package contractcourt import ( "fmt" "github.com/lightningnetwork/lnd/input" "io" "github.com/btcsuite/btcd/wire" "github.com/btcsuite/btcutil" "github.com/davecgh/go-spew/spew" "github.com/lightningnetwork/lnd/chainntnfs" ) // 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: // // * copy(preimage[:], spendingInput.Witness[3]) } else { // Otherwise, they'll be spending directly from our // commitment output. In which case the witness stack // looks like: // // * 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 ) // TODO(joostjager): output already set properly in // lnwallet.newOutgoingHtlcResolution? And script too? 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 = input.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") } } } // report returns a report on the resolution state of the contract. func (h *htlcOutgoingContestResolver) report() *ContractReport { // No locking needed as these values are read-only. finalAmt := h.htlcAmt.ToSatoshis() if h.htlcResolution.SignedTimeoutTx != nil { finalAmt = btcutil.Amount( h.htlcResolution.SignedTimeoutTx.TxOut[0].Value, ) } return &ContractReport{ Outpoint: h.htlcResolution.ClaimOutpoint, Incoming: false, Amount: finalAmt, MaturityHeight: h.htlcResolution.Expiry, LimboBalance: finalAmt, Stage: 1, } } // 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)