lnd.xprv/contractcourt/htlc_outgoing_contest_resolver.go

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package contractcourt
import (
"fmt"
"io"
"github.com/btcsuite/btcd/wire"
"github.com/davecgh/go-spew/spew"
"github.com/lightningnetwork/lnd/chainntnfs"
"github.com/lightningnetwork/lnd/lnwallet"
)
// htlcOutgoingContestResolver is a ContractResolver that's able to resolve an
// outgoing HTLC that is still contested. An HTLC is still contested, if at the
// time that we broadcast the commitment transaction, it isn't able to be fully
// resolved. In this case, we'll either wait for the HTLC to timeout, or for
// us to learn of the preimage.
type htlcOutgoingContestResolver struct {
// htlcTimeoutResolver is the inner solver that this resolver may turn
// into. This only happens if the HTLC expires on-chain.
htlcTimeoutResolver
}
// Resolve commences the resolution of this contract. As this contract hasn't
// yet timed out, we'll wait for one of two things to happen
//
// 1. The HTLC expires. In this case, we'll sweep the funds and send a clean
// up cancel message to outside sub-systems.
//
// 2. The remote party sweeps this HTLC on-chain, in which case we'll add the
// pre-image to our global cache, then send a clean up settle message
// backwards.
//
// When either of these two things happens, we'll create a new resolver which
// is able to handle the final resolution of the contract. We're only the pivot
// point.
func (h *htlcOutgoingContestResolver) Resolve() (ContractResolver, error) {
// If we're already full resolved, then we don't have anything further
// to do.
if h.resolved {
return nil, nil
}
// claimCleanUp is a helper function 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.
claimCleanUp := func(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 preimage [32]byte
if h.htlcResolution.SignedTimeoutTx == nil {
// The witness stack when the remote party sweeps the
// output to them looks like:
//
// * <sender sig> <recvr sig> <preimage> <witness script>
copy(preimage[:], spendingInput.Witness[3])
} else {
// Otherwise, they'll be spending directly from our
// commitment output. In which case the witness stack
// looks like:
//
// * <sig> <preimage> <witness script>
copy(preimage[:], spendingInput.Witness[1])
}
log.Infof("%T(%v): extracting preimage=%x 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.AddPreimage(preimage[:]); err != nil {
log.Errorf("%T(%v): unable to add witness to cache",
h, h.htlcResolution.ClaimOutpoint)
}
// Finally, we'll send the clean up message, mark ourselves as
// resolved, then exit.
if err := h.DeliverResolutionMsg(ResolutionMsg{
SourceChan: h.ShortChanID,
HtlcIndex: h.htlcIndex,
PreImage: &preimage,
}); err != nil {
return nil, err
}
h.resolved = true
return nil, h.Checkpoint(h)
}
// Otherwise, we'll watch for two external signals to decide if we'll
// morph into another resolver, or fully resolve the contract.
// The output we'll be watching for is the *direct* spend from the HTLC
// output. If this isn't our commitment transaction, it'll be right on
// the resolution. Otherwise, we fetch this pointer from the input of
// the time out transaction.
var (
outPointToWatch wire.OutPoint
scriptToWatch []byte
err error
)
if h.htlcResolution.SignedTimeoutTx == nil {
outPointToWatch = h.htlcResolution.ClaimOutpoint
scriptToWatch = h.htlcResolution.SweepSignDesc.Output.PkScript
} else {
// 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 pkScipt we need to watch.
outPointToWatch = h.htlcResolution.SignedTimeoutTx.TxIn[0].PreviousOutPoint
witness := h.htlcResolution.SignedTimeoutTx.TxIn[0].Witness
scriptToWatch, err = lnwallet.WitnessScriptHash(
witness[len(witness)-1],
)
if err != nil {
return nil, err
}
}
// First, we'll register for a spend notification for this output. If
// the remote party sweeps with the pre-image, we'll be notified.
spendNtfn, err := h.Notifier.RegisterSpendNtfn(
&outPointToWatch, scriptToWatch, h.broadcastHeight,
)
if err != nil {
return nil, err
}
// We'll quickly check to see if the output has already been spent.
select {
// If the output has already been spent, then we can stop early and
// sweep the pre-image from the output.
case commitSpend, ok := <-spendNtfn.Spend:
if !ok {
return nil, fmt.Errorf("quitting")
}
// TODO(roasbeef): Checkpoint?
return claimCleanUp(commitSpend)
// If it hasn't, then we'll watch for both the expiration, and the
// sweeping out this output.
default:
}
// We'll check the current height, if the HTLC has already expired,
// then we'll morph immediately into a resolver that can sweep the
// HTLC.
//
// TODO(roasbeef): use grace period instead?
_, currentHeight, err := h.ChainIO.GetBestBlock()
if err != nil {
return nil, err
}
// If the current height is >= expiry-1, then a spend will be valid to
// be included in the next block, and we can immediately return the
// resolver.
//
// TODO(joostjager): Statement above may not be valid. For CLTV locks,
// the expiry value is the last _invalid_ block. The likely reason that
// this does not create a problem, is that utxonursery is checking the
// expiry again (in the proper way). Same holds for minus one operation
// below.
//
// Source:
// https://github.com/btcsuite/btcd/blob/991d32e72fe84d5fbf9c47cd604d793a0cd3a072/blockchain/validate.go#L154
if uint32(currentHeight) >= h.htlcResolution.Expiry-1 {
log.Infof("%T(%v): HTLC has expired (height=%v, expiry=%v), "+
"transforming into timeout resolver", h,
h.htlcResolution.ClaimOutpoint, currentHeight,
h.htlcResolution.Expiry)
return &h.htlcTimeoutResolver, nil
}
// If we reach this point, then we can't fully act yet, so we'll await
// either of our signals triggering: the HTLC expires, or we learn of
// the preimage.
blockEpochs, err := h.Notifier.RegisterBlockEpochNtfn(nil)
if err != nil {
return nil, err
}
defer blockEpochs.Cancel()
for {
select {
// A new block has arrived, we'll check to see if this leads to
// HTLC expiration.
case newBlock, ok := <-blockEpochs.Epochs:
if !ok {
return nil, fmt.Errorf("quitting")
}
// If this new height expires the HTLC, then we can
// exit early and create a resolver that's capable of
// handling the time locked output.
newHeight := uint32(newBlock.Height)
if newHeight >= h.htlcResolution.Expiry-1 {
log.Infof("%T(%v): HTLC has expired "+
"(height=%v, expiry=%v), transforming "+
"into timeout resolver", h,
h.htlcResolution.ClaimOutpoint,
newHeight, h.htlcResolution.Expiry)
return &h.htlcTimeoutResolver, nil
}
// The output has been spent! This means the preimage has been
// revealed on-chain.
case commitSpend, ok := <-spendNtfn.Spend:
if !ok {
return nil, fmt.Errorf("quitting")
}
// The only way this output can be spent by the remote
// party is by revealing the preimage. So we'll perform
// our duties to clean up the contract once it has been
// claimed.
return claimCleanUp(commitSpend)
case <-h.Quit:
return nil, fmt.Errorf("resolver cancelled")
}
}
}
// Stop signals the resolver to cancel any current resolution processes, and
// suspend.
//
// NOTE: Part of the ContractResolver interface.
func (h *htlcOutgoingContestResolver) 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 *htlcOutgoingContestResolver) 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 *htlcOutgoingContestResolver) Encode(w io.Writer) error {
return h.htlcTimeoutResolver.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 *htlcOutgoingContestResolver) Decode(r io.Reader) error {
return h.htlcTimeoutResolver.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 *htlcOutgoingContestResolver) AttachResolverKit(r ResolverKit) {
h.ResolverKit = r
}
// A compile time assertion to ensure htlcOutgoingContestResolver meets the
// ContractResolver interface.
var _ ContractResolver = (*htlcOutgoingContestResolver)(nil)