lnd.xprv/contractcourt/htlc_incoming_contest_resolver.go

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package contractcourt
import (
"encoding/binary"
"errors"
"fmt"
"io"
"github.com/lightningnetwork/lnd/channeldb"
"github.com/lightningnetwork/lnd/invoices"
"github.com/btcsuite/btcutil"
"github.com/lightningnetwork/lnd/lntypes"
)
// htlcIncomingContestResolver is a ContractResolver that's able to resolve an
// incoming HTLC that is still contested. An HTLC is still contested, if at the
// time of commitment broadcast, we don't know of the preimage for it yet, and
// it hasn't expired. In this case, we can resolve the HTLC if we learn of the
// preimage, otherwise the remote party will sweep it after it expires.
//
// TODO(roasbeef): just embed the other resolver?
type htlcIncomingContestResolver struct {
// htlcExpiry is the absolute expiry of this incoming HTLC. We use this
// value to determine if we can exit early as if the HTLC times out,
// before we learn of the preimage then we can't claim it on chain
// successfully.
htlcExpiry uint32
// htlcSuccessResolver is the inner resolver that may be utilized if we
// learn of the preimage.
htlcSuccessResolver
}
// Resolve attempts to resolve this contract. As we don't yet know of the
// preimage for the contract, we'll wait for one of two things to happen:
//
// 1. We learn of the preimage! In this case, we can sweep the HTLC incoming
// and ensure that if this was a multi-hop HTLC we are made whole. In this
// case, an additional ContractResolver will be returned to finish the
// job.
//
// 2. The HTLC expires. If this happens, then the contract is fully resolved
// as we have no remaining actions left at our disposal.
//
// NOTE: Part of the ContractResolver interface.
func (h *htlcIncomingContestResolver) Resolve() (ContractResolver, error) {
// If we're already full resolved, then we don't have anything further
// to do.
if h.resolved {
return nil, nil
}
// Register for block epochs. After registration, the current height
// will be sent on the channel immediately.
blockEpochs, err := h.Notifier.RegisterBlockEpochNtfn(nil)
if err != nil {
return nil, err
}
defer blockEpochs.Cancel()
var currentHeight int32
select {
case newBlock, ok := <-blockEpochs.Epochs:
if !ok {
return nil, fmt.Errorf("quitting")
}
currentHeight = newBlock.Height
case <-h.Quit:
return nil, fmt.Errorf("resolver stopped")
}
// We'll first check if this HTLC has been timed out, if so, we can
// return now and mark ourselves as resolved. If we're past the point of
// expiry of the HTLC, then at this point the sender can sweep it, so
// we'll end our lifetime. Here we deliberately forego the chance that
// the sender doesn't sweep and we already have or will learn the
// preimage. Otherwise the resolver could potentially stay active
// indefinitely and the channel will never close properly.
if uint32(currentHeight) >= h.htlcExpiry {
// TODO(roasbeef): should also somehow check if outgoing is
// resolved or not
// * may need to hook into the circuit map
// * can't timeout before the outgoing has been
log.Infof("%T(%v): HTLC has timed out (expiry=%v, height=%v), "+
"abandoning", h, h.htlcResolution.ClaimOutpoint,
h.htlcExpiry, currentHeight)
h.resolved = true
return nil, h.Checkpoint(h)
}
// tryApplyPreimage is a helper function that will populate our internal
// resolver with the preimage we learn of. This should be called once
// the preimage is revealed so the inner resolver can properly complete
// its duties. The boolean return value indicates whether the preimage
// was properly applied.
applyPreimage := func(preimage lntypes.Preimage) error {
// Sanity check to see if this preimage matches our htlc. At
// this point it should never happen that it does not match.
if !preimage.Matches(h.payHash) {
return errors.New("preimage does not match hash")
}
// Update htlcResolution with the matching preimage.
h.htlcResolution.Preimage = preimage
log.Infof("%T(%v): extracted preimage=%v from beacon!", h,
h.htlcResolution.ClaimOutpoint, preimage)
// If this our commitment transaction, then we'll need to
// populate the witness for the second-level HTLC transaction.
if h.htlcResolution.SignedSuccessTx != nil {
// Within the witness for the success transaction, the
// preimage is the 4th element as it looks like:
//
// * <sender sig> <recvr sig> <preimage> <witness script>
//
// We'll populate it within the witness, as since this
// was a "contest" resolver, we didn't yet know of the
// preimage.
h.htlcResolution.SignedSuccessTx.TxIn[0].Witness[3] = preimage[:]
}
return nil
}
// If the HTLC hasn't expired yet, then we may still be able to claim
// it if we learn of the pre-image, so we'll subscribe to the preimage
// database to see if it turns up, or the HTLC times out.
//
// NOTE: This is done BEFORE opportunistically querying the db, to
// ensure the preimage can't be delivered between querying and
// registering for the preimage subscription.
preimageSubscription := h.PreimageDB.SubscribeUpdates()
defer preimageSubscription.CancelSubscription()
// Define closure to process hodl events either direct or triggered by
// later notifcation.
processHodlEvent := func(e invoices.HodlEvent) (ContractResolver,
error) {
if e.Preimage == nil {
log.Infof("%T(%v): Exit hop HTLC canceled "+
"(expiry=%v, height=%v), abandoning", h,
h.htlcResolution.ClaimOutpoint,
h.htlcExpiry, currentHeight)
h.resolved = true
return nil, h.Checkpoint(h)
}
if err := applyPreimage(*e.Preimage); err != nil {
return nil, err
}
return &h.htlcSuccessResolver, nil
}
// Create a buffered hodl chan to prevent deadlock.
hodlChan := make(chan interface{}, 1)
// Notify registry that we are potentially resolving as an exit hop
// on-chain. If this HTLC indeed pays to an existing invoice, the
// invoice registry will tell us what to do with the HTLC. This is
// identical to HTLC resolution in the link.
event, err := h.Registry.NotifyExitHopHtlc(
h.payHash, h.htlcAmt, h.htlcExpiry, currentHeight,
hodlChan,
)
switch err {
case channeldb.ErrInvoiceNotFound:
case nil:
defer h.Registry.HodlUnsubscribeAll(hodlChan)
// Resolve the htlc directly if possible.
if event != nil {
return processHodlEvent(*event)
}
default:
return nil, err
}
// With the epochs and preimage subscriptions initialized, we'll query
// to see if we already know the preimage.
preimage, ok := h.PreimageDB.LookupPreimage(h.payHash)
if ok {
// If we do, then this means we can claim the HTLC! However,
// we don't know how to ourselves, so we'll return our inner
// resolver which has the knowledge to do so.
if err := applyPreimage(preimage); err != nil {
return nil, err
}
return &h.htlcSuccessResolver, nil
}
for {
select {
case preimage := <-preimageSubscription.WitnessUpdates:
// We receive all new preimages, so we need to ignore
// all except the preimage we are waiting for.
if !preimage.Matches(h.payHash) {
continue
}
if err := applyPreimage(preimage); err != nil {
return nil, err
}
// We've learned of the preimage and this information
// has been added to our inner resolver. We return it so
// it can continue contract resolution.
return &h.htlcSuccessResolver, nil
case hodlItem := <-hodlChan:
hodlEvent := hodlItem.(invoices.HodlEvent)
return processHodlEvent(hodlEvent)
case newBlock, ok := <-blockEpochs.Epochs:
if !ok {
return nil, fmt.Errorf("quitting")
}
// If this new height expires the HTLC, then this means
// we never found out the preimage, so we can mark
// resolved and exit.
newHeight := uint32(newBlock.Height)
if newHeight >= h.htlcExpiry {
log.Infof("%T(%v): HTLC has timed out "+
"(expiry=%v, height=%v), abandoning", h,
h.htlcResolution.ClaimOutpoint,
h.htlcExpiry, currentHeight)
h.resolved = true
return nil, h.Checkpoint(h)
}
case <-h.Quit:
return nil, fmt.Errorf("resolver stopped")
}
}
}
// report returns a report on the resolution state of the contract.
func (h *htlcIncomingContestResolver) report() *ContractReport {
// No locking needed as these values are read-only.
finalAmt := h.htlcAmt.ToSatoshis()
if h.htlcResolution.SignedSuccessTx != nil {
finalAmt = btcutil.Amount(
h.htlcResolution.SignedSuccessTx.TxOut[0].Value,
)
}
return &ContractReport{
Outpoint: h.htlcResolution.ClaimOutpoint,
Incoming: true,
Amount: finalAmt,
MaturityHeight: h.htlcExpiry,
LimboBalance: finalAmt,
Stage: 1,
}
}
// Stop signals the resolver to cancel any current resolution processes, and
// suspend.
//
// NOTE: Part of the ContractResolver interface.
func (h *htlcIncomingContestResolver) 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 *htlcIncomingContestResolver) IsResolved() bool {
return h.resolved
}
// Encode writes an encoded version of the ContractResolver into the passed
// Writer.
//
// NOTE: Part of the ContractResolver interface.
func (h *htlcIncomingContestResolver) Encode(w io.Writer) error {
// We'll first write out the one field unique to this resolver.
if err := binary.Write(w, endian, h.htlcExpiry); err != nil {
return err
}
// Then we'll write out our internal resolver.
return h.htlcSuccessResolver.Encode(w)
}
// Decode attempts to decode an encoded ContractResolver from the passed Reader
// instance, returning an active ContractResolver instance.
//
// NOTE: Part of the ContractResolver interface.
func (h *htlcIncomingContestResolver) Decode(r io.Reader) error {
// We'll first read the one field unique to this resolver.
if err := binary.Read(r, endian, &h.htlcExpiry); err != nil {
return err
}
// Then we'll decode our internal resolver.
return h.htlcSuccessResolver.Decode(r)
}
// AttachResolverKit should be called once a resolved is successfully decoded
// from its stored format. This struct delivers a generic tool kit that
// resolvers need to complete their duty.
//
// NOTE: Part of the ContractResolver interface.
func (h *htlcIncomingContestResolver) AttachResolverKit(r ResolverKit) {
h.ResolverKit = r
}
// A compile time assertion to ensure htlcIncomingContestResolver meets the
// ContractResolver interface.
var _ ContractResolver = (*htlcIncomingContestResolver)(nil)