17830307614

Committed 18 Sep 2025 01:29PM UTC coverage: 54.617% (-12.0%) from 66.637%

Build # 17830307614

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Pull #10200

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web-flow

Commit Message

Merge 181a0a7bc into b34fc964b

Pull Request Pull Request #10200: github: change to form-based issue template

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71.47

/contractcourt/htlc_timeout_resolver.go

package contractcourt

import (
        "encoding/binary"
        "fmt"
        "io"
        "sync"

        "github.com/btcsuite/btcd/btcutil"
        "github.com/btcsuite/btcd/chaincfg/chainhash"
        "github.com/btcsuite/btcd/txscript"
        "github.com/btcsuite/btcd/wire"
        "github.com/lightningnetwork/lnd/chainntnfs"
        "github.com/lightningnetwork/lnd/channeldb"
        "github.com/lightningnetwork/lnd/fn/v2"
        "github.com/lightningnetwork/lnd/input"
        "github.com/lightningnetwork/lnd/lntypes"
        "github.com/lightningnetwork/lnd/lnutils"
        "github.com/lightningnetwork/lnd/lnwallet"
        "github.com/lightningnetwork/lnd/lnwire"
        "github.com/lightningnetwork/lnd/sweep"
)

// htlcTimeoutResolver is a ContractResolver that's capable of resolving an
// outgoing HTLC. The HTLC may be on our commitment transaction, or on the
// commitment transaction of the remote party. An output on our commitment
// transaction is considered fully resolved once the second-level transaction
// has been confirmed (and reached a sufficient depth). An output on the
// commitment transaction of the remote party is resolved once we detect a
// spend of the direct HTLC output using the timeout clause.
type htlcTimeoutResolver struct {
        // htlcResolution contains all the information required to properly
        // resolve this outgoing HTLC.
        htlcResolution lnwallet.OutgoingHtlcResolution

        // outputIncubating returns true if we've sent the output to the output
        // incubator (utxo nursery).
        outputIncubating bool

        // broadcastHeight is the height that the original contract was
        // broadcast to the main-chain at. We'll use this value to bound any
        // historical queries to the chain for spends/confirmations.
        //
        // TODO(roasbeef): wrap above into definite resolution embedding?
        broadcastHeight uint32

        // htlc contains information on the htlc that we are resolving on-chain.
        htlc channeldb.HTLC

        // currentReport stores the current state of the resolver for reporting
        // over the rpc interface. This should only be reported in case we have
        // a non-nil SignDetails on the htlcResolution, otherwise the nursery
        // will produce reports.
        currentReport ContractReport

        // reportLock prevents concurrent access to the resolver report.
        reportLock sync.Mutex

        contractResolverKit

        htlcLeaseResolver

        // incomingHTLCExpiryHeight is the absolute block height at which the
        // incoming HTLC will expire. This is used as the deadline height as
        // the outgoing HTLC must be swept before its incoming HTLC expires.
        incomingHTLCExpiryHeight fn.Option[int32]
}

// newTimeoutResolver instantiates a new timeout htlc resolver.
func newTimeoutResolver(res lnwallet.OutgoingHtlcResolution,
        broadcastHeight uint32, htlc channeldb.HTLC,
        resCfg ResolverConfig) *htlcTimeoutResolver {

        h := &htlcTimeoutResolver{
                contractResolverKit: *newContractResolverKit(resCfg),
                htlcResolution:      res,
                broadcastHeight:     broadcastHeight,
                htlc:                htlc,
        }

        h.initReport()
        h.initLogger(fmt.Sprintf("%T(%v)", h, h.outpoint()))

        return h
}

// isTaproot returns true if the htlc output is a taproot output.
func (h *htlcTimeoutResolver) isTaproot() bool {
        return txscript.IsPayToTaproot(
                h.htlcResolution.SweepSignDesc.Output.PkScript,
        )
}

// outpoint returns the outpoint of the HTLC output we're attempting to sweep.
func (h *htlcTimeoutResolver) outpoint() wire.OutPoint {
        // The primary key for this resolver will be the outpoint of the HTLC
        // on the commitment transaction itself. If this is our commitment,
        // then the output can be found within the signed timeout tx,
        // otherwise, it's just the ClaimOutpoint.
        if h.htlcResolution.SignedTimeoutTx != nil {
                return h.htlcResolution.SignedTimeoutTx.TxIn[0].PreviousOutPoint
        }

        return h.htlcResolution.ClaimOutpoint
}

// ResolverKey returns an identifier which should be globally unique for this
// particular resolver within the chain the original contract resides within.
//
// NOTE: Part of the ContractResolver interface.
func (h *htlcTimeoutResolver) ResolverKey() []byte {
        key := newResolverID(h.outpoint())
        return key[:]
}

const (
        // expectedRemoteWitnessSuccessSize is the expected size of the witness
        // on the remote commitment transaction for an outgoing HTLC that is
        // swept on-chain by them with pre-image.
        expectedRemoteWitnessSuccessSize = 5

        // expectedLocalWitnessSuccessSize is the expected size of the witness
        // on the local commitment transaction for an outgoing HTLC that is
        // swept on-chain by them with pre-image.
        expectedLocalWitnessSuccessSize = 3

        // remotePreimageIndex index within the witness on the remote
        // commitment transaction that will hold they pre-image if they go to
        // sweep it on chain.
        remotePreimageIndex = 3

        // localPreimageIndex is the index within the witness on the local
        // commitment transaction for an outgoing HTLC that will hold the
        // pre-image if the remote party sweeps it.
        localPreimageIndex = 1

        // remoteTaprootWitnessSuccessSize is the expected size of the witness
        // on the remote commitment for taproot channels. The spend path will
        // look like
        //   - <sender sig> <receiver sig> <preimage> <success_script>
        //     <control_block>
        remoteTaprootWitnessSuccessSize = 5

        // localTaprootWitnessSuccessSize is the expected size of the witness
        // on the local commitment for taproot channels. The spend path will
        // look like
        //  - <receiver sig> <preimage> <success_script> <control_block>
        localTaprootWitnessSuccessSize = 4

        // taprootRemotePreimageIndex is the index within the witness on the
        // taproot remote commitment spend that'll hold the pre-image if the
        // remote party sweeps it.
        taprootRemotePreimageIndex = 2
)

// claimCleanUp is a helper method that's called once the HTLC output is spent
// by the remote party. It'll extract the preimage, add it to the global cache,
// and finally send the appropriate clean up message.
func (h *htlcTimeoutResolver) claimCleanUp(
        commitSpend *chainntnfs.SpendDetail) error {

        // Depending on if this is our commitment or not, then we'll be looking
        // for a different witness pattern.
        spenderIndex := commitSpend.SpenderInputIndex
        spendingInput := commitSpend.SpendingTx.TxIn[spenderIndex]

        log.Infof("%T(%v): extracting preimage! remote party spent "+
                "HTLC with tx=%v", h, h.htlcResolution.ClaimOutpoint,
                lnutils.SpewLogClosure(commitSpend.SpendingTx))

        // If this is the remote party's commitment, then we'll be looking for
        // them to spend using the second-level success transaction.
        var preimageBytes []byte
        switch {
        // For taproot channels, if the remote party has swept the HTLC, then
        // the witness stack will look like:
        //
        //   - <sender sig> <receiver sig> <preimage> <success_script>
        //     <control_block>
        case h.isTaproot() && h.htlcResolution.SignedTimeoutTx == nil:
                //nolint:ll
                preimageBytes = spendingInput.Witness[taprootRemotePreimageIndex]

        // The witness stack when the remote party sweeps the output on a
        // regular channel to them looks like:
        //
        //  - <0> <sender sig> <recvr sig> <preimage> <witness script>
        case !h.isTaproot() && h.htlcResolution.SignedTimeoutTx == nil:
                preimageBytes = spendingInput.Witness[remotePreimageIndex]

        // If this is a taproot channel, and there's only a single witness
        // element, then we're actually on the losing side of a breach
        // attempt...
        case h.isTaproot() && len(spendingInput.Witness) == 1:
                return fmt.Errorf("breach attempt failed")

        // Otherwise, they'll be spending directly from our commitment output.
        // In which case the witness stack looks like:
        //
        //  - <sig> <preimage> <witness script>
        //
        // For taproot channels, this looks like:
        //  - <receiver sig> <preimage> <success_script> <control_block>
        //
        // So we can target the same index.
        default:
                preimageBytes = spendingInput.Witness[localPreimageIndex]
        }

        preimage, err := lntypes.MakePreimage(preimageBytes)
        if err != nil {
                return fmt.Errorf("unable to create pre-image from witness: %w",
                        err)
        }

        log.Infof("%T(%v): extracting preimage=%v from on-chain "+
                "spend!", h, h.htlcResolution.ClaimOutpoint, preimage)

        // With the preimage obtained, we can now add it to the global cache.
        if err := h.PreimageDB.AddPreimages(preimage); err != nil {
                log.Errorf("%T(%v): unable to add witness to cache",
                        h, h.htlcResolution.ClaimOutpoint)
        }

        var pre [32]byte
        copy(pre[:], preimage[:])

        // Finally, we'll send the clean up message, mark ourselves as
        // resolved, then exit.
        if err := h.DeliverResolutionMsg(ResolutionMsg{
                SourceChan: h.ShortChanID,
                HtlcIndex:  h.htlc.HtlcIndex,
                PreImage:   &pre,
        }); err != nil {
                return err
        }
        h.markResolved()

        // Checkpoint our resolver with a report which reflects the preimage
        // claim by the remote party.
        amt := btcutil.Amount(h.htlcResolution.SweepSignDesc.Output.Value)
        report := &channeldb.ResolverReport{
                OutPoint:        h.htlcResolution.ClaimOutpoint,
                Amount:          amt,
                ResolverType:    channeldb.ResolverTypeOutgoingHtlc,
                ResolverOutcome: channeldb.ResolverOutcomeClaimed,
                SpendTxID:       commitSpend.SpenderTxHash,
        }

        return h.Checkpoint(h, report)
}

// chainDetailsToWatch returns the output and script which we use to watch for
// spends from the direct HTLC output on the commitment transaction.
func (h *htlcTimeoutResolver) chainDetailsToWatch() (*wire.OutPoint, []byte, error) {
        // If there's no timeout transaction, it means we are spending from a
        // remote commit, then the claim output is the output directly on the
        // commitment transaction, so we'll just use that.
        if h.htlcResolution.SignedTimeoutTx == nil {
                outPointToWatch := h.htlcResolution.ClaimOutpoint
                scriptToWatch := h.htlcResolution.SweepSignDesc.Output.PkScript

                return &outPointToWatch, scriptToWatch, nil
        }

        // If SignedTimeoutTx is not nil, this is the local party's commitment,
        // and we'll need to grab watch the output that our timeout transaction
        // points to. We can directly grab the outpoint, then also extract the
        // witness script (the last element of the witness stack) to
        // re-construct the pkScript we need to watch.
        //
        //nolint:ll
        outPointToWatch := h.htlcResolution.SignedTimeoutTx.TxIn[0].PreviousOutPoint
        witness := h.htlcResolution.SignedTimeoutTx.TxIn[0].Witness

        var (
                scriptToWatch []byte
                err           error
        )
        switch {
        // For taproot channels, then final witness element is the control
        // block, and the one before it the witness script. We can use both of
        // these together to reconstruct the taproot output key, then map that
        // into a v1 witness program.
        case h.isTaproot():
                // First, we'll parse the control block into something we can
                // use.
                ctrlBlockBytes := witness[len(witness)-1]
                ctrlBlock, err := txscript.ParseControlBlock(ctrlBlockBytes)
                if err != nil {
                        return nil, nil, err
                }

                // With the control block, we'll grab the witness script, then
                // use that to derive the tapscript root.
                witnessScript := witness[len(witness)-2]
                tapscriptRoot := ctrlBlock.RootHash(witnessScript)

                // Once we have the root, then we can derive the output key
                // from the internal key, then turn that into a witness
                // program.
                outputKey := txscript.ComputeTaprootOutputKey(
                        ctrlBlock.InternalKey, tapscriptRoot,
                )
                scriptToWatch, err = txscript.PayToTaprootScript(outputKey)
                if err != nil {
                        return nil, nil, err
                }

        // For regular channels, the witness script is the last element on the
        // stack. We can then use this to re-derive the output that we're
        // watching on chain.
        default:
                scriptToWatch, err = input.WitnessScriptHash(
                        witness[len(witness)-1],
                )
        }
        if err != nil {
                return nil, nil, err
        }

        return &outPointToWatch, scriptToWatch, nil
}

// isPreimageSpend returns true if the passed spend on the specified commitment
// is a success spend that reveals the pre-image or not.
func isPreimageSpend(isTaproot bool, spend *chainntnfs.SpendDetail,
        localCommit bool) bool {

        // Based on the spending input index and transaction, obtain the
        // witness that tells us what type of spend this is.
        spenderIndex := spend.SpenderInputIndex
        spendingInput := spend.SpendingTx.TxIn[spenderIndex]
        spendingWitness := spendingInput.Witness

        switch {
        // If this is a taproot remote commitment, then we can detect the type
        // of spend via the leaf revealed in the control block and the witness
        // itself.
        //
        // The keyspend (revocation path) is just a single signature, while the
        // timeout and success paths are most distinct.
        //
        // The success path will look like:
        //
        //   - <sender sig> <receiver sig> <preimage> <success_script>
        //     <control_block>
        case isTaproot && !localCommit:
                return checkSizeAndIndex(
                        spendingWitness, remoteTaprootWitnessSuccessSize,
                        taprootRemotePreimageIndex,
                )

        // Otherwise, then if this is our local commitment transaction, then if
        // they're sweeping the transaction, it'll be directly from the output,
        // skipping the second level.
        //
        // In this case, then there're two main tapscript paths, with the
        // success case look like:
        //
        //  - <receiver sig> <preimage> <success_script> <control_block>
        case isTaproot && localCommit:
                return checkSizeAndIndex(
                        spendingWitness, localTaprootWitnessSuccessSize,
                        localPreimageIndex,
                )

        // If this is the non-taproot, remote commitment then the only possible
        // spends for outgoing HTLCs are:
        //
        //  RECVR: <0> <sender sig> <recvr sig> <preimage> (2nd level success spend)
        //  REVOK: <sig> <key>
        //  SENDR: <sig> 0
        //
        // In this case, if 5 witness elements are present (factoring the
        // witness script), and the 3rd element is the size of the pre-image,
        // then this is a remote spend. If not, then we swept it ourselves, or
        // revoked their output.
        case !isTaproot && !localCommit:
                return checkSizeAndIndex(
                        spendingWitness, expectedRemoteWitnessSuccessSize,
                        remotePreimageIndex,
                )

        // Otherwise, for our non-taproot commitment, the only possible spends
        // for an outgoing HTLC are:
        //
        //  SENDR: <0> <sendr sig>  <recvr sig> <0> (2nd level timeout)
        //  RECVR: <recvr sig>  <preimage>
        //  REVOK: <revoke sig> <revoke key>
        //
        // So the only success case has the pre-image as the 2nd (index 1)
        // element in the witness.
        case !isTaproot:
                fallthrough

        default:
                return checkSizeAndIndex(
                        spendingWitness, expectedLocalWitnessSuccessSize,
                        localPreimageIndex,
                )
        }
}

// checkSizeAndIndex checks that the witness is of the expected size and that
// the witness element at the specified index is of the expected size.
func checkSizeAndIndex(witness wire.TxWitness, size, index int) bool {
        if len(witness) != size {
                return false
        }

        return len(witness[index]) == lntypes.HashSize
}

// Resolve kicks off full resolution of an outgoing HTLC output. If it's our
// commitment, it isn't resolved until we see the second level HTLC txn
// confirmed. If it's the remote party's commitment, we don't resolve until we
// see a direct sweep via the timeout clause.
//
// NOTE: Part of the ContractResolver interface.
func (h *htlcTimeoutResolver) Resolve() (ContractResolver, error) {
        // If we're already resolved, then we can exit early.
        if h.IsResolved() {
                h.log.Errorf("already resolved")
                return nil, nil
        }

        // If this is an output on the remote party's commitment transaction,
        // use the direct-spend path to sweep the htlc.
        if h.isRemoteCommitOutput() {
                return nil, h.resolveRemoteCommitOutput()
        }

        // If this is a zero-fee HTLC, we now handle the spend from our
        // commitment transaction.
        if h.isZeroFeeOutput() {
                return nil, h.resolveTimeoutTx()
        }

        // If this is an output on our own commitment using pre-anchor channel
        // type, we will let the utxo nursery handle it.
        return nil, h.resolveSecondLevelTxLegacy()
}

// sweepTimeoutTx sends a second level timeout transaction to the sweeper.
// This transaction uses the SINGLE|ANYONECANPAY flag.
func (h *htlcTimeoutResolver) sweepTimeoutTx() error {
        var inp input.Input
        if h.isTaproot() {
                inp = lnutils.Ptr(input.MakeHtlcSecondLevelTimeoutTaprootInput(
                        h.htlcResolution.SignedTimeoutTx,
                        h.htlcResolution.SignDetails,
                        h.broadcastHeight,
                        input.WithResolutionBlob(
                                h.htlcResolution.ResolutionBlob,
                        ),
                ))
        } else {
                inp = lnutils.Ptr(input.MakeHtlcSecondLevelTimeoutAnchorInput(
                        h.htlcResolution.SignedTimeoutTx,
                        h.htlcResolution.SignDetails,
                        h.broadcastHeight,
                ))
        }

        // Calculate the budget.
        budget := calculateBudget(
                btcutil.Amount(inp.SignDesc().Output.Value),
                h.Budget.DeadlineHTLCRatio, h.Budget.DeadlineHTLC,
        )

        h.log.Infof("offering 2nd-level HTLC timeout tx to sweeper "+
                "with deadline=%v, budget=%v", h.incomingHTLCExpiryHeight,
                budget)

        // For an outgoing HTLC, it must be swept before the RefundTimeout of
        // its incoming HTLC is reached.
        _, err := h.Sweeper.SweepInput(
                inp,
                sweep.Params{
                        Budget:         budget,
                        DeadlineHeight: h.incomingHTLCExpiryHeight,
                },
        )
        if err != nil {
                return err
        }

        return nil
}

// resolveSecondLevelTxLegacy sends a second level timeout transaction to the
// utxo nursery. This transaction uses the legacy SIGHASH_ALL flag.
func (h *htlcTimeoutResolver) resolveSecondLevelTxLegacy() error {
        h.log.Debug("incubating htlc output")

        // The utxo nursery will take care of broadcasting the second-level
        // timeout tx and sweeping its output once it confirms.
        err := h.IncubateOutputs(
                h.ChanPoint, fn.Some(h.htlcResolution),
                fn.None[lnwallet.IncomingHtlcResolution](),
                h.broadcastHeight, h.incomingHTLCExpiryHeight,
        )
        if err != nil {
                return err
        }

        return h.resolveTimeoutTx()
}

// sweepDirectHtlcOutput sends the direct spend of the HTLC output to the
// sweeper. This is used when the remote party goes on chain, and we're able to
// sweep an HTLC we offered after a timeout. Only the CLTV encumbered outputs
// are resolved via this path.
func (h *htlcTimeoutResolver) sweepDirectHtlcOutput() error {
        var htlcWitnessType input.StandardWitnessType
        if h.isTaproot() {
                htlcWitnessType = input.TaprootHtlcOfferedRemoteTimeout
        } else {
                htlcWitnessType = input.HtlcOfferedRemoteTimeout
        }

        sweepInput := input.NewCsvInputWithCltv(
                &h.htlcResolution.ClaimOutpoint, htlcWitnessType,
                &h.htlcResolution.SweepSignDesc, h.broadcastHeight,
                h.htlcResolution.CsvDelay, h.htlcResolution.Expiry,
                input.WithResolutionBlob(h.htlcResolution.ResolutionBlob),
        )

        // Calculate the budget.
        budget := calculateBudget(
                btcutil.Amount(sweepInput.SignDesc().Output.Value),
                h.Budget.DeadlineHTLCRatio, h.Budget.DeadlineHTLC,
        )

        log.Infof("%T(%x): offering offered remote timeout HTLC output to "+
                "sweeper with deadline %v and budget=%v at height=%v",
                h, h.htlc.RHash[:], h.incomingHTLCExpiryHeight, budget,
                h.broadcastHeight)

        _, err := h.Sweeper.SweepInput(
                sweepInput,
                sweep.Params{
                        Budget: budget,

                        // This is an outgoing HTLC, so we want to make sure
                        // that we sweep it before the incoming HTLC expires.
                        DeadlineHeight: h.incomingHTLCExpiryHeight,
                },
        )
        if err != nil {
                return err
        }

        return nil
}

// watchHtlcSpend watches for a spend of the HTLC output. For neutrino backend,
// it will check blocks for the confirmed spend. For btcd and bitcoind, it will
// check both the mempool and the blocks.
func (h *htlcTimeoutResolver) watchHtlcSpend() (*chainntnfs.SpendDetail,
        error) {

        // TODO(yy): outpointToWatch is always h.HtlcOutpoint(), can refactor
        // to remove the redundancy.
        outpointToWatch, scriptToWatch, err := h.chainDetailsToWatch()
        if err != nil {
                return nil, err
        }

        // If there's no mempool configured, which is the case for SPV node
        // such as neutrino, then we will watch for confirmed spend only.
        if h.Mempool == nil {
                return h.waitForConfirmedSpend(outpointToWatch, scriptToWatch)
        }

        // Watch for a spend of the HTLC output in both the mempool and blocks.
        return h.waitForMempoolOrBlockSpend(*outpointToWatch, scriptToWatch)
}

// waitForConfirmedSpend waits for the HTLC output to be spent and confirmed in
// a block, returns the spend details.
func (h *htlcTimeoutResolver) waitForConfirmedSpend(op *wire.OutPoint,
        pkScript []byte) (*chainntnfs.SpendDetail, error) {

        // We'll block here until either we exit, or the HTLC output on the
        // commitment transaction has been spent.
        spend, err := waitForSpend(
                op, pkScript, h.broadcastHeight, h.Notifier, h.quit,
        )
        if err != nil {
                return nil, err
        }

        return spend, nil
}

// Stop signals the resolver to cancel any current resolution processes, and
// suspend.
//
// NOTE: Part of the ContractResolver interface.
func (h *htlcTimeoutResolver) Stop() {
        h.log.Debugf("stopping...")
        defer h.log.Debugf("stopped")

        close(h.quit)
}

// report returns a report on the resolution state of the contract.
func (h *htlcTimeoutResolver) report() *ContractReport {
        // If we have a SignedTimeoutTx but no SignDetails, this is a local
        // commitment for a non-anchor channel, which was handled by the utxo
        // nursery.
        if h.htlcResolution.SignDetails == nil && h.
                htlcResolution.SignedTimeoutTx != nil {
                return nil
        }

        h.reportLock.Lock()
        defer h.reportLock.Unlock()
        cpy := h.currentReport
        return &cpy
}

func (h *htlcTimeoutResolver) initReport() {
        // We create the initial report. This will only be reported for
        // resolvers not handled by the nursery.
        finalAmt := h.htlc.Amt.ToSatoshis()
        if h.htlcResolution.SignedTimeoutTx != nil {
                finalAmt = btcutil.Amount(
                        h.htlcResolution.SignedTimeoutTx.TxOut[0].Value,
                )
        }

        // If there's no timeout transaction, then we're already effectively in
        // level two.
        stage := uint32(1)
        if h.htlcResolution.SignedTimeoutTx == nil {
                stage = 2
        }

        h.currentReport = ContractReport{
                Outpoint:       h.htlcResolution.ClaimOutpoint,
                Type:           ReportOutputOutgoingHtlc,
                Amount:         finalAmt,
                MaturityHeight: h.htlcResolution.Expiry,
                LimboBalance:   finalAmt,
                Stage:          stage,
        }
}

// Encode writes an encoded version of the ContractResolver into the passed
// Writer.
//
// NOTE: Part of the ContractResolver interface.
func (h *htlcTimeoutResolver) Encode(w io.Writer) error {
        // First, we'll write out the relevant fields of the
        // OutgoingHtlcResolution to the writer.
        if err := encodeOutgoingResolution(w, &h.htlcResolution); err != nil {
                return err
        }

        // With that portion written, we can now write out the fields specific
        // to the resolver itself.
        if err := binary.Write(w, endian, h.outputIncubating); err != nil {
                return err
        }
        if err := binary.Write(w, endian, h.IsResolved()); err != nil {
                return err
        }
        if err := binary.Write(w, endian, h.broadcastHeight); err != nil {
                return err
        }

        if err := binary.Write(w, endian, h.htlc.HtlcIndex); err != nil {
                return err
        }

        // We encode the sign details last for backwards compatibility.
        err := encodeSignDetails(w, h.htlcResolution.SignDetails)
        if err != nil {
                return err
        }

        return nil
}

// newTimeoutResolverFromReader attempts to decode an encoded ContractResolver
// from the passed Reader instance, returning an active ContractResolver
// instance.
func newTimeoutResolverFromReader(r io.Reader, resCfg ResolverConfig) (
        *htlcTimeoutResolver, error) {

        h := &htlcTimeoutResolver{
                contractResolverKit: *newContractResolverKit(resCfg),
        }

        // First, we'll read out all the mandatory fields of the
        // OutgoingHtlcResolution that we store.
        if err := decodeOutgoingResolution(r, &h.htlcResolution); err != nil {
                return nil, err
        }

        // With those fields read, we can now read back the fields that are
        // specific to the resolver itself.
        if err := binary.Read(r, endian, &h.outputIncubating); err != nil {
                return nil, err
        }

        var resolved bool
        if err := binary.Read(r, endian, &resolved); err != nil {
                return nil, err
        }
        if resolved {
                h.markResolved()
        }

        if err := binary.Read(r, endian, &h.broadcastHeight); err != nil {
                return nil, err
        }

        if err := binary.Read(r, endian, &h.htlc.HtlcIndex); err != nil {
                return nil, err
        }

        // Sign details is a new field that was added to the htlc resolution,
        // so it is serialized last for backwards compatibility. We try to read
        // it, but don't error out if there are not bytes left.
        signDetails, err := decodeSignDetails(r)
        if err == nil {
                h.htlcResolution.SignDetails = signDetails
        } else if err != io.EOF && err != io.ErrUnexpectedEOF {
                return nil, err
        }

        h.initReport()
        h.initLogger(fmt.Sprintf("%T(%v)", h, h.outpoint()))

        return h, nil
}

// Supplement adds additional information to the resolver that is required
// before Resolve() is called.
//
// NOTE: Part of the htlcContractResolver interface.
func (h *htlcTimeoutResolver) Supplement(htlc channeldb.HTLC) {
        h.htlc = htlc
}

// HtlcPoint returns the htlc's outpoint on the commitment tx.
//
// NOTE: Part of the htlcContractResolver interface.
func (h *htlcTimeoutResolver) HtlcPoint() wire.OutPoint {
        return h.htlcResolution.HtlcPoint()
}

// SupplementDeadline sets the incomingHTLCExpiryHeight for this outgoing htlc
// resolver.
//
// NOTE: Part of the htlcContractResolver interface.
func (h *htlcTimeoutResolver) SupplementDeadline(d fn.Option[int32]) {
        h.incomingHTLCExpiryHeight = d
}

// A compile time assertion to ensure htlcTimeoutResolver meets the
// ContractResolver interface.
var _ htlcContractResolver = (*htlcTimeoutResolver)(nil)

// spendResult is used to hold the result of a spend event from either a
// mempool spend or a block spend.
type spendResult struct {
        // spend contains the details of the spend.
        spend *chainntnfs.SpendDetail

        // err is the error that occurred during the spend notification.
        err error
}

// waitForMempoolOrBlockSpend waits for the htlc output to be spent by a
// transaction that's either be found in the mempool or in a block.
func (h *htlcTimeoutResolver) waitForMempoolOrBlockSpend(op wire.OutPoint,
        pkScript []byte) (*chainntnfs.SpendDetail, error) {

        log.Infof("%T(%v): waiting for spent of HTLC output %v to be found "+
                "in mempool or block", h, h.htlcResolution.ClaimOutpoint, op)

        // Subscribe for block spent(confirmed).
        blockSpent, err := h.Notifier.RegisterSpendNtfn(
                &op, pkScript, h.broadcastHeight,
        )
        if err != nil {
                return nil, fmt.Errorf("register spend: %w", err)
        }

        // Subscribe for mempool spent(unconfirmed).
        mempoolSpent, err := h.Mempool.SubscribeMempoolSpent(op)
        if err != nil {
                return nil, fmt.Errorf("register mempool spend: %w", err)
        }

        // Create a result chan that will be used to receive the spending
        // events.
        result := make(chan *spendResult, 2)

        // Create a goroutine that will wait for either a mempool spend or a
        // block spend.
        //
        // NOTE: no need to use waitgroup here as when the resolver exits, the
        // goroutine will return on the quit channel.
        go h.consumeSpendEvents(result, blockSpent.Spend, mempoolSpent.Spend)

        // Wait for the spend event to be received.
        select {
        case event := <-result:
                // Cancel the mempool subscription as we don't need it anymore.
                h.Mempool.CancelMempoolSpendEvent(mempoolSpent)

                return event.spend, event.err

        case <-h.quit:
                return nil, errResolverShuttingDown
        }
}

// consumeSpendEvents consumes the spend events from the block and mempool
// subscriptions. It exits when a spend event is received from the block, or
// the resolver itself quits. When a spend event is received from the mempool,
// however, it won't exit but continuing to wait for a spend event from the
// block subscription.
//
// NOTE: there could be a case where we found the preimage in the mempool,
// which will be added to our preimage beacon and settle the incoming link,
// meanwhile the timeout sweep tx confirms. This outgoing HTLC is "free" money
// and is not swept here.
//
// TODO(yy): sweep the outgoing htlc if it's confirmed.
func (h *htlcTimeoutResolver) consumeSpendEvents(resultChan chan *spendResult,
        blockSpent, mempoolSpent <-chan *chainntnfs.SpendDetail) {

        op := h.HtlcPoint()

        // Create a result chan to hold the results.
        result := &spendResult{}

        // Wait for a spend event to arrive.
        for {
                select {
                // If a spend event is received from the block, this outgoing
                // htlc is spent either by the remote via the preimage or by us
                // via the timeout. We can exit the loop and `claimCleanUp`
                // will feed the preimage to the beacon if found. This treats
                // the block as the final judge and the preimage spent won't
                // appear in the mempool afterwards.
                //
                // NOTE: if a reorg happens, the preimage spend can appear in
                // the mempool again. Though a rare case, we should handle it
                // in a dedicated reorg system.
                case spendDetail, ok := <-blockSpent:
                        if !ok {
                                result.err = fmt.Errorf("block spent err: %w",
                                        errResolverShuttingDown)
                        } else {
                                log.Debugf("Found confirmed spend of HTLC "+
                                        "output %s in tx=%s", op,
                                        spendDetail.SpenderTxHash)

                                result.spend = spendDetail

                                // Once confirmed, persist the state on disk if
                                // we haven't seen the output's spending tx in
                                // mempool before.
                        }

                        // Send the result and exit the loop.
                        resultChan <- result

                        return

                // If a spend event is received from the mempool, this can be
                // either the 2nd stage timeout tx or a preimage spend from the
                // remote. We will further check whether the spend reveals the
                // preimage and add it to the preimage beacon to settle the
                // incoming link.
                //
                // NOTE: we won't exit the loop here so we can continue to
                // watch for the block spend to check point the resolution.
                case spendDetail, ok := <-mempoolSpent:
                        if !ok {
                                result.err = fmt.Errorf("mempool spent err: %w",
                                        errResolverShuttingDown)

                                // This is an internal error so we exit.
                                resultChan <- result

                                return
                        }

                        log.Debugf("Found mempool spend of HTLC output %s "+
                                "in tx=%s", op, spendDetail.SpenderTxHash)

                        // Check whether the spend reveals the preimage, if not
                        // continue the loop.
                        hasPreimage := isPreimageSpend(
                                h.isTaproot(), spendDetail,
                                !h.isRemoteCommitOutput(),
                        )
                        if !hasPreimage {
                                log.Debugf("HTLC output %s spent doesn't "+
                                        "reveal preimage", op)
                                continue
                        }

                        // Found the preimage spend, send the result and
                        // continue the loop.
                        result.spend = spendDetail
                        resultChan <- result

                        continue

                // If the resolver exits, we exit the goroutine.
                case <-h.quit:
                        result.err = errResolverShuttingDown
                        resultChan <- result

                        return
                }
        }
}

// isRemoteCommitOutput returns a bool to indicate whether the htlc output is
// on the remote commitment.
func (h *htlcTimeoutResolver) isRemoteCommitOutput() bool {
        // If we don't have a timeout transaction, then this means that this is
        // an output on the remote party's commitment transaction.
        return h.htlcResolution.SignedTimeoutTx == nil
}

// isZeroFeeOutput returns a boolean indicating whether the htlc output is from
// a anchor-enabled channel, which uses the sighash SINGLE|ANYONECANPAY.
func (h *htlcTimeoutResolver) isZeroFeeOutput() bool {
        // If we have non-nil SignDetails, this means it has a 2nd level HTLC
        // transaction that is signed using sighash SINGLE|ANYONECANPAY (the
        // case for anchor type channels). In this case we can re-sign it and
        // attach fees at will.
        return h.htlcResolution.SignedTimeoutTx != nil &&
                h.htlcResolution.SignDetails != nil
}

// waitHtlcSpendAndCheckPreimage waits for the htlc output to be spent and
// checks whether the spending reveals the preimage. If the preimage is found,
// it will be added to the preimage beacon to settle the incoming link, and a
// nil spend details will be returned. Otherwise, the spend details will be
// returned, indicating this is a non-preimage spend.
func (h *htlcTimeoutResolver) waitHtlcSpendAndCheckPreimage() (
        *chainntnfs.SpendDetail, error) {

        // Wait for the htlc output to be spent, which can happen in one of the
        // paths,
        // 1. The remote party spends the htlc output using the preimage.
        // 2. The local party spends the htlc timeout tx from the local
        //    commitment.
        // 3. The local party spends the htlc output directlt from the remote
        //    commitment.
        spend, err := h.watchHtlcSpend()
        if err != nil {
                return nil, err
        }

        // If the spend reveals the pre-image, then we'll enter the clean up
        // workflow to pass the preimage back to the incoming link, add it to
        // the witness cache, and exit.
        if isPreimageSpend(h.isTaproot(), spend, !h.isRemoteCommitOutput()) {
                return nil, h.claimCleanUp(spend)
        }

        return spend, nil
}

// sweepTimeoutTxOutput attempts to sweep the output of the second level
// timeout tx.
func (h *htlcTimeoutResolver) sweepTimeoutTxOutput() error {
        h.log.Debugf("sweeping output %v from 2nd-level HTLC timeout tx",
                h.htlcResolution.ClaimOutpoint)

        // This should be non-blocking as we will only attempt to sweep the
        // output when the second level tx has already been confirmed. In other
        // words, waitHtlcSpendAndCheckPreimage will return immediately.
        commitSpend, err := h.waitHtlcSpendAndCheckPreimage()
        if err != nil {
                return err
        }

        // Exit early if the spend is nil, as this means it's a remote spend
        // using the preimage path, which is handled in claimCleanUp.
        if commitSpend == nil {
                h.log.Infof("preimage spend detected, skipping 2nd-level " +
                        "HTLC output sweep")

                return nil
        }

        waitHeight := h.deriveWaitHeight(h.htlcResolution.CsvDelay, commitSpend)

        // Now that the sweeper has broadcasted the second-level transaction,
        // it has confirmed, and we have checkpointed our state, we'll sweep
        // the second level output. We report the resolver has moved the next
        // stage.
        h.reportLock.Lock()
        h.currentReport.Stage = 2
        h.currentReport.MaturityHeight = waitHeight
        h.reportLock.Unlock()

        if h.hasCLTV() {
                h.log.Infof("waiting for CSV and CLTV lock to expire at "+
                        "height %v", waitHeight)
        } else {
                h.log.Infof("waiting for CSV lock to expire at height %v",
                        waitHeight)
        }

        // We'll use this input index to determine the second-level output
        // index on the transaction, as the signatures requires the indexes to
        // be the same. We don't look for the second-level output script
        // directly, as there might be more than one HTLC output to the same
        // pkScript.
        op := &wire.OutPoint{
                Hash:  *commitSpend.SpenderTxHash,
                Index: commitSpend.SpenderInputIndex,
        }

        var witType input.StandardWitnessType
        if h.isTaproot() {
                witType = input.TaprootHtlcOfferedTimeoutSecondLevel
        } else {
                witType = input.HtlcOfferedTimeoutSecondLevel
        }

        // Let the sweeper sweep the second-level output now that the CSV/CLTV
        // locks have expired.
        inp := h.makeSweepInput(
                op, witType,
                input.LeaseHtlcOfferedTimeoutSecondLevel,
                &h.htlcResolution.SweepSignDesc,
                h.htlcResolution.CsvDelay, uint32(commitSpend.SpendingHeight),
                h.htlc.RHash, h.htlcResolution.ResolutionBlob,
        )

        // Calculate the budget for this sweep.
        budget := calculateBudget(
                btcutil.Amount(inp.SignDesc().Output.Value),
                h.Budget.NoDeadlineHTLCRatio,
                h.Budget.NoDeadlineHTLC,
        )

        h.log.Infof("offering output from 2nd-level timeout tx to sweeper "+
                "with no deadline and budget=%v", budget)

        // TODO(yy): use the result chan returned from SweepInput to get the
        // confirmation status of this sweeping tx so we don't need to make
        // another subscription via `RegisterSpendNtfn` for this outpoint here
        // in the resolver.
        _, err = h.Sweeper.SweepInput(
                inp,
                sweep.Params{
                        Budget: budget,

                        // For second level success tx, there's no rush
                        // to get it confirmed, so we use a nil
                        // deadline.
                        DeadlineHeight: fn.None[int32](),
                },
        )

        return err
}

// checkpointStageOne creates a checkpoint for the first stage of the htlc
// timeout transaction. This is used to ensure that the resolver can resume
// watching for the second stage spend in case of a restart.
func (h *htlcTimeoutResolver) checkpointStageOne(
        spendTxid chainhash.Hash) error {

        h.log.Debugf("checkpoint stage one spend of HTLC output %v, spent "+
                "in tx %v", h.outpoint(), spendTxid)

        // Now that the second-level transaction has confirmed, we checkpoint
        // the state so we'll go to the next stage in case of restarts.
        h.outputIncubating = true

        // Create stage-one report.
        report := &channeldb.ResolverReport{
                OutPoint:        h.outpoint(),
                Amount:          h.htlc.Amt.ToSatoshis(),
                ResolverType:    channeldb.ResolverTypeOutgoingHtlc,
                ResolverOutcome: channeldb.ResolverOutcomeFirstStage,
                SpendTxID:       &spendTxid,
        }

        // At this point, the second-level transaction is sufficiently
        // confirmed. We can now send back our clean up message, failing the
        // HTLC on the incoming link.
        failureMsg := &lnwire.FailPermanentChannelFailure{}
        err := h.DeliverResolutionMsg(ResolutionMsg{
                SourceChan: h.ShortChanID,
                HtlcIndex:  h.htlc.HtlcIndex,
                Failure:    failureMsg,
        })
        if err != nil {
                return err
        }

        return h.Checkpoint(h, report)
}

// checkpointClaim checkpoints the timeout resolver with the reports it needs.
func (h *htlcTimeoutResolver) checkpointClaim(
        spendDetail *chainntnfs.SpendDetail) error {

        h.log.Infof("resolving htlc with incoming fail msg, output=%v "+
                "confirmed in tx=%v", spendDetail.SpentOutPoint,
                spendDetail.SpenderTxHash)

        // Create a resolver report for the claiming of the HTLC.
        amt := btcutil.Amount(h.htlcResolution.SweepSignDesc.Output.Value)
        report := &channeldb.ResolverReport{
                OutPoint:        *spendDetail.SpentOutPoint,
                Amount:          amt,
                ResolverType:    channeldb.ResolverTypeOutgoingHtlc,
                ResolverOutcome: channeldb.ResolverOutcomeTimeout,
                SpendTxID:       spendDetail.SpenderTxHash,
        }

        // Finally, we checkpoint the resolver with our report(s).
        h.markResolved()

        return h.Checkpoint(h, report)
}

// resolveRemoteCommitOutput handles sweeping an HTLC output on the remote
// commitment with via the timeout path. In this case we can sweep the output
// directly, and don't have to broadcast a second-level transaction.
func (h *htlcTimeoutResolver) resolveRemoteCommitOutput() error {
        h.log.Debug("waiting for direct-timeout spend of the htlc to confirm")

        // Wait for the direct-timeout HTLC sweep tx to confirm.
        spend, err := h.watchHtlcSpend()
        if err != nil {
                return err
        }

        // If the spend reveals the preimage, then we'll enter the clean up
        // workflow to pass the preimage back to the incoming link, add it to
        // the witness cache, and exit.
        if isPreimageSpend(h.isTaproot(), spend, !h.isRemoteCommitOutput()) {
                return h.claimCleanUp(spend)
        }

        // Send the clean up msg to fail the incoming HTLC.
        failureMsg := &lnwire.FailPermanentChannelFailure{}
        err = h.DeliverResolutionMsg(ResolutionMsg{
                SourceChan: h.ShortChanID,
                HtlcIndex:  h.htlc.HtlcIndex,
                Failure:    failureMsg,
        })
        if err != nil {
                return err
        }

        // TODO(yy): should also update the `RecoveredBalance` and
        // `LimboBalance` like other paths?

        // Checkpoint the resolver, and write the outcome to disk.
        return h.checkpointClaim(spend)
}

// resolveTimeoutTx waits for the sweeping tx of the second-level
// timeout tx to confirm and offers the output from the timeout tx to the
// sweeper.
func (h *htlcTimeoutResolver) resolveTimeoutTx() error {
        h.log.Debug("waiting for first-stage 2nd-level HTLC timeout tx to " +
                "confirm")

        // Wait for the second level transaction to confirm.
        spend, err := h.watchHtlcSpend()
        if err != nil {
                return err
        }

        // If the spend reveals the preimage, then we'll enter the clean up
        // workflow to pass the preimage back to the incoming link, add it to
        // the witness cache, and exit.
        if isPreimageSpend(h.isTaproot(), spend, !h.isRemoteCommitOutput()) {
                return h.claimCleanUp(spend)
        }

        op := h.htlcResolution.ClaimOutpoint
        spenderTxid := *spend.SpenderTxHash

        // If the timeout tx is a re-signed tx, we will need to find the actual
        // spent outpoint from the spending tx.
        if h.isZeroFeeOutput() {
                op = wire.OutPoint{
                        Hash:  spenderTxid,
                        Index: spend.SpenderInputIndex,
                }
        }

        // If the 2nd-stage sweeping has already been started, we can
        // fast-forward to start the resolving process for the stage two
        // output.
        if h.outputIncubating {
                return h.resolveTimeoutTxOutput(op)
        }

        h.log.Infof("2nd-level HTLC timeout tx=%v confirmed", spenderTxid)

        // Start the process to sweep the output from the timeout tx.
        if h.isZeroFeeOutput() {
                err = h.sweepTimeoutTxOutput()
                if err != nil {
                        return err
                }
        }

        // Create a checkpoint since the timeout tx is confirmed and the sweep
        // request has been made.
        if err := h.checkpointStageOne(spenderTxid); err != nil {
                return err
        }

        // Start the resolving process for the stage two output.
        return h.resolveTimeoutTxOutput(op)
}

// resolveTimeoutTxOutput waits for the spend of the output from the 2nd-level
// timeout tx.
func (h *htlcTimeoutResolver) resolveTimeoutTxOutput(op wire.OutPoint) error {
        h.log.Debugf("waiting for second-stage 2nd-level timeout tx output %v "+
                "to be spent after csv_delay=%v", op, h.htlcResolution.CsvDelay)

        spend, err := waitForSpend(
                &op, h.htlcResolution.SweepSignDesc.Output.PkScript,
                h.broadcastHeight, h.Notifier, h.quit,
        )
        if err != nil {
                return err
        }

        h.reportLock.Lock()
        h.currentReport.RecoveredBalance = h.currentReport.LimboBalance
        h.currentReport.LimboBalance = 0
        h.reportLock.Unlock()

        return h.checkpointClaim(spend)
}

// Launch creates an input based on the details of the outgoing htlc resolution
// and offers it to the sweeper.
func (h *htlcTimeoutResolver) Launch() error {
        if h.isLaunched() {
                h.log.Tracef("already launched")
                return nil
        }

        h.log.Debugf("launching resolver...")
        h.markLaunched()

        switch {
        // If we're already resolved, then we can exit early.
        case h.IsResolved():
                h.log.Errorf("already resolved")
                return nil

        // If this is an output on the remote party's commitment transaction,
        // use the direct timeout spend path.
        //
        // NOTE: When the outputIncubating is false, it means that the output
        // has been offered to the utxo nursery as starting in 0.18.4, we
        // stopped marking this flag for direct timeout spends (#9062). In that
        // case, we will do nothing and let the utxo nursery handle it.
        case h.isRemoteCommitOutput() && !h.outputIncubating:
                return h.sweepDirectHtlcOutput()

        // If this is an anchor type channel, we now sweep either the
        // second-level timeout tx or the output from the second-level timeout
        // tx.
        case h.isZeroFeeOutput():
                // If the second-level timeout tx has already been swept, we
                // can go ahead and sweep its output.
                if h.outputIncubating {
                        return h.sweepTimeoutTxOutput()
                }

                // Otherwise, sweep the second level tx.
                return h.sweepTimeoutTx()

        // If this is an output on our own commitment using pre-anchor channel
        // type, we will let the utxo nursery handle it via Resolve.
        //
        // TODO(yy): handle the legacy output by offering it to the sweeper.
        default:
                return nil
        }
}

lightningnetwork / lnd / 17830307614

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