lnd.xprv/contractcourt/htlc_incoming_contest_resolver.go

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package contractcourt
import (
"bytes"
"crypto/sha256"
"encoding/binary"
"fmt"
"io"
)
// 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
}
// We'll first check if this HTLC has been timed out, if so, we can
// return now and mark ourselves as resolved.
_, currentHeight, err := h.ChainIO.GetBestBlock()
if err != nil {
return nil, err
}
// 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.
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)
}
// applyPreimage 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.
applyPreimage := func(preimage []byte) {
copy(h.htlcResolution.Preimage[:], preimage)
log.Infof("%T(%v): extracted preimage=%x 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[:]
}
copy(h.htlcResolution.Preimage[:], preimage[:])
}
// 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()
blockEpochs, err := h.Notifier.RegisterBlockEpochNtfn(nil)
if err != nil {
return nil, err
}
defer func() {
preimageSubscription.CancelSubscription()
blockEpochs.Cancel()
}()
// 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.
applyPreimage(preimage[:])
return &h.htlcSuccessResolver, nil
}
for {
select {
case preimage := <-preimageSubscription.WitnessUpdates:
// If this isn't our preimage, then we'll continue
// onwards.
newHash := sha256.Sum256(preimage)
preimageMatches := bytes.Equal(newHash[:], h.payHash[:])
if !preimageMatches {
continue
}
// Otherwise, we've learned of the preimage! We'll add
// this information to our inner resolver, then return
// it so it can continue contract resolution.
applyPreimage(preimage)
return &h.htlcSuccessResolver, nil
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")
}
}
}
// 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)