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htlc_timeout_resolver.go
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htlc_timeout_resolver.go
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package contractcourt
import (
"encoding/binary"
"fmt"
"io"
"sync"
"github.com/btcsuite/btcd/wire"
"github.com/btcsuite/btcutil"
"github.com/davecgh/go-spew/spew"
"github.com/lightningnetwork/lnd/chainntnfs"
"github.com/lightningnetwork/lnd/channeldb"
"github.com/lightningnetwork/lnd/input"
"github.com/lightningnetwork/lnd/lntypes"
"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
// resolved reflects if the contract has been fully resolved or not.
resolved 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
}
// 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()
return h
}
// 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 {
// 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.
var op wire.OutPoint
if h.htlcResolution.SignedTimeoutTx != nil {
op = h.htlcResolution.SignedTimeoutTx.TxIn[0].PreviousOutPoint
} else {
op = h.htlcResolution.ClaimOutpoint
}
key := newResolverID(op)
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
// 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
)
// 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) (ContractResolver, 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,
spew.Sdump(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
if h.htlcResolution.SignedTimeoutTx == nil {
// The witness stack when the remote party sweeps the output to
// them looks like:
//
// * <0> <sender sig> <recvr sig> <preimage> <witness script>
preimageBytes = spendingInput.Witness[remotePreimageIndex]
} else {
// Otherwise, they'll be spending directly from our commitment
// output. In which case the witness stack looks like:
//
// * <sig> <preimage> <witness script>
preimageBytes = spendingInput.Witness[localPreimageIndex]
}
preimage, err := lntypes.MakePreimage(preimageBytes)
if err != nil {
return nil, fmt.Errorf("unable to create pre-image from "+
"witness: %v", 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 nil, err
}
h.resolved = true
// 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 nil, 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.
//
// TODO(joostjager): output already set properly in
// lnwallet.newOutgoingHtlcResolution? And script too?
func (h *htlcTimeoutResolver) chainDetailsToWatch() (*wire.OutPoint, []byte, error) {
// If there's no timeout transaction, 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 this is the remote party's commitment, then 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.
outPointToWatch := h.htlcResolution.SignedTimeoutTx.TxIn[0].PreviousOutPoint
witness := h.htlcResolution.SignedTimeoutTx.TxIn[0].Witness
scriptToWatch, err := input.WitnessScriptHash(witness[len(witness)-1])
if err != nil {
return nil, nil, err
}
return &outPointToWatch, scriptToWatch, nil
}
// isSuccessSpend returns true if the passed spend on the specified commitment
// is a success spend that reveals the pre-image or not.
func isSuccessSpend(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
// If this is the 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.
if !localCommit {
return len(spendingWitness) == expectedRemoteWitnessSuccessSize &&
len(spendingWitness[remotePreimageIndex]) == lntypes.HashSize
}
// Otherwise, for our 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.
return len(spendingWitness[localPreimageIndex]) == 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.resolved {
return nil, nil
}
// Start by spending the HTLC output, either by broadcasting the
// second-level timeout transaction, or directly if this is the remote
// commitment.
commitSpend, err := h.spendHtlcOutput()
if err != nil {
return nil, err
}
// If the spend reveals the pre-image, then we'll enter the clean up
// workflow to pass the pre-image back to the incoming link, add it to
// the witness cache, and exit.
if isSuccessSpend(commitSpend, h.htlcResolution.SignedTimeoutTx != nil) {
log.Infof("%T(%v): HTLC has been swept with pre-image by "+
"remote party during timeout flow! Adding pre-image to "+
"witness cache", h.htlcResolution.ClaimOutpoint)
return h.claimCleanUp(commitSpend)
}
log.Infof("%T(%v): resolving htlc with incoming fail msg, fully "+
"confirmed", h, h.htlcResolution.ClaimOutpoint)
// At this point, the second-level transaction is sufficiently
// confirmed, or a transaction directly spending the output is.
// Therefore, we can now send back our clean up message, failing the
// HTLC on the incoming link.
failureMsg := &lnwire.FailPermanentChannelFailure{}
if err := h.DeliverResolutionMsg(ResolutionMsg{
SourceChan: h.ShortChanID,
HtlcIndex: h.htlc.HtlcIndex,
Failure: failureMsg,
}); err != nil {
return nil, err
}
// Depending on whether this was a local or remote commit, we must
// handle the spending transaction accordingly.
return h.handleCommitSpend(commitSpend)
}
// spendHtlcOutput handles the initial spend of an HTLC output via the timeout
// clause. If this is our local commitment, the second-level timeout TX will be
// used to spend the output into the next stage. If this is the remote
// commitment, the output will be swept directly without the timeout
// transaction.
func (h *htlcTimeoutResolver) spendHtlcOutput() (*chainntnfs.SpendDetail, error) {
switch {
// If we have non-nil SignDetails, this means that have 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. We let the sweeper handle this job.
case h.htlcResolution.SignDetails != nil && !h.outputIncubating:
log.Infof("%T(%x): offering second-layer timeout tx to "+
"sweeper: %v", h, h.htlc.RHash[:],
spew.Sdump(h.htlcResolution.SignedTimeoutTx))
inp := input.MakeHtlcSecondLevelTimeoutAnchorInput(
h.htlcResolution.SignedTimeoutTx,
h.htlcResolution.SignDetails,
h.broadcastHeight,
)
_, err := h.Sweeper.SweepInput(
&inp,
sweep.Params{
Fee: sweep.FeePreference{
ConfTarget: secondLevelConfTarget,
},
},
)
if err != nil {
return nil, err
}
// If we have no SignDetails, and we haven't already sent the output to
// the utxo nursery, then we'll do so now.
case h.htlcResolution.SignDetails == nil && !h.outputIncubating:
log.Tracef("%T(%v): incubating htlc output", h,
h.htlcResolution.ClaimOutpoint)
err := h.IncubateOutputs(
h.ChanPoint, &h.htlcResolution, nil,
h.broadcastHeight,
)
if err != nil {
return nil, err
}
h.outputIncubating = true
if err := h.Checkpoint(h); err != nil {
log.Errorf("unable to Checkpoint: %v", err)
return nil, err
}
}
// Now that we've handed off the HTLC to the nursery or sweeper, we'll
// watch for a spend of the output, and make our next move off of that.
// Depending on if this is our commitment, or the remote party's
// commitment, we'll be watching a different outpoint and script.
outpointToWatch, scriptToWatch, err := h.chainDetailsToWatch()
if err != nil {
return nil, err
}
log.Infof("%T(%v): waiting for HTLC output %v to be spent"+
"fully confirmed", h, h.htlcResolution.ClaimOutpoint,
outpointToWatch)
// We'll block here until either we exit, or the HTLC output on the
// commitment transaction has been spent.
spend, err := waitForSpend(
outpointToWatch, scriptToWatch, h.broadcastHeight,
h.Notifier, h.quit,
)
if err != nil {
return nil, err
}
// If this was the second level transaction published by the sweeper,
// we can checkpoint the resolver now that it's confirmed.
if h.htlcResolution.SignDetails != nil && !h.outputIncubating {
h.outputIncubating = true
if err := h.Checkpoint(h); err != nil {
log.Errorf("unable to Checkpoint: %v", err)
return nil, err
}
}
return spend, err
}
// handleCommitSpend handles the spend of the HTLC output on the commitment
// transaction. If this was our local commitment, the spend will be he
// confirmed second-level timeout transaction, and we'll sweep that into our
// wallet. If the was a remote commitment, the resolver will resolve
// immetiately.
func (h *htlcTimeoutResolver) handleCommitSpend(
commitSpend *chainntnfs.SpendDetail) (ContractResolver, error) {
var (
// claimOutpoint will be the outpoint of the second level
// transaction, or on the remote commitment directly. It will
// start out as set in the resolution, but we'll update it if
// the second-level goes through the sweeper and changes its
// txid.
claimOutpoint = h.htlcResolution.ClaimOutpoint
// spendTxID will be the ultimate spend of the claimOutpoint.
// We set it to the commit spend for now, as this is the
// ultimate spend in case this is a remote commitment. If we go
// through the second-level transaction, we'll update this
// accordingly.
spendTxID = commitSpend.SpenderTxHash
reports []*channeldb.ResolverReport
)
switch {
// If the sweeper is handling the second level transaction, wait for
// the CSV lock to expire, before sweeping the output on the
// second-level.
case h.htlcResolution.SignDetails != nil:
waitHeight := uint32(commitSpend.SpendingHeight) +
h.htlcResolution.CsvDelay - 1
h.reportLock.Lock()
h.currentReport.Stage = 2
h.currentReport.MaturityHeight = waitHeight
h.reportLock.Unlock()
log.Infof("%T(%x): waiting for CSV lock to expire at height %v",
h, h.htlc.RHash[:], waitHeight)
err := waitForHeight(waitHeight, h.Notifier, h.quit)
if err != nil {
return nil, err
}
// 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,
}
// Let the sweeper sweep the second-level output now that the
// CSV delay has passed.
log.Infof("%T(%x): CSV lock expired, offering second-layer "+
"output to sweeper: %v", h, h.htlc.RHash[:], op)
inp := input.NewCsvInput(
op, input.HtlcOfferedTimeoutSecondLevel,
&h.htlcResolution.SweepSignDesc,
h.broadcastHeight,
h.htlcResolution.CsvDelay,
)
_, err = h.Sweeper.SweepInput(
inp,
sweep.Params{
Fee: sweep.FeePreference{
ConfTarget: sweepConfTarget,
},
},
)
if err != nil {
return nil, err
}
// Update the claim outpoint to point to the second-level
// transaction created by the sweeper.
claimOutpoint = *op
fallthrough
// Finally, if this was an output on our commitment transaction, we'll
// wait for the second-level HTLC output to be spent, and for that
// transaction itself to confirm.
case h.htlcResolution.SignedTimeoutTx != nil:
log.Infof("%T(%v): waiting for nursery/sweeper to spend CSV "+
"delayed output", h, claimOutpoint)
sweep, err := waitForSpend(
&claimOutpoint,
h.htlcResolution.SweepSignDesc.Output.PkScript,
h.broadcastHeight, h.Notifier, h.quit,
)
if err != nil {
return nil, err
}
// Update the spend txid to the hash of the sweep transaction.
spendTxID = sweep.SpenderTxHash
// Once our sweep of the timeout tx has confirmed, we add a
// resolution for our timeoutTx tx first stage transaction.
timeoutTx := commitSpend.SpendingTx
index := commitSpend.SpenderInputIndex
spendHash := commitSpend.SpenderTxHash
reports = append(reports, &channeldb.ResolverReport{
OutPoint: timeoutTx.TxIn[index].PreviousOutPoint,
Amount: h.htlc.Amt.ToSatoshis(),
ResolverType: channeldb.ResolverTypeOutgoingHtlc,
ResolverOutcome: channeldb.ResolverOutcomeFirstStage,
SpendTxID: spendHash,
})
}
// With the clean up message sent, we'll now mark the contract
// resolved, update the recovered balance, record the timeout and the
// sweep txid on disk, and wait.
h.resolved = true
h.reportLock.Lock()
h.currentReport.RecoveredBalance = h.currentReport.LimboBalance
h.currentReport.LimboBalance = 0
h.reportLock.Unlock()
amt := btcutil.Amount(h.htlcResolution.SweepSignDesc.Output.Value)
reports = append(reports, &channeldb.ResolverReport{
OutPoint: claimOutpoint,
Amount: amt,
ResolverType: channeldb.ResolverTypeOutgoingHtlc,
ResolverOutcome: channeldb.ResolverOutcomeTimeout,
SpendTxID: spendTxID,
})
return nil, h.Checkpoint(h, reports...)
}
// Stop signals the resolver to cancel any current resolution processes, and
// suspend.
//
// NOTE: Part of the ContractResolver interface.
func (h *htlcTimeoutResolver) Stop() {
close(h.quit)
}
// IsResolved returns true if the stored state in the resolve is fully
// resolved. In this case the target output can be forgotten.
//
// NOTE: Part of the ContractResolver interface.
func (h *htlcTimeoutResolver) IsResolved() bool {
return h.resolved
}
// report returns a report on the resolution state of the contract.
func (h *htlcTimeoutResolver) report() *ContractReport {
// If the sign details are nil, the report will be created by handled
// by the nursery.
if h.htlcResolution.SignDetails == nil {
return nil
}
h.reportLock.Lock()
defer h.reportLock.Unlock()
copy := h.currentReport
return ©
}
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,
)
}
h.currentReport = ContractReport{
Outpoint: h.htlcResolution.ClaimOutpoint,
Type: ReportOutputOutgoingHtlc,
Amount: finalAmt,
MaturityHeight: h.htlcResolution.Expiry,
LimboBalance: finalAmt,
Stage: 1,
}
}
// 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.resolved); 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
}
if err := binary.Read(r, endian, &h.resolved); err != nil {
return nil, err
}
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()
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()
}
// A compile time assertion to ensure htlcTimeoutResolver meets the
// ContractResolver interface.
var _ htlcContractResolver = (*htlcTimeoutResolver)(nil)