment_tx, &anchor_tx]);
                Ok(())
        }

        /// Handles a [`BumpTransactionEvent::HTLCResolution`] event variant by producing a
        /// fully-signed, fee-bumped HTLC transaction that is broadcast to the network.
        fn handle_htlc_resolution(
                &self, claim_id: ClaimId, target_feerate_sat_per_1000_weight: u32,
                htlc_descriptors: &[HTLCDescriptor], tx_lock_time: LockTime,
        ) -> Result<(), ()> {
                let mut htlc_tx = Transaction {
                        version: 2,
                        lock_time: tx_lock_time,
                        input: vec![],
                        output: vec![],
                };
                let mut must_spend = Vec::with_capacity(htlc_descriptors.len());
                for htlc_descriptor in htlc_descriptors {
                        let htlc_input = htlc_descriptor.unsigned_tx_input();
                        must_spend.push(Input {
                                outpoint: htlc_input.previous_output.clone(),
                                previous_utxo: htlc_descriptor.previous_utxo(&self.secp),
                                satisfaction_weight: EMPTY_SCRIPT_SIG_WEIGHT + if htlc_descriptor.preimage.is_some() {
                                        HTLC_SUCCESS_INPUT_ANCHOR_WITNESS_WEIGHT
                                } else {
                                        HTLC_TIMEOUT_INPUT_ANCHOR_WITNESS_WEIGHT
                                },
                        });
                        htlc_tx.input.push(htlc_input);
                        let htlc_output = htlc_descriptor.tx_output(&self.secp);
                        htlc_tx.output.push(htlc_output);
                }

                log_debug!(self.logger, "Peforming coin selection for HTLC transaction targeting {} sat/kW",
                        target_feerate_sat_per_1000_weight);

                #[cfg(debug_assertions)]
                let must_spend_satisfaction_weight =
                        must_spend.iter().map(|input| input.satisfaction_weight).sum::<u64>();
                #[cfg(debug_assertions)]
                let must_spend_amount = must_spend.iter().map(|input| input.previous_utxo.value).sum::<u64>();

                let coin_selection = self.utxo_source.select_confirmed_utxos(
                        claim_id, must_spend, &htlc_tx.output, target_feerate_sat_per_1000_weight,
                )?;

                #[cfg(debug_assertions)]
                let total_satisfaction_weight = must_spend_satisfaction_weight +
                        coin_selection.confirmed_utxos.iter().map(|utxo| utxo.satisfaction_weight).sum::<u64>();
                #[cfg(debug_assertions)]
                let total_input_amount = must_spend_amount +
                        coin_selection.confirmed_utxos.iter().map(|utxo| utxo.output.value).sum::<u64>();

                self.process_coin_selection(&mut htlc_tx, coin_selection);

                #[cfg(debug_assertions)]
                let unsigned_tx_weight = htlc_tx.weight().to_wu() - (htlc_tx.input.len() as u64 * EMPTY_SCRIPT_SIG_WEIGHT);

                log_debug!(self.logger, "Signing HTLC transaction {}", htlc_tx.txid());
                htlc_tx = self.utxo_source.sign_tx(htlc_tx)?;

                let mut signers = BTreeMap::new();
                for (idx, htlc_descriptor) in htlc_descriptors.iter().enumerate() {
                        let signer = signers.entry(htlc_descriptor.channel_derivation_parameters.keys_id)
                                .or_insert_with(|| htlc_descriptor.derive_channel_signer(&self.signer_provider));
                        let htlc_sig = signer.sign_holder_htlc_transaction(&htlc_tx, idx, htlc_descriptor, &self.secp)?;
                        let witness_script = htlc_descriptor.witness_script(&self.secp);
                        htlc_tx.input[idx].witness = htlc_descriptor.tx_input_witness(&htlc_sig, &witness_script);
                }

                #[cfg(debug_assertions)] {
                        let signed_tx_weight = htlc_tx.weight().to_wu();
                        let expected_signed_tx_weight = unsigned_tx_weight + total_satisfaction_weight;
                        // Our estimate should be within a 1% error margin of the actual weight and we should
                        // never underestimate.
                        assert!(expected_signed_tx_weight >= signed_tx_weight &&
                                expected_signed_tx_weight - (expected_signed_tx_weight / 100) <= signed_tx_weight);

                        let expected_signed_tx_fee = fee_for_weight(target_feerate_sat_per_1000_weight, signed_tx_weight);
                        let signed_tx_fee = total_input_amount -
                                htlc_tx.output.iter().map(|output| output.value).sum::<u64>();
                        // Our fee should be within a 5% error margin of the expected fee based on the
                        // feerate and transaction weight and we should never pay less than required.
                        let fee_error_margin = expected_signed_tx_fee * 5 / 100;
                        assert!(signed_tx_fee >= expected_signed_tx_fee &&
                                signed_tx_fee - fee_error_margin <= expected_signed_tx_fee);
                }

                log_info!(self.logger, "Broadcasting {}", log_tx!(htlc_tx));
                self.broadcaster.broadcast_transactions(&[&htlc_tx]);
                Ok(())
        }

        /// Handles all variants of [`BumpTransactionEvent`].
        pub fn handle_event(&self, event: &BumpTransactionEvent) {
                match event {
                        BumpTransactionEvent::ChannelClose {
                                claim_id, package_target_feerate_sat_per_1000_weight, commitment_tx,
                                commitment_tx_fee_satoshis, anchor_descriptor, ..
                        } => {
                                log_info!(self.logger, "Handling channel close bump (claim_id = {}, commitment_txid = {})",
                                        log_bytes!(claim_id.0), commitment_tx.txid());
                                if let Err(_) = self.handle_channel_close(
                                        *claim_id, *package_target_feerate_sat_per_1000_weight, commitment_tx,
                                        *commitment_tx_fee_satoshis, anchor_descriptor,
                                ) {
                                        log_error!(self.logger, "Failed bumping commitment transaction fee for {}",
                                                commitment_tx.txid());
                                }
                        }
                        BumpTransactionEvent::HTLCResolution {
                                claim_id, target_feerate_sat_per_1000_weight, htlc_descriptors, tx_lock_time,
                        } => {
                                log_info!(self.logger, "Handling HTLC bump (claim_id = {}, htlcs_to_claim = {})",
                                        log_bytes!(claim_id.0), log_iter!(htlc_descriptors.iter().map(|d| d.outpoint())));
                                if let Err(_) = self.handle_htlc_resolution(
                                        *claim_id, *target_feerate_sat_per_1000_weight, htlc_descriptors, *tx_lock_time,
                                ) {
                                        log_error!(self.logger, "Failed bumping HTLC transaction fee for commitment {}",
                                                htlc_descriptors[0].commitment_txid);
                                }
                        }
                }
        }
}