package contractcourt import ( "encoding/binary" "fmt" "io" "github.com/btcsuite/btcd/wire" "github.com/davecgh/go-spew/spew" "github.com/lightningnetwork/lnd/chainntnfs" "github.com/lightningnetwork/lnd/input" "github.com/lightningnetwork/lnd/lntypes" "github.com/lightningnetwork/lnd/lnwallet" "github.com/lightningnetwork/lnd/lnwire" ) // 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 // htlcIndex is the index of this HTLC within the trace of the // additional commitment state machine. htlcIndex uint64 // htlcAmt is the original amount of the htlc, not taking into // account any fees that may have to be paid if it goes on chain. htlcAmt lnwire.MilliSatoshi ResolverKit } // 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> preimageBytes = spendingInput.Witness[remotePreimageIndex] } else { // Otherwise, they'll be spending directly from our commitment // output. In which case the witness stack looks like: // // * 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.htlcIndex, PreImage: &pre, }); err != nil { return nil, err } h.resolved = true return nil, h.Checkpoint(h) } // 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> (2nd level success spend) // REVOK: // SENDR: 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> <0> (2nd level timeout) // RECVR: // REVOK: // // 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 } // If we haven't already sent the output to the utxo nursery, then // we'll do so now. if !h.outputIncubating { log.Tracef("%T(%v): incubating htlc output", h, h.htlcResolution.ClaimOutpoint) err := h.IncubateOutputs( h.ChanPoint, nil, &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 } } // waitForOutputResolution waits for the HTLC output to be fully // resolved. The output is considered fully resolved once it has been // spent, and the spending transaction has been fully confirmed. waitForOutputResolution := func() error { // We first need to register to see when the HTLC output itself // has been spent by a confirmed transaction. spendNtfn, err := h.Notifier.RegisterSpendNtfn( &h.htlcResolution.ClaimOutpoint, h.htlcResolution.SweepSignDesc.Output.PkScript, h.broadcastHeight, ) if err != nil { return err } select { case _, ok := <-spendNtfn.Spend: if !ok { return errResolverShuttingDown } case <-h.Quit: return errResolverShuttingDown } return nil } // Now that we've handed off the HTLC to the nursery, 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 } spendNtfn, err := h.Notifier.RegisterSpendNtfn( outpointToWatch, scriptToWatch, h.broadcastHeight, ) 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. var ( spend *chainntnfs.SpendDetail ok bool ) select { case spend, ok = <-spendNtfn.Spend: if !ok { return nil, errResolverShuttingDown } case <-h.Quit: return nil, errResolverShuttingDown } // 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(spend, 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(spend) } 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.htlcIndex, Failure: failureMsg, }); err != nil { return nil, err } // 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. if h.htlcResolution.SignedTimeoutTx != nil { log.Infof("%T(%v): waiting for nursery to spend CSV delayed "+ "output", h, h.htlcResolution.ClaimOutpoint) if err := waitForOutputResolution(); err != nil { return nil, err } } // With the clean up message sent, we'll now mark the contract // resolved, and wait. h.resolved = true return nil, h.Checkpoint(h) } // 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 } // 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.htlcIndex); err != nil { return err } return nil } // 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 *htlcTimeoutResolver) Decode(r io.Reader) error { // First, we'll read out all the mandatory fields of the // OutgoingHtlcResolution that we store. if err := decodeOutgoingResolution(r, &h.htlcResolution); err != nil { return 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 err } if err := binary.Read(r, endian, &h.resolved); err != nil { return err } if err := binary.Read(r, endian, &h.broadcastHeight); err != nil { return err } if err := binary.Read(r, endian, &h.htlcIndex); err != nil { return err } return nil } // 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 *htlcTimeoutResolver) AttachResolverKit(r ResolverKit) { h.ResolverKit = r } // A compile time assertion to ensure htlcTimeoutResolver meets the // ContractResolver interface. var _ ContractResolver = (*htlcTimeoutResolver)(nil)