16183445888

Committed 10 Jul 2025 12:49AM UTC coverage: 57.64% (+0.03%) from 57.611%

Build # 16183445888

Build Type

Pull #10060

github

Committed by

web-flow

Commit Message

Merge f652051f3 into 0e830da9d

Pull Request Pull Request #10060: sweep: fix expected spending events being missed

Run Details

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87.11

/sweep/fee_bumper.go

package sweep

import (
        "errors"
        "fmt"
        "sync"
        "sync/atomic"
        "time"

        "github.com/btcsuite/btcd/btcutil"
        "github.com/btcsuite/btcd/chaincfg/chainhash"
        "github.com/btcsuite/btcd/rpcclient"
        "github.com/btcsuite/btcd/txscript"
        "github.com/btcsuite/btcd/wire"
        "github.com/btcsuite/btcwallet/chain"
        "github.com/lightningnetwork/lnd/chainio"
        "github.com/lightningnetwork/lnd/chainntnfs"
        "github.com/lightningnetwork/lnd/fn/v2"
        "github.com/lightningnetwork/lnd/input"
        "github.com/lightningnetwork/lnd/labels"
        "github.com/lightningnetwork/lnd/lntypes"
        "github.com/lightningnetwork/lnd/lnutils"
        "github.com/lightningnetwork/lnd/lnwallet"
        "github.com/lightningnetwork/lnd/lnwallet/btcwallet"
        "github.com/lightningnetwork/lnd/lnwallet/chainfee"
        "github.com/lightningnetwork/lnd/tlv"
)

// spentNotificationTimeout defines the time to wait for a spending event from
// `RegisterSpendNtfn` when an immediate response is expected.
const spentNotificationTimeout = 1 * time.Second

var (
        // ErrInvalidBumpResult is returned when the bump result is invalid.
        ErrInvalidBumpResult = errors.New("invalid bump result")

        // ErrNotEnoughBudget is returned when the fee bumper decides the
        // current budget cannot cover the fee.
        ErrNotEnoughBudget = errors.New("not enough budget")

        // ErrLocktimeImmature is returned when sweeping an input whose
        // locktime is not reached.
        ErrLocktimeImmature = errors.New("immature input")

        // ErrTxNoOutput is returned when an output cannot be created during tx
        // preparation, usually due to the output being dust.
        ErrTxNoOutput = errors.New("tx has no output")

        // ErrUnknownSpent is returned when an unknown tx has spent an input in
        // the sweeping tx.
        ErrUnknownSpent = errors.New("unknown spend of input")

        // ErrInputMissing is returned when a given input no longer exists,
        // e.g., spending from an orphan tx.
        ErrInputMissing = errors.New("input no longer exists")
)

var (
        // dummyChangePkScript is a dummy tapscript change script that's used
        // when we don't need a real address, just something that can be used
        // for fee estimation.
        dummyChangePkScript = []byte{
                0x51, 0x20,
                0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
                0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
                0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
                0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        }
)

// Bumper defines an interface that can be used by other subsystems for fee
// bumping.
type Bumper interface {
        // Broadcast is used to publish the tx created from the given inputs
        // specified in the request. It handles the tx creation, broadcasts it,
        // and monitors its confirmation status for potential fee bumping. It
        // returns a chan that the caller can use to receive updates about the
        // broadcast result and potential RBF attempts.
        Broadcast(req *BumpRequest) <-chan *BumpResult
}

// BumpEvent represents the event of a fee bumping attempt.
type BumpEvent uint8

const (
        // TxPublished is sent when the broadcast attempt is finished.
        TxPublished BumpEvent = iota

        // TxFailed is sent when the tx has encountered a fee-related error
        // during its creation or broadcast, or an internal error from the fee
        // bumper. In either case the inputs in this tx should be retried with
        // either a different grouping strategy or an increased budget.
        //
        // TODO(yy): Remove the above usage once we remove sweeping non-CPFP
        // anchors.
        TxFailed

        // TxReplaced is sent when the original tx is replaced by a new one.
        TxReplaced

        // TxConfirmed is sent when the tx is confirmed.
        TxConfirmed

        // TxUnknownSpend is sent when at least one of the inputs is spent but
        // not by the current sweeping tx, this can happen when,
        // - a remote party has replaced our sweeping tx by spending the
        //   input(s), e.g., via the direct preimage spend on our outgoing HTLC.
        // - a third party has replaced our sweeping tx, e.g., the anchor output
        //   after 16 blocks.
        // - A previous sweeping tx has confirmed but the fee bumper is not
        //   aware of it, e.g., a restart happens right after the sweeping tx is
        //   broadcast and confirmed.
        TxUnknownSpend

        // TxFatal is sent when the inputs in this tx cannot be retried. Txns
        // will end up in this state if they have encountered a non-fee related
        // error, which means they cannot be retried with increased budget.
        TxFatal

        // sentinelEvent is used to check if an event is unknown.
        sentinelEvent
)

// String returns a human-readable string for the event.
func (e BumpEvent) String() string {
        switch e {
        case TxPublished:
                return "Published"
        case TxFailed:
                return "Failed"
        case TxReplaced:
                return "Replaced"
        case TxConfirmed:
                return "Confirmed"
        case TxUnknownSpend:
                return "UnknownSpend"
        case TxFatal:
                return "Fatal"
        default:
                return "Unknown"
        }
}

// Unknown returns true if the event is unknown.
func (e BumpEvent) Unknown() bool {
        return e >= sentinelEvent
}

// BumpRequest is used by the caller to give the Bumper the necessary info to
// create and manage potential fee bumps for a set of inputs.
type BumpRequest struct {
        // Budget gives the total amount that can be used as fees by these
        // inputs.
        Budget btcutil.Amount

        // Inputs is the set of inputs to sweep.
        Inputs []input.Input

        // DeadlineHeight is the block height at which the tx should be
        // confirmed.
        DeadlineHeight int32

        // DeliveryAddress is the script to send the change output to.
        DeliveryAddress lnwallet.AddrWithKey

        // MaxFeeRate is the maximum fee rate that can be used for fee bumping.
        MaxFeeRate chainfee.SatPerKWeight

        // StartingFeeRate is an optional parameter that can be used to specify
        // the initial fee rate to use for the fee function.
        StartingFeeRate fn.Option[chainfee.SatPerKWeight]

        // ExtraTxOut tracks if this bump request has an optional set of extra
        // outputs to add to the transaction.
        ExtraTxOut fn.Option[SweepOutput]

        // Immediate is used to specify that the tx should be broadcast
        // immediately.
        Immediate bool
}

// MaxFeeRateAllowed returns the maximum fee rate allowed for the given
// request. It calculates the feerate using the supplied budget and the weight,
// compares it with the specified MaxFeeRate, and returns the smaller of the
// two.
func (r *BumpRequest) MaxFeeRateAllowed() (chainfee.SatPerKWeight, error) {
        // We'll want to know if we have any blobs, as we need to factor this
        // into the max fee rate for this bump request.
        hasBlobs := fn.Any(r.Inputs, func(i input.Input) bool {
                return fn.MapOptionZ(
                        i.ResolutionBlob(), func(b tlv.Blob) bool {
                                return len(b) > 0
                        },
                )
        })

        sweepAddrs := [][]byte{
                r.DeliveryAddress.DeliveryAddress,
        }

        // If we have blobs, then we'll add an extra sweep addr for the size
        // estimate below. We know that these blobs will also always be based on
        // p2tr addrs.
        if hasBlobs {
                // We need to pass in a real address, so we'll use a dummy
                // tapscript change script that's used elsewhere for tests.
                sweepAddrs = append(sweepAddrs, dummyChangePkScript)
        }

        // Get the size of the sweep tx, which will be used to calculate the
        // budget fee rate.
        size, err := calcSweepTxWeight(
                r.Inputs, sweepAddrs,
        )
        if err != nil {
                return 0, err
        }

        // Use the budget and MaxFeeRate to decide the max allowed fee rate.
        // This is needed as, when the input has a large value and the user
        // sets the budget to be proportional to the input value, the fee rate
        // can be very high and we need to make sure it doesn't exceed the max
        // fee rate.
        maxFeeRateAllowed := chainfee.NewSatPerKWeight(r.Budget, size)
        if maxFeeRateAllowed > r.MaxFeeRate {
                log.Debugf("Budget feerate %v exceeds MaxFeeRate %v, use "+
                        "MaxFeeRate instead, txWeight=%v", maxFeeRateAllowed,
                        r.MaxFeeRate, size)

                return r.MaxFeeRate, nil
        }

        log.Debugf("Budget feerate %v below MaxFeeRate %v, use budget feerate "+
                "instead, txWeight=%v", maxFeeRateAllowed, r.MaxFeeRate, size)

        return maxFeeRateAllowed, nil
}

// calcSweepTxWeight calculates the weight of the sweep tx. It assumes a
// sweeping tx always has a single output(change).
func calcSweepTxWeight(inputs []input.Input,
        outputPkScript [][]byte) (lntypes.WeightUnit, error) {

        // Use a const fee rate as we only use the weight estimator to
        // calculate the size.
        const feeRate = 1

        // Initialize the tx weight estimator with,
        // - nil outputs as we only have one single change output.
        // - const fee rate as we don't care about the fees here.
        // - 0 maxfeerate as we don't care about fees here.
        //
        // TODO(yy): we should refactor the weight estimator to not require a
        // fee rate and max fee rate and make it a pure tx weight calculator.
        _, estimator, err := getWeightEstimate(
                inputs, nil, feeRate, 0, outputPkScript,
        )
        if err != nil {
                return 0, err
        }

        return estimator.weight(), nil
}

// BumpResult is used by the Bumper to send updates about the tx being
// broadcast.
type BumpResult struct {
        // Event is the type of event that the result is for.
        Event BumpEvent

        // Tx is the tx being broadcast.
        Tx *wire.MsgTx

        // ReplacedTx is the old, replaced tx if a fee bump is attempted.
        ReplacedTx *wire.MsgTx

        // FeeRate is the fee rate used for the new tx.
        FeeRate chainfee.SatPerKWeight

        // Fee is the fee paid by the new tx.
        Fee btcutil.Amount

        // Err is the error that occurred during the broadcast.
        Err error

        // SpentInputs are the inputs spent by another tx which caused the
        // current tx to be failed.
        SpentInputs map[wire.OutPoint]*wire.MsgTx

        // requestID is the ID of the request that created this record.
        requestID uint64
}

// String returns a human-readable string for the result.
func (b *BumpResult) String() string {
        desc := fmt.Sprintf("Event=%v", b.Event)
        if b.Tx != nil {
                desc += fmt.Sprintf(", Tx=%v", b.Tx.TxHash())
        }

        return fmt.Sprintf("[%s]", desc)
}

// Validate validates the BumpResult so it's safe to use.
func (b *BumpResult) Validate() error {
        isFailureEvent := b.Event == TxFailed || b.Event == TxFatal ||
                b.Event == TxUnknownSpend

        // Every result must have a tx except the fatal or failed case.
        if b.Tx == nil && !isFailureEvent {
                return fmt.Errorf("%w: nil tx", ErrInvalidBumpResult)
        }

        // Every result must have a known event.
        if b.Event.Unknown() {
                return fmt.Errorf("%w: unknown event", ErrInvalidBumpResult)
        }

        // If it's a replacing event, it must have a replaced tx.
        if b.Event == TxReplaced && b.ReplacedTx == nil {
                return fmt.Errorf("%w: nil replacing tx", ErrInvalidBumpResult)
        }

        // If it's a failed or fatal event, it must have an error.
        if isFailureEvent && b.Err == nil {
                return fmt.Errorf("%w: nil error", ErrInvalidBumpResult)
        }

        // If it's a confirmed event, it must have a fee rate and fee.
        if b.Event == TxConfirmed && (b.FeeRate == 0 || b.Fee == 0) {
                return fmt.Errorf("%w: missing fee rate or fee",
                        ErrInvalidBumpResult)
        }

        return nil
}

// TxPublisherConfig is the config used to create a new TxPublisher.
type TxPublisherConfig struct {
        // Signer is used to create the tx signature.
        Signer input.Signer

        // Wallet is used primarily to publish the tx.
        Wallet Wallet

        // Estimator is used to estimate the fee rate for the new tx based on
        // its deadline conf target.
        Estimator chainfee.Estimator

        // Notifier is used to monitor the confirmation status of the tx.
        Notifier chainntnfs.ChainNotifier

        // ChainIO represents an abstraction over a source that can query the
        // blockchain.
        ChainIO lnwallet.BlockChainIO

        // AuxSweeper is an optional interface that can be used to modify the
        // way sweep transaction are generated.
        AuxSweeper fn.Option[AuxSweeper]
}

// TxPublisher is an implementation of the Bumper interface. It utilizes the
// `testmempoolaccept` RPC to bump the fee of txns it created based on
// different fee function selected or configed by the caller. Its purpose is to
// take a list of inputs specified, and create a tx that spends them to a
// specified output. It will then monitor the confirmation status of the tx,
// and if it's not confirmed within a certain time frame, it will attempt to
// bump the fee of the tx by creating a new tx that spends the same inputs to
// the same output, but with a higher fee rate. It will continue to do this
// until the tx is confirmed or the fee rate reaches the maximum fee rate
// specified by the caller.
type TxPublisher struct {
        started atomic.Bool
        stopped atomic.Bool

        // Embed the blockbeat consumer struct to get access to the method
        // `NotifyBlockProcessed` and the `BlockbeatChan`.
        chainio.BeatConsumer

        wg sync.WaitGroup

        // cfg specifies the configuration of the TxPublisher.
        cfg *TxPublisherConfig

        // currentHeight is the current block height.
        currentHeight atomic.Int32

        // records is a map keyed by the requestCounter and the value is the tx
        // being monitored.
        records lnutils.SyncMap[uint64, *monitorRecord]

        // requestCounter is a monotonically increasing counter used to keep
        // track of how many requests have been made.
        requestCounter atomic.Uint64

        // subscriberChans is a map keyed by the requestCounter, each item is
        // the chan that the publisher sends the fee bump result to.
        subscriberChans lnutils.SyncMap[uint64, chan *BumpResult]

        // quit is used to signal the publisher to stop.
        quit chan struct{}
}

// Compile-time constraint to ensure TxPublisher implements Bumper.
var _ Bumper = (*TxPublisher)(nil)

// Compile-time check for the chainio.Consumer interface.
var _ chainio.Consumer = (*TxPublisher)(nil)

// NewTxPublisher creates a new TxPublisher.
func NewTxPublisher(cfg TxPublisherConfig) *TxPublisher {
        tp := &TxPublisher{
                cfg:             &cfg,
                records:         lnutils.SyncMap[uint64, *monitorRecord]{},
                subscriberChans: lnutils.SyncMap[uint64, chan *BumpResult]{},
                quit:            make(chan struct{}),
        }

        // Mount the block consumer.
        tp.BeatConsumer = chainio.NewBeatConsumer(tp.quit, tp.Name())

        return tp
}

// isNeutrinoBackend checks if the wallet backend is neutrino.
func (t *TxPublisher) isNeutrinoBackend() bool {
        return t.cfg.Wallet.BackEnd() == "neutrino"
}

// Broadcast is used to publish the tx created from the given inputs. It will
// register the broadcast request and return a chan to the caller to subscribe
// the broadcast result. The initial broadcast is guaranteed to be
// RBF-compliant unless the budget specified cannot cover the fee.
//
// NOTE: part of the Bumper interface.
func (t *TxPublisher) Broadcast(req *BumpRequest) <-chan *BumpResult {
        log.Tracef("Received broadcast request: %s",
                lnutils.SpewLogClosure(req))

        // Store the request.
        record := t.storeInitialRecord(req)

        // Create a chan to send the result to the caller.
        subscriber := make(chan *BumpResult, 1)
        t.subscriberChans.Store(record.requestID, subscriber)

        // Publish the tx immediately if specified.
        if req.Immediate {
                t.handleInitialBroadcast(record)
        }

        return subscriber
}

// storeInitialRecord initializes a monitor record and saves it in the map.
func (t *TxPublisher) storeInitialRecord(req *BumpRequest) *monitorRecord {
        // Increase the request counter.
        //
        // NOTE: this is the only place where we increase the counter.
        requestID := t.requestCounter.Add(1)

        // Register the record.
        record := &monitorRecord{
                requestID: requestID,
                req:       req,
        }
        t.records.Store(requestID, record)

        return record
}

// updateRecord updates the given record's tx and fee, and saves it in the
// records map.
func (t *TxPublisher) updateRecord(r *monitorRecord,
        sweepCtx *sweepTxCtx) *monitorRecord {

        r.tx = sweepCtx.tx
        r.fee = sweepCtx.fee
        r.outpointToTxIndex = sweepCtx.outpointToTxIndex

        // Register the record.
        t.records.Store(r.requestID, r)

        return r
}

// NOTE: part of the `chainio.Consumer` interface.
func (t *TxPublisher) Name() string {
        return "TxPublisher"
}

// initializeTx initializes a fee function and creates an RBF-compliant tx. If
// succeeded, the initial tx is stored in the records map.
func (t *TxPublisher) initializeTx(r *monitorRecord) (*monitorRecord, error) {
        // Create a fee bumping algorithm to be used for future RBF.
        feeAlgo, err := t.initializeFeeFunction(r.req)
        if err != nil {
                return nil, fmt.Errorf("init fee function: %w", err)
        }

        // Attach the newly created fee function.
        //
        // TODO(yy): current we'd initialize a monitorRecord before creating the
        // fee function, while we could instead create the fee function first
        // then save it to the record. To make this happen we need to change the
        // conf target calculation below since we would be initializing the fee
        // function one block before.
        r.feeFunction = feeAlgo

        // Create the initial tx to be broadcasted. This tx is guaranteed to
        // comply with the RBF restrictions.
        record, err := t.createRBFCompliantTx(r)
        if err != nil {
                return nil, fmt.Errorf("create RBF-compliant tx: %w", err)
        }

        return record, nil
}

// initializeFeeFunction initializes a fee function to be used for this request
// for future fee bumping.
func (t *TxPublisher) initializeFeeFunction(
        req *BumpRequest) (FeeFunction, error) {

        // Get the max allowed feerate.
        maxFeeRateAllowed, err := req.MaxFeeRateAllowed()
        if err != nil {
                return nil, err
        }

        // Get the initial conf target.
        confTarget := calcCurrentConfTarget(
                t.currentHeight.Load(), req.DeadlineHeight,
        )

        log.Debugf("Initializing fee function with conf target=%v, budget=%v, "+
                "maxFeeRateAllowed=%v", confTarget, req.Budget,
                maxFeeRateAllowed)

        // Initialize the fee function and return it.
        //
        // TODO(yy): return based on differet req.Strategy?
        return NewLinearFeeFunction(
                maxFeeRateAllowed, confTarget, t.cfg.Estimator,
                req.StartingFeeRate,
        )
}

// createRBFCompliantTx creates a tx that is compliant with RBF rules. It does
// so by creating a tx, validate it using `TestMempoolAccept`, and bump its fee
// and redo the process until the tx is valid, or return an error when non-RBF
// related errors occur or the budget has been used up.
func (t *TxPublisher) createRBFCompliantTx(
        r *monitorRecord) (*monitorRecord, error) {

        f := r.feeFunction

        for {
                // Create a new tx with the given fee rate and check its
                // mempool acceptance.
                sweepCtx, err := t.createAndCheckTx(r)

                switch {
                case err == nil:
                        // The tx is valid, store it.
                        record := t.updateRecord(r, sweepCtx)

                        log.Infof("Created initial sweep tx=%v for %v inputs: "+
                                "feerate=%v, fee=%v, inputs:\n%v",
                                sweepCtx.tx.TxHash(), len(r.req.Inputs),
                                f.FeeRate(), sweepCtx.fee,
                                inputTypeSummary(r.req.Inputs))

                        return record, nil

                // If the error indicates the fees paid is not enough, we will
                // ask the fee function to increase the fee rate and retry.
                case errors.Is(err, lnwallet.ErrMempoolFee):
                        // We should at least start with a feerate above the
                        // mempool min feerate, so if we get this error, it
                        // means something is wrong earlier in the pipeline.
                        log.Errorf("Current fee=%v, feerate=%v, %v",
                                sweepCtx.fee, f.FeeRate(), err)

                        fallthrough

                // We are not paying enough fees so we increase it.
                case errors.Is(err, chain.ErrInsufficientFee):
                        increased := false

                        // Keep calling the fee function until the fee rate is
                        // increased or maxed out.
                        for !increased {
                                log.Debugf("Increasing fee for next round, "+
                                        "current fee=%v, feerate=%v",
                                        sweepCtx.fee, f.FeeRate())

                                // If the fee function tells us that we have
                                // used up the budget, we will return an error
                                // indicating this tx cannot be made. The
                                // sweeper should handle this error and try to
                                // cluster these inputs differently.
                                increased, err = f.Increment()
                                if err != nil {
                                        return nil, err
                                }
                        }

                // TODO(yy): suppose there's only one bad input, we can do a
                // binary search to find out which input is causing this error
                // by recreating a tx using half of the inputs and check its
                // mempool acceptance.
                default:
                        log.Debugf("Failed to create RBF-compliant tx: %v", err)
                        return nil, err
                }
        }
}

// createAndCheckTx creates a tx based on the given inputs, change output
// script, and the fee rate. In addition, it validates the tx's mempool
// acceptance before returning a tx that can be published directly, along with
// its fee.
func (t *TxPublisher) createAndCheckTx(r *monitorRecord) (*sweepTxCtx, error) {
        req := r.req
        f := r.feeFunction

        // Create the sweep tx with max fee rate of 0 as the fee function
        // guarantees the fee rate used here won't exceed the max fee rate.
        sweepCtx, err := t.createSweepTx(
                req.Inputs, req.DeliveryAddress, f.FeeRate(),
        )
        if err != nil {
                return sweepCtx, fmt.Errorf("create sweep tx: %w", err)
        }

        // Sanity check the budget still covers the fee.
        if sweepCtx.fee > req.Budget {
                return sweepCtx, fmt.Errorf("%w: budget=%v, fee=%v",
                        ErrNotEnoughBudget, req.Budget, sweepCtx.fee)
        }

        // If we had an extra txOut, then we'll update the result to include
        // it.
        req.ExtraTxOut = sweepCtx.extraTxOut

        // Validate the tx's mempool acceptance.
        err = t.cfg.Wallet.CheckMempoolAcceptance(sweepCtx.tx)

        // Exit early if the tx is valid.
        if err == nil {
                return sweepCtx, nil
        }

        // Print an error log if the chain backend doesn't support the mempool
        // acceptance test RPC.
        if errors.Is(err, rpcclient.ErrBackendVersion) {
                log.Errorf("TestMempoolAccept not supported by backend, " +
                        "consider upgrading it to a newer version")
                return sweepCtx, nil
        }

        // We are running on a backend that doesn't implement the RPC
        // testmempoolaccept, eg, neutrino, so we'll skip the check.
        if errors.Is(err, chain.ErrUnimplemented) {
                log.Debug("Skipped testmempoolaccept due to not implemented")
                return sweepCtx, nil
        }

        // If the inputs are spent by another tx, we will exit with the latest
        // sweepCtx and an error.
        if errors.Is(err, chain.ErrMissingInputs) {
                log.Debugf("Tx %v missing inputs, it's likely the input has "+
                        "been spent by others", sweepCtx.tx.TxHash())

                // Make sure to update the record with the latest attempt.
                t.updateRecord(r, sweepCtx)

                return sweepCtx, ErrInputMissing
        }

        return sweepCtx, fmt.Errorf("tx=%v failed mempool check: %w",
                sweepCtx.tx.TxHash(), err)
}

// handleMissingInputs handles the case when the chain backend reports back a
// missing inputs error, which could happen when one of the input has been spent
// in another tx, or the input is referencing an orphan. When the input is
// spent, it will be handled via the TxUnknownSpend flow by creating a
// TxUnknownSpend bump result, otherwise, a TxFatal bump result is returned.
func (t *TxPublisher) handleMissingInputs(r *monitorRecord) *BumpResult {
        // Get the spending txns.
        spends := t.getSpentInputs(r)

        // Attach the spending txns.
        r.spentInputs = spends

        // If there are no spending txns found and the input is missing, the
        // input is referencing an orphan tx that's no longer valid, e.g., the
        // spending the anchor output from the remote commitment after the local
        // commitment has confirmed. In this case we will mark it as fatal and
        // exit.
        if len(spends) == 0 {
                log.Warnf("Failing record=%v: found orphan inputs: %v\n",
                        r.requestID, inputTypeSummary(r.req.Inputs))

                // Create a result that will be sent to the resultChan which is
                // listened by the caller.
                result := &BumpResult{
                        Event:     TxFatal,
                        Tx:        r.tx,
                        requestID: r.requestID,
                        Err:       ErrInputMissing,
                }

                return result
        }

        // Check that the spending tx matches the sweeping tx - given that the
        // current sweeping tx has been failed due to missing inputs, the
        // spending tx must be a different tx, thus it should NOT be matched. We
        // perform a sanity check here to catch the unexpected state.
        if !t.isUnknownSpent(r, spends) {
                log.Errorf("Sweeping tx %v has missing inputs, yet the "+
                        "spending tx is the sweeping tx itself: %v",
                        r.tx.TxHash(), r.spentInputs)
        }

        return t.createUnknownSpentBumpResult(r)
}

// broadcast takes a monitored tx and publishes it to the network. Prior to the
// broadcast, it will subscribe the tx's confirmation notification and attach
// the event channel to the record. Any broadcast-related errors will not be
// returned here, instead, they will be put inside the `BumpResult` and
// returned to the caller.
func (t *TxPublisher) broadcast(record *monitorRecord) (*BumpResult, error) {
        txid := record.tx.TxHash()

        tx := record.tx
        log.Debugf("Publishing sweep tx %v, num_inputs=%v, height=%v",
                txid, len(tx.TxIn), t.currentHeight.Load())

        // Before we go to broadcast, we'll notify the aux sweeper, if it's
        // present of this new broadcast attempt.
        err := fn.MapOptionZ(t.cfg.AuxSweeper, func(aux AuxSweeper) error {
                return aux.NotifyBroadcast(
                        record.req, tx, record.fee, record.outpointToTxIndex,
                )
        })
        if err != nil {
                return nil, fmt.Errorf("unable to notify aux sweeper: %w", err)
        }

        // Set the event, and change it to TxFailed if the wallet fails to
        // publish it.
        event := TxPublished

        // Publish the sweeping tx with customized label. If the publish fails,
        // this error will be saved in the `BumpResult` and it will be removed
        // from being monitored.
        err = t.cfg.Wallet.PublishTransaction(
                tx, labels.MakeLabel(labels.LabelTypeSweepTransaction, nil),
        )
        if err != nil {
                // NOTE: we decide to attach this error to the result instead
                // of returning it here because by the time the tx reaches
                // here, it should have passed the mempool acceptance check. If
                // it still fails to be broadcast, it's likely a non-RBF
                // related error happened. So we send this error back to the
                // caller so that it can handle it properly.
                //
                // TODO(yy): find out which input is causing the failure.
                log.Errorf("Failed to publish tx %v: %v", txid, err)
                event = TxFailed
        }

        result := &BumpResult{
                Event:     event,
                Tx:        record.tx,
                Fee:       record.fee,
                FeeRate:   record.feeFunction.FeeRate(),
                Err:       err,
                requestID: record.requestID,
        }

        return result, nil
}

// notifyResult sends the result to the resultChan specified by the requestID.
// This channel is expected to be read by the caller.
func (t *TxPublisher) notifyResult(result *BumpResult) {
        id := result.requestID
        subscriber, ok := t.subscriberChans.Load(id)
        if !ok {
                log.Errorf("Result chan for id=%v not found", id)
                return
        }

        log.Debugf("Sending result %v for requestID=%v", result, id)

        select {
        // Send the result to the subscriber.
        //
        // TODO(yy): Add timeout in case it's blocking?
        case subscriber <- result:
        case <-t.quit:
                log.Debug("Fee bumper stopped")
        }
}

// removeResult removes the tracking of the result if the result contains a
// non-nil error, or the tx is confirmed, the record will be removed from the
// maps.
func (t *TxPublisher) removeResult(result *BumpResult) {
        id := result.requestID

        var txid chainhash.Hash
        if result.Tx != nil {
                txid = result.Tx.TxHash()
        }

        // Remove the record from the maps if there's an error or the tx is
        // confirmed. When there's an error, it means this tx has failed its
        // broadcast and cannot be retried. There are two cases it may fail,
        // - when the budget cannot cover the increased fee calculated by the
        //   fee function, hence the budget is used up.
        // - when a non-fee related error returned from PublishTransaction.
        switch result.Event {
        case TxFailed:
                log.Errorf("Removing monitor record=%v, tx=%v, due to err: %v",
                        id, txid, result.Err)

        case TxConfirmed:
                // Remove the record if the tx is confirmed.
                log.Debugf("Removing confirmed monitor record=%v, tx=%v", id,
                        txid)

        case TxFatal:
                // Remove the record if there's an error.
                log.Debugf("Removing monitor record=%v due to fatal err: %v",
                        id, result.Err)

        case TxUnknownSpend:
                // Remove the record if there's an unknown spend.
                log.Debugf("Removing monitor record=%v due unknown spent: "+
                        "%v", id, result.Err)

        // Do nothing if it's neither failed or confirmed.
        default:
                log.Tracef("Skipping record removal for id=%v, event=%v", id,
                        result.Event)

                return
        }

        t.records.Delete(id)
        t.subscriberChans.Delete(id)
}

// handleResult handles the result of a tx broadcast. It will notify the
// subscriber and remove the record if the tx is confirmed or failed to be
// broadcast.
func (t *TxPublisher) handleResult(result *BumpResult) {
        // Notify the subscriber.
        t.notifyResult(result)

        // Remove the record if it's failed or confirmed.
        t.removeResult(result)
}

// monitorRecord is used to keep track of the tx being monitored by the
// publisher internally.
type monitorRecord struct {
        // requestID is the ID of the request that created this record.
        requestID uint64

        // tx is the tx being monitored.
        tx *wire.MsgTx

        // req is the original request.
        req *BumpRequest

        // feeFunction is the fee bumping algorithm used by the publisher.
        feeFunction FeeFunction

        // fee is the fee paid by the tx.
        fee btcutil.Amount

        // outpointToTxIndex is a map of outpoint to tx index.
        outpointToTxIndex map[wire.OutPoint]int

        // spentInputs are the inputs spent by another tx which caused the
        // current tx failed.
        spentInputs map[wire.OutPoint]*wire.MsgTx
}

// Start starts the publisher by subscribing to block epoch updates and kicking
// off the monitor loop.
func (t *TxPublisher) Start(beat chainio.Blockbeat) error {
        log.Info("TxPublisher starting...")

        if t.started.Swap(true) {
                return fmt.Errorf("TxPublisher started more than once")
        }

        // Set the current height.
        t.currentHeight.Store(beat.Height())

        t.wg.Add(1)
        go t.monitor()

        log.Debugf("TxPublisher started")

        return nil
}

// Stop stops the publisher and waits for the monitor loop to exit.
func (t *TxPublisher) Stop() error {
        log.Info("TxPublisher stopping...")

        if t.stopped.Swap(true) {
                return fmt.Errorf("TxPublisher stopped more than once")
        }

        close(t.quit)
        t.wg.Wait()

        log.Debug("TxPublisher stopped")

        return nil
}

// monitor is the main loop driven by new blocks. Whevenr a new block arrives,
// it will examine all the txns being monitored, and check if any of them needs
// to be bumped. If so, it will attempt to bump the fee of the tx.
//
// NOTE: Must be run as a goroutine.
func (t *TxPublisher) monitor() {
        defer t.wg.Done()

        for {
                select {
                case beat := <-t.BlockbeatChan:
                        height := beat.Height()
                        log.Debugf("TxPublisher received new block: %v", height)

                        // Update the best known height for the publisher.
                        t.currentHeight.Store(height)

                        // Check all monitored txns to see if any of them needs
                        // to be bumped.
                        t.processRecords()

                        // Notify we've processed the block.
                        t.NotifyBlockProcessed(beat, nil)

                case <-t.quit:
                        log.Debug("Fee bumper stopped, exit monitor")
                        return
                }
        }
}

// processRecords checks all the txns being monitored, and checks if any of
// them needs to be bumped. If so, it will attempt to bump the fee of the tx.
func (t *TxPublisher) processRecords() {
        // confirmedRecords stores a map of the records which have been
        // confirmed.
        confirmedRecords := make(map[uint64]*monitorRecord)

        // feeBumpRecords stores a map of records which need to be bumped.
        feeBumpRecords := make(map[uint64]*monitorRecord)

        // failedRecords stores a map of records which has inputs being spent
        // by a third party.
        failedRecords := make(map[uint64]*monitorRecord)

        // initialRecords stores a map of records which are being created and
        // published for the first time.
        initialRecords := make(map[uint64]*monitorRecord)

        // visitor is a helper closure that visits each record and divides them
        // into two groups.
        visitor := func(requestID uint64, r *monitorRecord) error {
                log.Tracef("Checking monitor recordID=%v", requestID)

                // Check whether the inputs have already been spent.
                spends := t.getSpentInputs(r)

                // If the any of the inputs has been spent, the record will be
                // marked as failed or confirmed.
                if len(spends) != 0 {
                        // Attach the spending txns.
                        r.spentInputs = spends

                        // When tx is nil, it means we haven't tried the initial
                        // broadcast yet the input is already spent. This could
                        // happen when the node shuts down, a previous sweeping
                        // tx confirmed, then the node comes back online and
                        // reoffers the inputs. Another case is the remote node
                        // spends the input quickly before we even attempt the
                        // sweep. In either case we will fail the record and let
                        // the sweeper handles it.
                        if r.tx == nil {
                                failedRecords[requestID] = r
                                return nil
                        }

                        // Check whether the inputs has been spent by a unknown
                        // tx.
                        if t.isUnknownSpent(r, spends) {
                                failedRecords[requestID] = r

                                // Move to the next record.
                                return nil
                        }

                        // The tx is ours, we can move it to the confirmed queue
                        // and stop monitoring it.
                        confirmedRecords[requestID] = r

                        // Move to the next record.
                        return nil
                }

                // This is the first time we see this record, so we put it in
                // the initial queue.
                if r.tx == nil {
                        initialRecords[requestID] = r

                        return nil
                }

                // We can only get here when the inputs are not spent and a
                // previous sweeping tx has been attempted. In this case we will
                // perform an RBF on it in the current block.
                feeBumpRecords[requestID] = r

                // Return nil to move to the next record.
                return nil
        }

        // Iterate through all the records and divide them into four groups.
        t.records.ForEach(visitor)

        // Handle the initial broadcast.
        for _, r := range initialRecords {
                t.handleInitialBroadcast(r)
        }

        // For records that are confirmed, we'll notify the caller about this
        // result.
        for _, r := range confirmedRecords {
                t.wg.Add(1)
                go t.handleTxConfirmed(r)
        }

        // Get the current height to be used in the following goroutines.
        currentHeight := t.currentHeight.Load()

        // For records that are not confirmed, we perform a fee bump if needed.
        for _, r := range feeBumpRecords {
                t.wg.Add(1)
                go t.handleFeeBumpTx(r, currentHeight)
        }

        // For records that are failed, we'll notify the caller about this
        // result.
        for _, r := range failedRecords {
                t.wg.Add(1)
                go t.handleUnknownSpent(r)
        }
}

// handleTxConfirmed is called when a monitored tx is confirmed. It will
// notify the subscriber then remove the record from the maps .
//
// NOTE: Must be run as a goroutine to avoid blocking on sending the result.
func (t *TxPublisher) handleTxConfirmed(r *monitorRecord) {
        defer t.wg.Done()

        log.Debugf("Record %v is spent in tx=%v", r.requestID, r.tx.TxHash())

        // Create a result that will be sent to the resultChan which is
        // listened by the caller.
        result := &BumpResult{
                Event:     TxConfirmed,
                Tx:        r.tx,
                requestID: r.requestID,
                Fee:       r.fee,
                FeeRate:   r.feeFunction.FeeRate(),
        }

        // Notify that this tx is confirmed and remove the record from the map.
        t.handleResult(result)
}

// handleInitialTxError takes the error from `initializeTx` and decides the
// bump event. It will construct a BumpResult and handles it.
func (t *TxPublisher) handleInitialTxError(r *monitorRecord, err error) {
        // Create a bump result to be sent to the sweeper.
        result := &BumpResult{
                Err:       err,
                requestID: r.requestID,
        }

        // We now decide what type of event to send.
        switch {
        // When the error is due to a dust output, we'll send a TxFailed so
        // these inputs can be retried with a different group in the next
        // block.
        case errors.Is(err, ErrTxNoOutput):
                result.Event = TxFailed

        // When the error is due to zero fee rate delta, we'll send a TxFailed
        // so these inputs can be retried in the next block.
        case errors.Is(err, ErrZeroFeeRateDelta):
                result.Event = TxFailed

        // When the error is due to budget being used up, we'll send a TxFailed
        // so these inputs can be retried with a different group in the next
        // block.
        case errors.Is(err, ErrMaxPosition):
                fallthrough

        // If the tx doesn't not have enough budget, or if the inputs amounts
        // are not sufficient to cover the budget, we will return a TxFailed
        // event so the sweeper can handle it by re-clustering the utxos.
        case errors.Is(err, ErrNotEnoughInputs),
                errors.Is(err, ErrNotEnoughBudget):

                result.Event = TxFailed

                // Calculate the starting fee rate to be used when retry
                // sweeping these inputs.
                feeRate, err := t.calculateRetryFeeRate(r)
                if err != nil {
                        result.Event = TxFatal
                        result.Err = err
                }

                // Attach the new fee rate.
                result.FeeRate = feeRate

        // When there are missing inputs, we'll create a TxUnknownSpend bump
        // result here so the rest of the inputs can be retried.
        case errors.Is(err, ErrInputMissing):
                result = t.handleMissingInputs(r)

        // Otherwise this is not a fee-related error and the tx cannot be
        // retried. In that case we will fail ALL the inputs in this tx, which
        // means they will be removed from the sweeper and never be tried
        // again.
        //
        // TODO(yy): Find out which input is causing the failure and fail that
        // one only.
        default:
                result.Event = TxFatal
        }

        t.handleResult(result)
}

// handleInitialBroadcast is called when a new request is received. It will
// handle the initial tx creation and broadcast. In details,
// 1. init a fee function based on the given strategy.
// 2. create an RBF-compliant tx and monitor it for confirmation.
// 3. notify the initial broadcast result back to the caller.
func (t *TxPublisher) handleInitialBroadcast(r *monitorRecord) {
        log.Debugf("Initial broadcast for requestID=%v", r.requestID)

        var (
                result *BumpResult
                err    error
        )

        // Attempt an initial broadcast which is guaranteed to comply with the
        // RBF rules.
        //
        // Create the initial tx to be broadcasted.
        record, err := t.initializeTx(r)
        if err != nil {
                log.Errorf("Initial broadcast failed: %v", err)

                // We now handle the initialization error and exit.
                t.handleInitialTxError(r, err)

                return
        }

        // Successfully created the first tx, now broadcast it.
        result, err = t.broadcast(record)
        if err != nil {
                // The broadcast failed, which can only happen if the tx record
                // cannot be found or the aux sweeper returns an error. In
                // either case, we will send back a TxFail event so these
                // inputs can be retried.
                result = &BumpResult{
                        Event:     TxFailed,
                        Err:       err,
                        requestID: r.requestID,
                }
        }

        t.handleResult(result)
}

// handleFeeBumpTx checks if the tx needs to be bumped, and if so, it will
// attempt to bump the fee of the tx.
//
// NOTE: Must be run as a goroutine to avoid blocking on sending the result.
func (t *TxPublisher) handleFeeBumpTx(r *monitorRecord, currentHeight int32) {
        defer t.wg.Done()

        log.Debugf("Attempting to fee bump tx=%v in record %v", r.tx.TxHash(),
                r.requestID)

        oldTxid := r.tx.TxHash()

        // Get the current conf target for this record.
        confTarget := calcCurrentConfTarget(currentHeight, r.req.DeadlineHeight)

        // Ask the fee function whether a bump is needed. We expect the fee
        // function to increase its returned fee rate after calling this
        // method.
        increased, err := r.feeFunction.IncreaseFeeRate(confTarget)
        if err != nil {
                // TODO(yy): send this error back to the sweeper so it can
                // re-group the inputs?
                log.Errorf("Failed to increase fee rate for tx %v at "+
                        "height=%v: %v", oldTxid, t.currentHeight.Load(), err)

                return
        }

        // If the fee rate was not increased, there's no need to bump the fee.
        if !increased {
                log.Tracef("Skip bumping tx %v at height=%v", oldTxid,
                        t.currentHeight.Load())

                return
        }

        // The fee function now has a new fee rate, we will use it to bump the
        // fee of the tx.
        resultOpt := t.createAndPublishTx(r)

        // If there's a result, we will notify the caller about the result.
        resultOpt.WhenSome(func(result BumpResult) {
                // Notify the new result.
                t.handleResult(&result)
        })
}

// handleUnknownSpent is called when the inputs are spent by a unknown tx. It
// will notify the subscriber then remove the record from the maps and send a
// TxUnknownSpend event to the subscriber.
//
// NOTE: Must be run as a goroutine to avoid blocking on sending the result.
func (t *TxPublisher) handleUnknownSpent(r *monitorRecord) {
        defer t.wg.Done()

        log.Debugf("Record %v has inputs spent by a tx unknown to the fee "+
                "bumper, failing it now:\n%v", r.requestID,
                inputTypeSummary(r.req.Inputs))

        // Create a result that will be sent to the resultChan which is listened
        // by the caller.
        result := t.createUnknownSpentBumpResult(r)

        // Notify the sweeper about this result in the end.
        t.handleResult(result)
}

// createUnknownSpentBumpResult creates and returns a BumpResult given the
// monitored record has unknown spends.
func (t *TxPublisher) createUnknownSpentBumpResult(
        r *monitorRecord) *BumpResult {

        // Create a result that will be sent to the resultChan which is listened
        // by the caller.
        result := &BumpResult{
                Event:       TxUnknownSpend,
                Tx:          r.tx,
                requestID:   r.requestID,
                Err:         ErrUnknownSpent,
                SpentInputs: r.spentInputs,
        }

        // Calculate the next fee rate for the retry.
        feeRate, err := t.calculateRetryFeeRate(r)
        if err != nil {
                // Overwrite the event and error so the sweeper will
                // remove this input.
                result.Event = TxFatal
                result.Err = err
        }

        // Attach the new fee rate to be used for the next sweeping attempt.
        result.FeeRate = feeRate

        return result
}

// createAndPublishTx creates a new tx with a higher fee rate and publishes it
// to the network. It will update the record with the new tx and fee rate if
// successfully created, and return the result when published successfully.
func (t *TxPublisher) createAndPublishTx(
        r *monitorRecord) fn.Option[BumpResult] {

        // Fetch the old tx.
        oldTx := r.tx

        // Create a new tx with the new fee rate.
        //
        // NOTE: The fee function is expected to have increased its returned
        // fee rate after calling the SkipFeeBump method. So we can use it
        // directly here.
        sweepCtx, err := t.createAndCheckTx(r)

        // If there's an error creating the replacement tx, we need to abort the
        // flow and handle it.
        if err != nil {
                return t.handleReplacementTxError(r, oldTx, err)
        }

        // The tx has been created without any errors, we now register a new
        // record by overwriting the same requestID.
        record := t.updateRecord(r, sweepCtx)

        // Attempt to broadcast this new tx.
        result, err := t.broadcast(record)
        if err != nil {
                log.Infof("Failed to broadcast replacement tx %v: %v",
                        sweepCtx.tx.TxHash(), err)

                return fn.None[BumpResult]()
        }

        // If the result error is fee related, we will return no error and let
        // the fee bumper retry it at next block.
        //
        // NOTE: we may get this error if we've bypassed the mempool check,
        // which means we are using neutrino backend.
        if errors.Is(result.Err, chain.ErrInsufficientFee) ||
                errors.Is(result.Err, lnwallet.ErrMempoolFee) {

                log.Debugf("Failed to bump tx %v: %v", oldTx.TxHash(),
                        result.Err)

                return fn.None[BumpResult]()
        }

        // A successful replacement tx is created, attach the old tx.
        result.ReplacedTx = oldTx

        // If the new tx failed to be published, we will return the result so
        // the caller can handle it.
        if result.Event == TxFailed {
                return fn.Some(*result)
        }

        log.Debugf("Replaced tx=%v with new tx=%v", oldTx.TxHash(),
                sweepCtx.tx.TxHash())

        // Otherwise, it's a successful RBF, set the event and return.
        result.Event = TxReplaced

        return fn.Some(*result)
}

// isUnknownSpent checks whether the inputs of the tx has already been spent by
// a tx not known to us. When a tx is not confirmed, yet its inputs has been
// spent, then it must be spent by a different tx other than the sweeping tx
// here.
func (t *TxPublisher) isUnknownSpent(r *monitorRecord,
        spends map[wire.OutPoint]*wire.MsgTx) bool {

        txid := r.tx.TxHash()

        // Iterate all the spending txns and check if they match the sweeping
        // tx.
        for op, spendingTx := range spends {
                spendingTxID := spendingTx.TxHash()

                // If the spending tx is the same as the sweeping tx then we are
                // good.
                if spendingTxID == txid {
                        continue
                }

                log.Warnf("Detected unknown spend of input=%v in tx=%v", op,
                        spendingTx.TxHash())

                return true
        }

        return false
}

// getSpentInputs performs a non-blocking read on the spending subscriptions to
// see whether any of the monitored inputs has been spent. A map of inputs with
// their spending txns are returned if found.
func (t *TxPublisher) getSpentInputs(
        r *monitorRecord) map[wire.OutPoint]*wire.MsgTx {

        // Create a slice to record the inputs spent.
        spentInputs := make(map[wire.OutPoint]*wire.MsgTx, len(r.req.Inputs))

        // Iterate all the inputs and check if they have been spent already.
        for _, inp := range r.req.Inputs {
                op := inp.OutPoint()

                // For wallet utxos, the height hint is not set - we don't need
                // to monitor them for third party spend.
                //
                // TODO(yy): We need to properly lock wallet utxos before
                // skipping this check as the same wallet utxo can be used by
                // different sweeping txns.
                heightHint := inp.HeightHint()
                if heightHint == 0 {
                        heightHint = uint32(t.currentHeight.Load())
                        log.Debugf("Checking wallet input %v using heightHint "+
                                "%v", op, heightHint)
                }

                // Check whether the input has been spent or not.
                utxo, err := t.cfg.ChainIO.GetUtxo(
                        &op, inp.SignDesc().Output.PkScript, heightHint, t.quit,
                )
                if err != nil {
                        // GetUtxo will return `ErrOutputSpent` when the input
                        // has already been spent. In that case, the returned
                        // `utxo` must be nil, which will move us to subscribe
                        // its spending event below.
                        if !errors.Is(err, btcwallet.ErrOutputSpent) {
                                log.Errorf("Failed to get utxo for input=%v: "+
                                        "%v", op, err)

                                // If this is an unexpected error, move to check
                                // the next input.
                                continue
                        }

                        log.Tracef("GetUtxo for input=%v, err: %v", op, err)
                }

                // If a non-nil utxo is returned it means this input is still
                // unspent. Thus we can continue to the next input as there's no
                // need to register spend notification for it.
                if utxo != nil {
                        log.Tracef("Input=%v not spent yet", op)
                        continue
                }

                log.Debugf("Input=%v already spent, fetching its spending "+
                        "tx...", op)

                // If the input has already been spent after the height hint, a
                // spend event is sent back immediately.
                spendEvent, err := t.cfg.Notifier.RegisterSpendNtfn(
                        &op, inp.SignDesc().Output.PkScript, heightHint,
                )
                if err != nil {
                        log.Criticalf("Failed to register spend ntfn for "+
                                "input=%v: %v", op, err)

                        return spentInputs
                }

                // Remove the subscription when exit.
                defer spendEvent.Cancel()

                // Do a blocking read to receive the spent event.
                select {
                case spend, ok := <-spendEvent.Spend:
                        if !ok {
                                log.Debugf("Spend ntfn for %v canceled", op)

                                continue
                        }

                        spendingTx := spend.SpendingTx

                        log.Debugf("Detected spent of input=%v in tx=%v", op,
                                spendingTx.TxHash())

                        spentInputs[op] = spendingTx

                // The above spent event should be returned immediately, yet we
                // still perform a timeout check here in case it blocks forever.
                //
                // TODO(yy): The proper way to fix this is to redesign the area
                // so we use the async flow for checking whether a given input
                // is spent or not. A better approach is to implement a new
                // synchronous method to check for spending, which should be
                // attempted when implementing SQL into btcwallet.
                case <-time.After(spentNotificationTimeout):
                        log.Warnf("Input is reported as spent by GetUtxo, "+
                                "but spending notification is not returned "+
                                "immediately: input=%v, heightHint=%v", op,
                                heightHint)
                }
        }

        return spentInputs
}

// calcCurrentConfTarget calculates the current confirmation target based on
// the deadline height. The conf target is capped at 0 if the deadline has
// already been past.
func calcCurrentConfTarget(currentHeight, deadline int32) uint32 {
        var confTarget uint32

        // Calculate how many blocks left until the deadline.
        deadlineDelta := deadline - currentHeight

        // If we are already past the deadline, we will set the conf target to
        // be 1.
        if deadlineDelta < 0 {
                log.Warnf("Deadline is %d blocks behind current height %v",
                        -deadlineDelta, currentHeight)

                confTarget = 0
        } else {
                confTarget = uint32(deadlineDelta)
        }

        return confTarget
}

// sweepTxCtx houses a sweep transaction with additional context.
type sweepTxCtx struct {
        tx *wire.MsgTx

        fee btcutil.Amount

        extraTxOut fn.Option[SweepOutput]

        // outpointToTxIndex maps the outpoint of the inputs to their index in
        // the sweep transaction.
        outpointToTxIndex map[wire.OutPoint]int
}

// createSweepTx creates a sweeping tx based on the given inputs, change
// address and fee rate.
func (t *TxPublisher) createSweepTx(inputs []input.Input,
        changePkScript lnwallet.AddrWithKey,
        feeRate chainfee.SatPerKWeight) (*sweepTxCtx, error) {

        // Validate and calculate the fee and change amount.
        txFee, changeOutputsOpt, locktimeOpt, err := prepareSweepTx(
                inputs, changePkScript, feeRate, t.currentHeight.Load(),
                t.cfg.AuxSweeper,
        )
        if err != nil {
                return nil, err
        }

        var (
                // Create the sweep transaction that we will be building. We
                // use version 2 as it is required for CSV.
                sweepTx = wire.NewMsgTx(2)

                // We'll add the inputs as we go so we know the final ordering
                // of inputs to sign.
                idxs []input.Input
        )

        // We start by adding all inputs that commit to an output. We do this
        // since the input and output index must stay the same for the
        // signatures to be valid.
        outpointToTxIndex := make(map[wire.OutPoint]int)
        for _, o := range inputs {
                if o.RequiredTxOut() == nil {
                        continue
                }

                idxs = append(idxs, o)
                sweepTx.AddTxIn(&wire.TxIn{
                        PreviousOutPoint: o.OutPoint(),
                        Sequence:         o.BlocksToMaturity(),
                })
                sweepTx.AddTxOut(o.RequiredTxOut())

                outpointToTxIndex[o.OutPoint()] = len(sweepTx.TxOut) - 1
        }

        // Sum up the value contained in the remaining inputs, and add them to
        // the sweep transaction.
        for _, o := range inputs {
                if o.RequiredTxOut() != nil {
                        continue
                }

                idxs = append(idxs, o)
                sweepTx.AddTxIn(&wire.TxIn{
                        PreviousOutPoint: o.OutPoint(),
                        Sequence:         o.BlocksToMaturity(),
                })
        }

        // If we have change outputs to add, then add it the sweep transaction
        // here.
        changeOutputsOpt.WhenSome(func(changeOuts []SweepOutput) {
                for i := range changeOuts {
                        sweepTx.AddTxOut(&changeOuts[i].TxOut)
                }
        })

        // We'll default to using the current block height as locktime, if none
        // of the inputs commits to a different locktime.
        sweepTx.LockTime = uint32(locktimeOpt.UnwrapOr(t.currentHeight.Load()))

        prevInputFetcher, err := input.MultiPrevOutFetcher(inputs)
        if err != nil {
                return nil, fmt.Errorf("error creating prev input fetcher "+
                        "for hash cache: %v", err)
        }
        hashCache := txscript.NewTxSigHashes(sweepTx, prevInputFetcher)

        // With all the inputs in place, use each output's unique input script
        // function to generate the final witness required for spending.
        addInputScript := func(idx int, tso input.Input) error {
                inputScript, err := tso.CraftInputScript(
                        t.cfg.Signer, sweepTx, hashCache, prevInputFetcher, idx,
                )
                if err != nil {
                        return err
                }

                sweepTx.TxIn[idx].Witness = inputScript.Witness

                if len(inputScript.SigScript) == 0 {
                        return nil
                }

                sweepTx.TxIn[idx].SignatureScript = inputScript.SigScript

                return nil
        }

        for idx, inp := range idxs {
                if err := addInputScript(idx, inp); err != nil {
                        return nil, err
                }
        }

        log.Debugf("Created sweep tx %v for inputs:\n%v", sweepTx.TxHash(),
                inputTypeSummary(inputs))

        // Try to locate the extra change output, though there might be None.
        extraTxOut := fn.MapOption(
                func(sweepOuts []SweepOutput) fn.Option[SweepOutput] {
                        for _, sweepOut := range sweepOuts {
                                if !sweepOut.IsExtra {
                                        continue
                                }

                                // If we sweep outputs of a custom channel, the
                                // custom leaves in those outputs will be merged
                                // into a single output, even if we sweep
                                // multiple outputs (e.g. to_remote and breached
                                // to_local of a breached channel) at the same
                                // time. So there will only ever be one extra
                                // output.
                                log.Debugf("Sweep produced extra_sweep_out=%v",
                                        lnutils.SpewLogClosure(sweepOut))

                                return fn.Some(sweepOut)
                        }

                        return fn.None[SweepOutput]()
                },
        )(changeOutputsOpt)

        return &sweepTxCtx{
                tx:                sweepTx,
                fee:               txFee,
                extraTxOut:        fn.FlattenOption(extraTxOut),
                outpointToTxIndex: outpointToTxIndex,
        }, nil
}

// prepareSweepTx returns the tx fee, a set of optional change outputs and an
// optional locktime after a series of validations:
// 1. check the locktime has been reached.
// 2. check the locktimes are the same.
// 3. check the inputs cover the outputs.
//
// NOTE: if the change amount is below dust, it will be added to the tx fee.
func prepareSweepTx(inputs []input.Input, changePkScript lnwallet.AddrWithKey,
        feeRate chainfee.SatPerKWeight, currentHeight int32,
        auxSweeper fn.Option[AuxSweeper]) (
        btcutil.Amount, fn.Option[[]SweepOutput], fn.Option[int32], error) {

        noChange := fn.None[[]SweepOutput]()
        noLocktime := fn.None[int32]()

        // Given the set of inputs we have, if we have an aux sweeper, then
        // we'll attempt to see if we have any other change outputs we'll need
        // to add to the sweep transaction.
        changePkScripts := [][]byte{changePkScript.DeliveryAddress}

        var extraChangeOut fn.Option[SweepOutput]
        err := fn.MapOptionZ(
                auxSweeper, func(aux AuxSweeper) error {
                        extraOut := aux.DeriveSweepAddr(inputs, changePkScript)
                        if err := extraOut.Err(); err != nil {
                                return err
                        }

                        extraChangeOut = extraOut.LeftToSome()

                        return nil
                },
        )
        if err != nil {
                return 0, noChange, noLocktime, err
        }

        // Creating a weight estimator with nil outputs and zero max fee rate.
        // We don't allow adding customized outputs in the sweeping tx, and the
        // fee rate is already being managed before we get here.
        inputs, estimator, err := getWeightEstimate(
                inputs, nil, feeRate, 0, changePkScripts,
        )
        if err != nil {
                return 0, noChange, noLocktime, err
        }

        txFee := estimator.fee()

        var (
                // Track whether any of the inputs require a certain locktime.
                locktime = int32(-1)

                // We keep track of total input amount, and required output
                // amount to use for calculating the change amount below.
                totalInput     btcutil.Amount
                requiredOutput btcutil.Amount
        )

        // If we have an extra change output, then we'll add it as a required
        // output amt.
        extraChangeOut.WhenSome(func(o SweepOutput) {
                requiredOutput += btcutil.Amount(o.Value)
        })

        // Go through each input and check if the required lock times have
        // reached and are the same.
        for _, o := range inputs {
                // If the input has a required output, we'll add it to the
                // required output amount.
                if o.RequiredTxOut() != nil {
                        requiredOutput += btcutil.Amount(
                                o.RequiredTxOut().Value,
                        )
                }

                // Update the total input amount.
                totalInput += btcutil.Amount(o.SignDesc().Output.Value)

                lt, ok := o.RequiredLockTime()

                // Skip if the input doesn't require a lock time.
                if !ok {
                        continue
                }

                // Check if the lock time has reached
                if lt > uint32(currentHeight) {
                        return 0, noChange, noLocktime,
                                fmt.Errorf("%w: current height is %v, "+
                                        "locktime is %v", ErrLocktimeImmature,
                                        currentHeight, lt)
                }

                // If another input commits to a different locktime, they
                // cannot be combined in the same transaction.
                if locktime != -1 && locktime != int32(lt) {
                        return 0, noChange, noLocktime, ErrLocktimeConflict
                }

                // Update the locktime for next iteration.
                locktime = int32(lt)
        }

        // Make sure total output amount is less than total input amount.
        if requiredOutput+txFee > totalInput {
                log.Errorf("Insufficient input to create sweep tx: "+
                        "input_sum=%v, output_sum=%v", totalInput,
                        requiredOutput+txFee)

                return 0, noChange, noLocktime, ErrNotEnoughInputs
        }

        // The value remaining after the required output and fees is the
        // change output.
        changeAmt := totalInput - requiredOutput - txFee
        changeOuts := make([]SweepOutput, 0, 2)

        extraChangeOut.WhenSome(func(o SweepOutput) {
                changeOuts = append(changeOuts, o)
        })

        // We'll calculate the dust limit for the given changePkScript since it
        // is variable.
        changeFloor := lnwallet.DustLimitForSize(
                len(changePkScript.DeliveryAddress),
        )

        switch {
        // If the change amount is dust, we'll move it into the fees, and
        // ignore it.
        case changeAmt < changeFloor:
                log.Infof("Change amt %v below dustlimit %v, not adding "+
                        "change output", changeAmt, changeFloor)

                // If there's no required output, and the change output is a
                // dust, it means we are creating a tx without any outputs. In
                // this case we'll return an error. This could happen when
                // creating a tx that has an anchor as the only input.
                if requiredOutput == 0 {
                        return 0, noChange, noLocktime, ErrTxNoOutput
                }

                // The dust amount is added to the fee.
                txFee += changeAmt

        // Otherwise, we'll actually recognize it as a change output.
        default:
                changeOuts = append(changeOuts, SweepOutput{
                        TxOut: wire.TxOut{
                                Value:    int64(changeAmt),
                                PkScript: changePkScript.DeliveryAddress,
                        },
                        IsExtra:     false,
                        InternalKey: changePkScript.InternalKey,
                })
        }

        // Optionally set the locktime.
        locktimeOpt := fn.Some(locktime)
        if locktime == -1 {
                locktimeOpt = noLocktime
        }

        var changeOutsOpt fn.Option[[]SweepOutput]
        if len(changeOuts) > 0 {
                changeOutsOpt = fn.Some(changeOuts)
        }

        log.Debugf("Creating sweep tx for %v inputs (%s) using %v, "+
                "tx_weight=%v, tx_fee=%v, locktime=%v, parents_count=%v, "+
                "parents_fee=%v, parents_weight=%v, current_height=%v",
                len(inputs), inputTypeSummary(inputs), feeRate,
                estimator.weight(), txFee, locktimeOpt, len(estimator.parents),
                estimator.parentsFee, estimator.parentsWeight, currentHeight)

        return txFee, changeOutsOpt, locktimeOpt, nil
}

// handleReplacementTxError handles the error returned from creating the
// replacement tx. It returns a BumpResult that should be notified to the
// sweeper.
func (t *TxPublisher) handleReplacementTxError(r *monitorRecord,
        oldTx *wire.MsgTx, err error) fn.Option[BumpResult] {

        // If the error is fee related, we will return no error and let the fee
        // bumper retry it at next block.
        //
        // NOTE: we can check the RBF error here and ask the fee function to
        // recalculate the fee rate. However, this would defeat the purpose of
        // using a deadline based fee function:
        // - if the deadline is far away, there's no rush to RBF the tx.
        // - if the deadline is close, we expect the fee function to give us a
        //   higher fee rate. If the fee rate cannot satisfy the RBF rules, it
        //   means the budget is not enough.
        if errors.Is(err, chain.ErrInsufficientFee) ||
                errors.Is(err, lnwallet.ErrMempoolFee) {

                log.Debugf("Failed to bump tx %v: %v", oldTx.TxHash(), err)
                return fn.None[BumpResult]()
        }

        // At least one of the inputs is missing, which means it has already
        // been spent by another tx and confirmed. In this case we will handle
        // it by returning a TxUnknownSpend bump result.
        if errors.Is(err, ErrInputMissing) {
                log.Warnf("Fail to fee bump tx %v: %v", oldTx.TxHash(), err)
                bumpResult := t.handleMissingInputs(r)

                return fn.Some(*bumpResult)
        }

        // Return a failed event to retry the sweep.
        event := TxFailed

        // Calculate the next fee rate for the retry.
        feeRate, ferr := t.calculateRetryFeeRate(r)
        if ferr != nil {
                // If there's an error with the fee calculation, we need to
                // abort the sweep.
                event = TxFatal
        }

        // If the error is not fee related, we will return a `TxFailed` event so
        // this input can be retried.
        result := fn.Some(BumpResult{
                Event:     event,
                Tx:        oldTx,
                Err:       err,
                requestID: r.requestID,
                FeeRate:   feeRate,
        })

        // If the tx doesn't not have enough budget, or if the inputs amounts
        // are not sufficient to cover the budget, we will return a result so
        // the sweeper can handle it by re-clustering the utxos.
        if errors.Is(err, ErrNotEnoughBudget) ||
                errors.Is(err, ErrNotEnoughInputs) {

                log.Warnf("Fail to fee bump tx %v: %v", oldTx.TxHash(), err)
                return result
        }

        // Otherwise, an unexpected error occurred, we will log an error and let
        // the sweeper retry the whole process.
        log.Errorf("Failed to bump tx %v: %v", oldTx.TxHash(), err)

        return result
}

// calculateRetryFeeRate calculates a new fee rate to be used as the starting
// fee rate for the next sweep attempt if the inputs are to be retried. When the
// fee function is nil it will be created here, and an error is returned if the
// fee func cannot be initialized.
func (t *TxPublisher) calculateRetryFeeRate(
        r *monitorRecord) (chainfee.SatPerKWeight, error) {

        // Get the fee function, which will be used to decided the next fee rate
        // to use if the sweeper decides to retry sweeping this input.
        feeFunc := r.feeFunction

        // When the record is failed before the initial broadcast is attempted,
        // it will have a nil fee func. In this case, we'll create the fee func
        // here.
        //
        // NOTE: Since the current record is failed and will be deleted, we
        // don't need to update the record on this fee function. We only need
        // the fee rate data so the sweeper can pick up where we left off.
        if feeFunc == nil {
                f, err := t.initializeFeeFunction(r.req)

                // TODO(yy): The only error we would receive here is when the
                // pkScript is not recognized by the weightEstimator. What we
                // should do instead is to check the pkScript immediately after
                // receiving a sweep request so we don't need to check it again,
                // which will also save us from error checking from several
                // callsites.
                if err != nil {
                        log.Errorf("Failed to create fee func for record %v: "+
                                "%v", r.requestID, err)

                        return 0, err
                }

                feeFunc = f
        }

        // Since we failed to sweep the inputs, either the sweeping tx has been
        // replaced by another party's tx, or the current output values cannot
        // cover the budget, we missed this block window to increase its fee
        // rate. To make sure the fee rate stays in the initial line, we now ask
        // the fee function to give us the next fee rate as if the sweeping tx
        // were RBFed. This new fee rate will be used as the starting fee rate
        // if the upper system decides to continue sweeping the rest of the
        // inputs.
        _, err := feeFunc.Increment()
        if err != nil {
                // The fee function has reached its max position - nothing we
                // can do here other than letting the user increase the budget.
                log.Errorf("Failed to calculate the next fee rate for "+
                        "Record(%v): %v", r.requestID, err)
        }

        return feeFunc.FeeRate(), nil
}

lightningnetwork / lnd / 16183445888

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