lnd.xprv/contractcourt/htlc_outgoing_contest_resolver.go

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)