package lnwallet import ( "bytes" "crypto/sha256" "errors" "fmt" "math" "net" "sync" "sync/atomic" "github.com/btcsuite/btcd/blockchain" "github.com/btcsuite/btcd/btcec" "github.com/btcsuite/btcd/chaincfg/chainhash" "github.com/btcsuite/btcd/txscript" "github.com/btcsuite/btcd/wire" "github.com/btcsuite/btcutil" "github.com/btcsuite/btcutil/txsort" "github.com/davecgh/go-spew/spew" "github.com/lightningnetwork/lnd/channeldb" "github.com/lightningnetwork/lnd/input" "github.com/lightningnetwork/lnd/keychain" "github.com/lightningnetwork/lnd/lnwire" "github.com/lightningnetwork/lnd/shachain" ) const ( // The size of the buffered queue of requests to the wallet from the // outside word. msgBufferSize = 100 ) // ErrInsufficientFunds is a type matching the error interface which is // returned when coin selection for a new funding transaction fails to due // having an insufficient amount of confirmed funds. type ErrInsufficientFunds struct { amountAvailable btcutil.Amount amountSelected btcutil.Amount } func (e *ErrInsufficientFunds) Error() string { return fmt.Sprintf("not enough witness outputs to create funding transaction,"+ " need %v only have %v available", e.amountAvailable, e.amountSelected) } // InitFundingReserveMsg is the first message sent to initiate the workflow // required to open a payment channel with a remote peer. The initial required // parameters are configurable across channels. These parameters are to be // chosen depending on the fee climate within the network, and time value of // funds to be locked up within the channel. Upon success a ChannelReservation // will be created in order to track the lifetime of this pending channel. // Outputs selected will be 'locked', making them unavailable, for any other // pending reservations. Therefore, all channels in reservation limbo will be // periodically timed out after an idle period in order to avoid "exhaustion" // attacks. type InitFundingReserveMsg struct { // ChainHash denotes that chain to be used to ultimately open the // target channel. ChainHash *chainhash.Hash // NodeID is the ID of the remote node we would like to open a channel // with. NodeID *btcec.PublicKey // NodeAddr is the address port that we used to either establish or // accept the connection which led to the negotiation of this funding // workflow. NodeAddr net.Addr // SubtractFees should be set if we intend to spend exactly // LocalFundingAmt when opening the channel, subtracting the fees from // the funding output. This can be used for instance to use all our // remaining funds to open the channel, since it will take fees into // account. SubtractFees bool // LocalFundingAmt is the amount of funds requested from us for this // channel. LocalFundingAmt btcutil.Amount // RemoteFundingAmnt is the amount of funds the remote will contribute // to this channel. RemoteFundingAmt btcutil.Amount // CommitFeePerKw is the starting accepted satoshis/Kw fee for the set // of initial commitment transactions. In order to ensure timely // confirmation, it is recommended that this fee should be generous, // paying some multiple of the accepted base fee rate of the network. CommitFeePerKw SatPerKWeight // FundingFeePerKw is the fee rate in sat/kw to use for the initial // funding transaction. FundingFeePerKw SatPerKWeight // PushMSat is the number of milli-satoshis that should be pushed over // the responder as part of the initial channel creation. PushMSat lnwire.MilliSatoshi // Flags are the channel flags specified by the initiator in the // open_channel message. Flags lnwire.FundingFlag // MinConfs indicates the minimum number of confirmations that each // output selected to fund the channel should satisfy. MinConfs int32 // err is a channel in which all errors will be sent across. Will be // nil if this initial set is successful. // // NOTE: In order to avoid deadlocks, this channel MUST be buffered. err chan error // resp is channel in which a ChannelReservation with our contributions // filled in will be sent across this channel in the case of a // successfully reservation initiation. In the case of an error, this // will read a nil pointer. // // NOTE: In order to avoid deadlocks, this channel MUST be buffered. resp chan *ChannelReservation } // fundingReserveCancelMsg is a message reserved for cancelling an existing // channel reservation identified by its reservation ID. Cancelling a reservation // frees its locked outputs up, for inclusion within further reservations. type fundingReserveCancelMsg struct { pendingFundingID uint64 // NOTE: In order to avoid deadlocks, this channel MUST be buffered. err chan error // Buffered } // addContributionMsg represents a message executing the second phase of the // channel reservation workflow. This message carries the counterparty's // "contribution" to the payment channel. In the case that this message is // processed without generating any errors, then channel reservation will then // be able to construct the funding tx, both commitment transactions, and // finally generate signatures for all our inputs to the funding transaction, // and for the remote node's version of the commitment transaction. type addContributionMsg struct { pendingFundingID uint64 // TODO(roasbeef): Should also carry SPV proofs in we're in SPV mode contribution *ChannelContribution // NOTE: In order to avoid deadlocks, this channel MUST be buffered. err chan error } // addSingleContributionMsg represents a message executing the second phase of // a single funder channel reservation workflow. This messages carries the // counterparty's "contribution" to the payment channel. As this message is // sent when on the responding side to a single funder workflow, no further // action apart from storing the provided contribution is carried out. type addSingleContributionMsg struct { pendingFundingID uint64 contribution *ChannelContribution // NOTE: In order to avoid deadlocks, this channel MUST be buffered. err chan error } // addCounterPartySigsMsg represents the final message required to complete, // and 'open' a payment channel. This message carries the counterparty's // signatures for each of their inputs to the funding transaction, and also a // signature allowing us to spend our version of the commitment transaction. // If we're able to verify all the signatures are valid, the funding transaction // will be broadcast to the network. After the funding transaction gains a // configurable number of confirmations, the channel is officially considered // 'open'. type addCounterPartySigsMsg struct { pendingFundingID uint64 // Should be order of sorted inputs that are theirs. Sorting is done // in accordance to BIP-69: // https://github.com/bitcoin/bips/blob/master/bip-0069.mediawiki. theirFundingInputScripts []*input.Script // This should be 1/2 of the signatures needed to successfully spend our // version of the commitment transaction. theirCommitmentSig []byte // This channel is used to return the completed channel after the wallet // has completed all of its stages in the funding process. completeChan chan *channeldb.OpenChannel // NOTE: In order to avoid deadlocks, this channel MUST be buffered. err chan error } // addSingleFunderSigsMsg represents the next-to-last message required to // complete a single-funder channel workflow. Once the initiator is able to // construct the funding transaction, they send both the outpoint and a // signature for our version of the commitment transaction. Once this message // is processed we (the responder) are able to construct both commitment // transactions, signing the remote party's version. type addSingleFunderSigsMsg struct { pendingFundingID uint64 // fundingOutpoint is the outpoint of the completed funding // transaction as assembled by the workflow initiator. fundingOutpoint *wire.OutPoint // theirCommitmentSig are the 1/2 of the signatures needed to // successfully spend our version of the commitment transaction. theirCommitmentSig []byte // This channel is used to return the completed channel after the wallet // has completed all of its stages in the funding process. completeChan chan *channeldb.OpenChannel // NOTE: In order to avoid deadlocks, this channel MUST be buffered. err chan error } // LightningWallet is a domain specific, yet general Bitcoin wallet capable of // executing workflow required to interact with the Lightning Network. It is // domain specific in the sense that it understands all the fancy scripts used // within the Lightning Network, channel lifetimes, etc. However, it embeds a // general purpose Bitcoin wallet within it. Therefore, it is also able to // serve as a regular Bitcoin wallet which uses HD keys. The wallet is highly // concurrent internally. All communication, and requests towards the wallet // are dispatched as messages over channels, ensuring thread safety across all // operations. Interaction has been designed independent of any peer-to-peer // communication protocol, allowing the wallet to be self-contained and // embeddable within future projects interacting with the Lightning Network. // // NOTE: At the moment the wallet requires a btcd full node, as it's dependent // on btcd's websockets notifications as event triggers during the lifetime of a // channel. However, once the chainntnfs package is complete, the wallet will // be compatible with multiple RPC/notification services such as Electrum, // Bitcoin Core + ZeroMQ, etc. Eventually, the wallet won't require a full-node // at all, as SPV support is integrated into btcwallet. type LightningWallet struct { started int32 // To be used atomically. shutdown int32 // To be used atomically. nextFundingID uint64 // To be used atomically. // Cfg is the configuration struct that will be used by the wallet to // access the necessary interfaces and default it needs to carry on its // duties. Cfg Config // WalletController is the core wallet, all non Lightning Network // specific interaction is proxied to the internal wallet. WalletController // SecretKeyRing is the interface we'll use to derive any keys related // to our purpose within the network including: multi-sig keys, node // keys, revocation keys, etc. keychain.SecretKeyRing // This mutex is to be held when generating external keys to be used as // multi-sig, and commitment keys within the channel. keyGenMtx sync.RWMutex // This mutex MUST be held when performing coin selection in order to // avoid inadvertently creating multiple funding transaction which // double spend inputs across each other. coinSelectMtx sync.RWMutex // All messages to the wallet are to be sent across this channel. msgChan chan interface{} // Incomplete payment channels are stored in the map below. An intent // to create a payment channel is tracked as a "reservation" within // limbo. Once the final signatures have been exchanged, a reservation // is removed from limbo. Each reservation is tracked by a unique // monotonically integer. All requests concerning the channel MUST // carry a valid, active funding ID. fundingLimbo map[uint64]*ChannelReservation limboMtx sync.RWMutex // lockedOutPoints is a set of the currently locked outpoint. This // information is kept in order to provide an easy way to unlock all // the currently locked outpoints. lockedOutPoints map[wire.OutPoint]struct{} quit chan struct{} wg sync.WaitGroup // TODO(roasbeef): handle wallet lock/unlock } // NewLightningWallet creates/opens and initializes a LightningWallet instance. // If the wallet has never been created (according to the passed dataDir), first-time // setup is executed. func NewLightningWallet(Cfg Config) (*LightningWallet, error) { return &LightningWallet{ Cfg: Cfg, SecretKeyRing: Cfg.SecretKeyRing, WalletController: Cfg.WalletController, msgChan: make(chan interface{}, msgBufferSize), nextFundingID: 0, fundingLimbo: make(map[uint64]*ChannelReservation), lockedOutPoints: make(map[wire.OutPoint]struct{}), quit: make(chan struct{}), }, nil } // Startup establishes a connection to the RPC source, and spins up all // goroutines required to handle incoming messages. func (l *LightningWallet) Startup() error { // Already started? if atomic.AddInt32(&l.started, 1) != 1 { return nil } // Start the underlying wallet controller. if err := l.Start(); err != nil { return err } l.wg.Add(1) // TODO(roasbeef): multiple request handlers? go l.requestHandler() return nil } // Shutdown gracefully stops the wallet, and all active goroutines. func (l *LightningWallet) Shutdown() error { if atomic.AddInt32(&l.shutdown, 1) != 1 { return nil } // Signal the underlying wallet controller to shutdown, waiting until // all active goroutines have been shutdown. if err := l.Stop(); err != nil { return err } close(l.quit) l.wg.Wait() return nil } // LockedOutpoints returns a list of all currently locked outpoint. func (l *LightningWallet) LockedOutpoints() []*wire.OutPoint { outPoints := make([]*wire.OutPoint, 0, len(l.lockedOutPoints)) for outPoint := range l.lockedOutPoints { outPoints = append(outPoints, &outPoint) } return outPoints } // ResetReservations reset the volatile wallet state which tracks all currently // active reservations. func (l *LightningWallet) ResetReservations() { l.nextFundingID = 0 l.fundingLimbo = make(map[uint64]*ChannelReservation) for outpoint := range l.lockedOutPoints { l.UnlockOutpoint(outpoint) } l.lockedOutPoints = make(map[wire.OutPoint]struct{}) } // ActiveReservations returns a slice of all the currently active // (non-cancelled) reservations. func (l *LightningWallet) ActiveReservations() []*ChannelReservation { reservations := make([]*ChannelReservation, 0, len(l.fundingLimbo)) for _, reservation := range l.fundingLimbo { reservations = append(reservations, reservation) } return reservations } // requestHandler is the primary goroutine(s) responsible for handling, and // dispatching replies to all messages. func (l *LightningWallet) requestHandler() { out: for { select { case m := <-l.msgChan: switch msg := m.(type) { case *InitFundingReserveMsg: l.handleFundingReserveRequest(msg) case *fundingReserveCancelMsg: l.handleFundingCancelRequest(msg) case *addSingleContributionMsg: l.handleSingleContribution(msg) case *addContributionMsg: l.handleContributionMsg(msg) case *addSingleFunderSigsMsg: l.handleSingleFunderSigs(msg) case *addCounterPartySigsMsg: l.handleFundingCounterPartySigs(msg) } case <-l.quit: // TODO: do some clean up break out } } l.wg.Done() } // InitChannelReservation kicks off the 3-step workflow required to successfully // open a payment channel with a remote node. As part of the funding // reservation, the inputs selected for the funding transaction are 'locked'. // This ensures that multiple channel reservations aren't double spending the // same inputs in the funding transaction. If reservation initialization is // successful, a ChannelReservation containing our completed contribution is // returned. Our contribution contains all the items necessary to allow the // counterparty to build the funding transaction, and both versions of the // commitment transaction. Otherwise, an error occurred and a nil pointer along // with an error are returned. // // Once a ChannelReservation has been obtained, two additional steps must be // processed before a payment channel can be considered 'open'. The second step // validates, and processes the counterparty's channel contribution. The third, // and final step verifies all signatures for the inputs of the funding // transaction, and that the signature we record for our version of the // commitment transaction is valid. func (l *LightningWallet) InitChannelReservation( req *InitFundingReserveMsg) (*ChannelReservation, error) { req.resp = make(chan *ChannelReservation, 1) req.err = make(chan error, 1) select { case l.msgChan <- req: case <-l.quit: return nil, errors.New("wallet shutting down") } return <-req.resp, <-req.err } // handleFundingReserveRequest processes a message intending to create, and // validate a funding reservation request. func (l *LightningWallet) handleFundingReserveRequest(req *InitFundingReserveMsg) { // It isn't possible to create a channel with zero funds committed. if req.LocalFundingAmt+req.RemoteFundingAmt == 0 { err := ErrZeroCapacity() req.err <- err req.resp <- nil return } // If the funding request is for a different chain than the one the // wallet is aware of, then we'll reject the request. if !bytes.Equal(l.Cfg.NetParams.GenesisHash[:], req.ChainHash[:]) { err := ErrChainMismatch( l.Cfg.NetParams.GenesisHash, req.ChainHash, ) req.err <- err req.resp <- nil return } localFundingAmt := req.LocalFundingAmt var ( selected *coinSelection err error ) // If we're on the receiving end of a single funder channel then we // don't need to perform any coin selection, and the remote contributes // all funds. Otherwise, attempt to obtain enough coins to meet the // required funding amount. if req.LocalFundingAmt != 0 { // Coin selection is done on the basis of sat/kw, so we'll use // the fee rate passed in to perform coin selection. var err error selected, err = l.selectCoinsAndChange( req.FundingFeePerKw, req.LocalFundingAmt, req.MinConfs, req.SubtractFees, ) if err != nil { req.err <- err req.resp <- nil return } localFundingAmt = selected.fundingAmt } // The total channel capacity will be the size of the funding output we // created plus the remote contribution. capacity := localFundingAmt + req.RemoteFundingAmt id := atomic.AddUint64(&l.nextFundingID, 1) reservation, err := NewChannelReservation( capacity, localFundingAmt, req.CommitFeePerKw, l, id, req.PushMSat, l.Cfg.NetParams.GenesisHash, req.Flags, ) if err != nil { selected.unlockCoins() req.err <- err req.resp <- nil return } err = l.initOurContribution( reservation, selected, req.NodeAddr, req.NodeID, ) if err != nil { selected.unlockCoins() req.err <- err req.resp <- nil return } // Create a limbo and record entry for this newly pending funding // request. l.limboMtx.Lock() l.fundingLimbo[id] = reservation l.limboMtx.Unlock() // Funding reservation request successfully handled. The funding inputs // will be marked as unavailable until the reservation is either // completed, or cancelled. req.resp <- reservation req.err <- nil } // initOurContribution initializes the given ChannelReservation with our coins // and change reserved for the channel, and derives the keys to use for this // channel. func (l *LightningWallet) initOurContribution(reservation *ChannelReservation, selected *coinSelection, nodeAddr net.Addr, nodeID *btcec.PublicKey) error { // Grab the mutex on the ChannelReservation to ensure thread-safety reservation.Lock() defer reservation.Unlock() if selected != nil { reservation.ourContribution.Inputs = selected.coins reservation.ourContribution.ChangeOutputs = selected.change } reservation.nodeAddr = nodeAddr reservation.partialState.IdentityPub = nodeID // Next, we'll grab a series of keys from the wallet which will be used // for the duration of the channel. The keys include: our multi-sig // key, the base revocation key, the base htlc key,the base payment // key, and the delayed payment key. // // TODO(roasbeef): "salt" each key as well? var err error reservation.ourContribution.MultiSigKey, err = l.DeriveNextKey( keychain.KeyFamilyMultiSig, ) if err != nil { return err } reservation.ourContribution.RevocationBasePoint, err = l.DeriveNextKey( keychain.KeyFamilyRevocationBase, ) if err != nil { return err } reservation.ourContribution.HtlcBasePoint, err = l.DeriveNextKey( keychain.KeyFamilyHtlcBase, ) if err != nil { return err } reservation.ourContribution.PaymentBasePoint, err = l.DeriveNextKey( keychain.KeyFamilyPaymentBase, ) if err != nil { return err } reservation.ourContribution.DelayBasePoint, err = l.DeriveNextKey( keychain.KeyFamilyDelayBase, ) if err != nil { return err } // With the above keys created, we'll also need to initialization our // initial revocation tree state. nextRevocationKeyDesc, err := l.DeriveNextKey( keychain.KeyFamilyRevocationRoot, ) if err != nil { return err } revocationRoot, err := l.DerivePrivKey(nextRevocationKeyDesc) if err != nil { return err } // Once we have the root, we can then generate our shachain producer // and from that generate the per-commitment point. revRoot, err := chainhash.NewHash(revocationRoot.Serialize()) if err != nil { return err } producer := shachain.NewRevocationProducer(*revRoot) firstPreimage, err := producer.AtIndex(0) if err != nil { return err } reservation.ourContribution.FirstCommitmentPoint = input.ComputeCommitmentPoint( firstPreimage[:], ) reservation.partialState.RevocationProducer = producer reservation.ourContribution.ChannelConstraints = l.Cfg.DefaultConstraints return nil } // handleFundingReserveCancel cancels an existing channel reservation. As part // of the cancellation, outputs previously selected as inputs for the funding // transaction via coin selection are freed allowing future reservations to // include them. func (l *LightningWallet) handleFundingCancelRequest(req *fundingReserveCancelMsg) { // TODO(roasbeef): holding lock too long l.limboMtx.Lock() defer l.limboMtx.Unlock() pendingReservation, ok := l.fundingLimbo[req.pendingFundingID] if !ok { // TODO(roasbeef): make new error, "unknown funding state" or something req.err <- fmt.Errorf("attempted to cancel non-existent funding state") return } // Grab the mutex on the ChannelReservation to ensure thread-safety pendingReservation.Lock() defer pendingReservation.Unlock() // Mark all previously locked outpoints as useable for future funding // requests. for _, unusedInput := range pendingReservation.ourContribution.Inputs { delete(l.lockedOutPoints, unusedInput.PreviousOutPoint) l.UnlockOutpoint(unusedInput.PreviousOutPoint) } // TODO(roasbeef): is it even worth it to keep track of unused keys? // TODO(roasbeef): Is it possible to mark the unused change also as // available? delete(l.fundingLimbo, req.pendingFundingID) req.err <- nil } // CreateCommitmentTxns is a helper function that creates the initial // commitment transaction for both parties. This function is used during the // initial funding workflow as both sides must generate a signature for the // remote party's commitment transaction, and verify the signature for their // version of the commitment transaction. func CreateCommitmentTxns(localBalance, remoteBalance btcutil.Amount, ourChanCfg, theirChanCfg *channeldb.ChannelConfig, localCommitPoint, remoteCommitPoint *btcec.PublicKey, fundingTxIn wire.TxIn) (*wire.MsgTx, *wire.MsgTx, error) { localCommitmentKeys := deriveCommitmentKeys(localCommitPoint, true, ourChanCfg, theirChanCfg) remoteCommitmentKeys := deriveCommitmentKeys(remoteCommitPoint, false, ourChanCfg, theirChanCfg) ourCommitTx, err := CreateCommitTx(fundingTxIn, localCommitmentKeys, uint32(ourChanCfg.CsvDelay), localBalance, remoteBalance, ourChanCfg.DustLimit) if err != nil { return nil, nil, err } otxn := btcutil.NewTx(ourCommitTx) if err := blockchain.CheckTransactionSanity(otxn); err != nil { return nil, nil, err } theirCommitTx, err := CreateCommitTx(fundingTxIn, remoteCommitmentKeys, uint32(theirChanCfg.CsvDelay), remoteBalance, localBalance, theirChanCfg.DustLimit) if err != nil { return nil, nil, err } ttxn := btcutil.NewTx(theirCommitTx) if err := blockchain.CheckTransactionSanity(ttxn); err != nil { return nil, nil, err } return ourCommitTx, theirCommitTx, nil } // handleContributionMsg processes the second workflow step for the lifetime of // a channel reservation. Upon completion, the reservation will carry a // completed funding transaction (minus the counterparty's input signatures), // both versions of the commitment transaction, and our signature for their // version of the commitment transaction. func (l *LightningWallet) handleContributionMsg(req *addContributionMsg) { l.limboMtx.Lock() pendingReservation, ok := l.fundingLimbo[req.pendingFundingID] l.limboMtx.Unlock() if !ok { req.err <- fmt.Errorf("attempted to update non-existent funding state") return } // Grab the mutex on the ChannelReservation to ensure thread-safety pendingReservation.Lock() defer pendingReservation.Unlock() // Create a blank, fresh transaction. Soon to be a complete funding // transaction which will allow opening a lightning channel. pendingReservation.fundingTx = wire.NewMsgTx(1) fundingTx := pendingReservation.fundingTx // Some temporary variables to cut down on the resolution verbosity. pendingReservation.theirContribution = req.contribution theirContribution := req.contribution ourContribution := pendingReservation.ourContribution // Add all multi-party inputs and outputs to the transaction. for _, ourInput := range ourContribution.Inputs { fundingTx.AddTxIn(ourInput) } for _, theirInput := range theirContribution.Inputs { fundingTx.AddTxIn(theirInput) } for _, ourChangeOutput := range ourContribution.ChangeOutputs { fundingTx.AddTxOut(ourChangeOutput) } for _, theirChangeOutput := range theirContribution.ChangeOutputs { fundingTx.AddTxOut(theirChangeOutput) } ourKey := pendingReservation.ourContribution.MultiSigKey theirKey := theirContribution.MultiSigKey // Finally, add the 2-of-2 multi-sig output which will set up the lightning // channel. channelCapacity := int64(pendingReservation.partialState.Capacity) witnessScript, multiSigOut, err := input.GenFundingPkScript( ourKey.PubKey.SerializeCompressed(), theirKey.PubKey.SerializeCompressed(), channelCapacity, ) if err != nil { req.err <- err return } // Sort the transaction. Since both side agree to a canonical ordering, // by sorting we no longer need to send the entire transaction. Only // signatures will be exchanged. fundingTx.AddTxOut(multiSigOut) txsort.InPlaceSort(pendingReservation.fundingTx) // Next, sign all inputs that are ours, collecting the signatures in // order of the inputs. pendingReservation.ourFundingInputScripts = make([]*input.Script, 0, len(ourContribution.Inputs)) signDesc := input.SignDescriptor{ HashType: txscript.SigHashAll, SigHashes: txscript.NewTxSigHashes(fundingTx), } for i, txIn := range fundingTx.TxIn { info, err := l.FetchInputInfo(&txIn.PreviousOutPoint) if err == ErrNotMine { continue } else if err != nil { req.err <- err return } signDesc.Output = info signDesc.InputIndex = i inputScript, err := l.Cfg.Signer.ComputeInputScript( fundingTx, &signDesc, ) if err != nil { req.err <- err return } txIn.SignatureScript = inputScript.SigScript txIn.Witness = inputScript.Witness pendingReservation.ourFundingInputScripts = append( pendingReservation.ourFundingInputScripts, inputScript, ) } // Locate the index of the multi-sig outpoint in order to record it // since the outputs are canonically sorted. If this is a single funder // workflow, then we'll also need to send this to the remote node. fundingTxID := fundingTx.TxHash() _, multiSigIndex := input.FindScriptOutputIndex(fundingTx, multiSigOut.PkScript) fundingOutpoint := wire.NewOutPoint(&fundingTxID, multiSigIndex) pendingReservation.partialState.FundingOutpoint = *fundingOutpoint walletLog.Debugf("Funding tx for ChannelPoint(%v) generated: %v", fundingOutpoint, spew.Sdump(fundingTx)) // Initialize an empty sha-chain for them, tracking the current pending // revocation hash (we don't yet know the preimage so we can't add it // to the chain). s := shachain.NewRevocationStore() pendingReservation.partialState.RevocationStore = s // Store their current commitment point. We'll need this after the // first state transition in order to verify the authenticity of the // revocation. chanState := pendingReservation.partialState chanState.RemoteCurrentRevocation = theirContribution.FirstCommitmentPoint // Create the txin to our commitment transaction; required to construct // the commitment transactions. fundingTxIn := wire.TxIn{ PreviousOutPoint: wire.OutPoint{ Hash: fundingTxID, Index: multiSigIndex, }, } // With the funding tx complete, create both commitment transactions. localBalance := pendingReservation.partialState.LocalCommitment.LocalBalance.ToSatoshis() remoteBalance := pendingReservation.partialState.LocalCommitment.RemoteBalance.ToSatoshis() ourCommitTx, theirCommitTx, err := CreateCommitmentTxns( localBalance, remoteBalance, ourContribution.ChannelConfig, theirContribution.ChannelConfig, ourContribution.FirstCommitmentPoint, theirContribution.FirstCommitmentPoint, fundingTxIn, ) if err != nil { req.err <- err return } // With both commitment transactions constructed, generate the state // obfuscator then use it to encode the current state number within // both commitment transactions. var stateObfuscator [StateHintSize]byte if chanState.ChanType == channeldb.SingleFunder { stateObfuscator = DeriveStateHintObfuscator( ourContribution.PaymentBasePoint.PubKey, theirContribution.PaymentBasePoint.PubKey, ) } else { ourSer := ourContribution.PaymentBasePoint.PubKey.SerializeCompressed() theirSer := theirContribution.PaymentBasePoint.PubKey.SerializeCompressed() switch bytes.Compare(ourSer, theirSer) { case -1: stateObfuscator = DeriveStateHintObfuscator( ourContribution.PaymentBasePoint.PubKey, theirContribution.PaymentBasePoint.PubKey, ) default: stateObfuscator = DeriveStateHintObfuscator( theirContribution.PaymentBasePoint.PubKey, ourContribution.PaymentBasePoint.PubKey, ) } } err = initStateHints(ourCommitTx, theirCommitTx, stateObfuscator) if err != nil { req.err <- err return } // Sort both transactions according to the agreed upon canonical // ordering. This lets us skip sending the entire transaction over, // instead we'll just send signatures. txsort.InPlaceSort(ourCommitTx) txsort.InPlaceSort(theirCommitTx) walletLog.Debugf("Local commit tx for ChannelPoint(%v): %v", fundingOutpoint, spew.Sdump(ourCommitTx)) walletLog.Debugf("Remote commit tx for ChannelPoint(%v): %v", fundingOutpoint, spew.Sdump(theirCommitTx)) // Record newly available information within the open channel state. chanState.FundingOutpoint = *fundingOutpoint chanState.LocalCommitment.CommitTx = ourCommitTx chanState.RemoteCommitment.CommitTx = theirCommitTx // Generate a signature for their version of the initial commitment // transaction. signDesc = input.SignDescriptor{ WitnessScript: witnessScript, KeyDesc: ourKey, Output: multiSigOut, HashType: txscript.SigHashAll, SigHashes: txscript.NewTxSigHashes(theirCommitTx), InputIndex: 0, } sigTheirCommit, err := l.Cfg.Signer.SignOutputRaw(theirCommitTx, &signDesc) if err != nil { req.err <- err return } pendingReservation.ourCommitmentSig = sigTheirCommit req.err <- nil } // handleSingleContribution is called as the second step to a single funder // workflow to which we are the responder. It simply saves the remote peer's // contribution to the channel, as solely the remote peer will contribute any // funds to the channel. func (l *LightningWallet) handleSingleContribution(req *addSingleContributionMsg) { l.limboMtx.Lock() pendingReservation, ok := l.fundingLimbo[req.pendingFundingID] l.limboMtx.Unlock() if !ok { req.err <- fmt.Errorf("attempted to update non-existent funding state") return } // Grab the mutex on the channelReservation to ensure thread-safety. pendingReservation.Lock() defer pendingReservation.Unlock() // TODO(roasbeef): verify sanity of remote party's parameters, fail if // disagree // Simply record the counterparty's contribution into the pending // reservation data as they'll be solely funding the channel entirely. pendingReservation.theirContribution = req.contribution theirContribution := pendingReservation.theirContribution chanState := pendingReservation.partialState // Initialize an empty sha-chain for them, tracking the current pending // revocation hash (we don't yet know the preimage so we can't add it // to the chain). remotePreimageStore := shachain.NewRevocationStore() chanState.RevocationStore = remotePreimageStore // Now that we've received their first commitment point, we'll store it // within the channel state so we can sync it to disk once the funding // process is complete. chanState.RemoteCurrentRevocation = theirContribution.FirstCommitmentPoint req.err <- nil return } // openChanDetails contains a "finalized" channel which can be considered // "open" according to the requested confirmation depth at reservation // initialization. Additionally, the struct contains additional details // pertaining to the exact location in the main chain in-which the transaction // was confirmed. type openChanDetails struct { channel *LightningChannel blockHeight uint32 txIndex uint32 } // handleFundingCounterPartySigs is the final step in the channel reservation // workflow. During this step, we validate *all* the received signatures for // inputs to the funding transaction. If any of these are invalid, we bail, // and forcibly cancel this funding request. Additionally, we ensure that the // signature we received from the counterparty for our version of the commitment // transaction allows us to spend from the funding output with the addition of // our signature. func (l *LightningWallet) handleFundingCounterPartySigs(msg *addCounterPartySigsMsg) { l.limboMtx.RLock() res, ok := l.fundingLimbo[msg.pendingFundingID] l.limboMtx.RUnlock() if !ok { msg.err <- fmt.Errorf("attempted to update non-existent funding state") return } // Grab the mutex on the ChannelReservation to ensure thread-safety res.Lock() defer res.Unlock() // Now we can complete the funding transaction by adding their // signatures to their inputs. res.theirFundingInputScripts = msg.theirFundingInputScripts inputScripts := msg.theirFundingInputScripts fundingTx := res.fundingTx sigIndex := 0 fundingHashCache := txscript.NewTxSigHashes(fundingTx) for i, txin := range fundingTx.TxIn { if len(inputScripts) != 0 && len(txin.Witness) == 0 { // Attach the input scripts so we can verify it below. txin.Witness = inputScripts[sigIndex].Witness txin.SignatureScript = inputScripts[sigIndex].SigScript // Fetch the alleged previous output along with the // pkscript referenced by this input. // // TODO(roasbeef): when dual funder pass actual // height-hint pkScript, err := input.WitnessScriptHash( txin.Witness[len(txin.Witness)-1], ) if err != nil { msg.err <- fmt.Errorf("cannot create script: "+ "%v", err) msg.completeChan <- nil return } output, err := l.Cfg.ChainIO.GetUtxo( &txin.PreviousOutPoint, pkScript, 0, l.quit, ) if output == nil { msg.err <- fmt.Errorf("input to funding tx "+ "does not exist: %v", err) msg.completeChan <- nil return } // Ensure that the witness+sigScript combo is valid. vm, err := txscript.NewEngine(output.PkScript, fundingTx, i, txscript.StandardVerifyFlags, nil, fundingHashCache, output.Value) if err != nil { msg.err <- fmt.Errorf("cannot create script "+ "engine: %s", err) msg.completeChan <- nil return } if err = vm.Execute(); err != nil { msg.err <- fmt.Errorf("cannot validate "+ "transaction: %s", err) msg.completeChan <- nil return } sigIndex++ } } // At this point, we can also record and verify their signature for our // commitment transaction. res.theirCommitmentSig = msg.theirCommitmentSig commitTx := res.partialState.LocalCommitment.CommitTx ourKey := res.ourContribution.MultiSigKey theirKey := res.theirContribution.MultiSigKey // Re-generate both the witnessScript and p2sh output. We sign the // witnessScript script, but include the p2sh output as the subscript // for verification. witnessScript, _, err := input.GenFundingPkScript( ourKey.PubKey.SerializeCompressed(), theirKey.PubKey.SerializeCompressed(), int64(res.partialState.Capacity), ) if err != nil { msg.err <- err msg.completeChan <- nil return } // Next, create the spending scriptSig, and then verify that the script // is complete, allowing us to spend from the funding transaction. channelValue := int64(res.partialState.Capacity) hashCache := txscript.NewTxSigHashes(commitTx) sigHash, err := txscript.CalcWitnessSigHash(witnessScript, hashCache, txscript.SigHashAll, commitTx, 0, channelValue) if err != nil { msg.err <- err msg.completeChan <- nil return } // Verify that we've received a valid signature from the remote party // for our version of the commitment transaction. theirCommitSig := msg.theirCommitmentSig sig, err := btcec.ParseSignature(theirCommitSig, btcec.S256()) if err != nil { msg.err <- err msg.completeChan <- nil return } else if !sig.Verify(sigHash, theirKey.PubKey) { msg.err <- fmt.Errorf("counterparty's commitment signature is invalid") msg.completeChan <- nil return } res.partialState.LocalCommitment.CommitSig = theirCommitSig // Funding complete, this entry can be removed from limbo. l.limboMtx.Lock() delete(l.fundingLimbo, res.reservationID) l.limboMtx.Unlock() // As we're about to broadcast the funding transaction, we'll take note // of the current height for record keeping purposes. // // TODO(roasbeef): this info can also be piped into light client's // basic fee estimation? _, bestHeight, err := l.Cfg.ChainIO.GetBestBlock() if err != nil { msg.err <- err msg.completeChan <- nil return } // As we've completed the funding process, we'll no convert the // contribution structs into their underlying channel config objects to // he stored within the database. res.partialState.LocalChanCfg = res.ourContribution.toChanConfig() res.partialState.RemoteChanCfg = res.theirContribution.toChanConfig() // We'll also record the finalized funding txn, which will allow us to // rebroadcast on startup in case we fail. res.partialState.FundingTxn = fundingTx // Add the complete funding transaction to the DB, in its open bucket // which will be used for the lifetime of this channel. nodeAddr := res.nodeAddr err = res.partialState.SyncPending(nodeAddr, uint32(bestHeight)) if err != nil { msg.err <- err msg.completeChan <- nil return } msg.completeChan <- res.partialState msg.err <- nil } // handleSingleFunderSigs is called once the remote peer who initiated the // single funder workflow has assembled the funding transaction, and generated // a signature for our version of the commitment transaction. This method // progresses the workflow by generating a signature for the remote peer's // version of the commitment transaction. func (l *LightningWallet) handleSingleFunderSigs(req *addSingleFunderSigsMsg) { l.limboMtx.RLock() pendingReservation, ok := l.fundingLimbo[req.pendingFundingID] l.limboMtx.RUnlock() if !ok { req.err <- fmt.Errorf("attempted to update non-existent funding state") req.completeChan <- nil return } // Grab the mutex on the ChannelReservation to ensure thread-safety pendingReservation.Lock() defer pendingReservation.Unlock() chanState := pendingReservation.partialState chanState.FundingOutpoint = *req.fundingOutpoint fundingTxIn := wire.NewTxIn(req.fundingOutpoint, nil, nil) // Now that we have the funding outpoint, we can generate both versions // of the commitment transaction, and generate a signature for the // remote node's commitment transactions. localBalance := pendingReservation.partialState.LocalCommitment.LocalBalance.ToSatoshis() remoteBalance := pendingReservation.partialState.LocalCommitment.RemoteBalance.ToSatoshis() ourCommitTx, theirCommitTx, err := CreateCommitmentTxns( localBalance, remoteBalance, pendingReservation.ourContribution.ChannelConfig, pendingReservation.theirContribution.ChannelConfig, pendingReservation.ourContribution.FirstCommitmentPoint, pendingReservation.theirContribution.FirstCommitmentPoint, *fundingTxIn, ) if err != nil { req.err <- err req.completeChan <- nil return } // With both commitment transactions constructed, we can now use the // generator state obfuscator to encode the current state number within // both commitment transactions. stateObfuscator := DeriveStateHintObfuscator( pendingReservation.theirContribution.PaymentBasePoint.PubKey, pendingReservation.ourContribution.PaymentBasePoint.PubKey, ) err = initStateHints(ourCommitTx, theirCommitTx, stateObfuscator) if err != nil { req.err <- err req.completeChan <- nil return } // Sort both transactions according to the agreed upon canonical // ordering. This ensures that both parties sign the same sighash // without further synchronization. txsort.InPlaceSort(ourCommitTx) txsort.InPlaceSort(theirCommitTx) chanState.LocalCommitment.CommitTx = ourCommitTx chanState.RemoteCommitment.CommitTx = theirCommitTx walletLog.Debugf("Local commit tx for ChannelPoint(%v): %v", req.fundingOutpoint, spew.Sdump(ourCommitTx)) walletLog.Debugf("Remote commit tx for ChannelPoint(%v): %v", req.fundingOutpoint, spew.Sdump(theirCommitTx)) channelValue := int64(pendingReservation.partialState.Capacity) hashCache := txscript.NewTxSigHashes(ourCommitTx) theirKey := pendingReservation.theirContribution.MultiSigKey ourKey := pendingReservation.ourContribution.MultiSigKey witnessScript, _, err := input.GenFundingPkScript( ourKey.PubKey.SerializeCompressed(), theirKey.PubKey.SerializeCompressed(), channelValue, ) if err != nil { req.err <- err req.completeChan <- nil return } sigHash, err := txscript.CalcWitnessSigHash(witnessScript, hashCache, txscript.SigHashAll, ourCommitTx, 0, channelValue) if err != nil { req.err <- err req.completeChan <- nil return } // Verify that we've received a valid signature from the remote party // for our version of the commitment transaction. sig, err := btcec.ParseSignature(req.theirCommitmentSig, btcec.S256()) if err != nil { req.err <- err req.completeChan <- nil return } else if !sig.Verify(sigHash, theirKey.PubKey) { req.err <- fmt.Errorf("counterparty's commitment signature " + "is invalid") req.completeChan <- nil return } chanState.LocalCommitment.CommitSig = req.theirCommitmentSig // With their signature for our version of the commitment transactions // verified, we can now generate a signature for their version, // allowing the funding transaction to be safely broadcast. p2wsh, err := input.WitnessScriptHash(witnessScript) if err != nil { req.err <- err req.completeChan <- nil return } signDesc := input.SignDescriptor{ WitnessScript: witnessScript, KeyDesc: ourKey, Output: &wire.TxOut{ PkScript: p2wsh, Value: channelValue, }, HashType: txscript.SigHashAll, SigHashes: txscript.NewTxSigHashes(theirCommitTx), InputIndex: 0, } sigTheirCommit, err := l.Cfg.Signer.SignOutputRaw(theirCommitTx, &signDesc) if err != nil { req.err <- err req.completeChan <- nil return } pendingReservation.ourCommitmentSig = sigTheirCommit _, bestHeight, err := l.Cfg.ChainIO.GetBestBlock() if err != nil { req.err <- err req.completeChan <- nil return } // Add the complete funding transaction to the DB, in it's open bucket // which will be used for the lifetime of this channel. chanState.LocalChanCfg = pendingReservation.ourContribution.toChanConfig() chanState.RemoteChanCfg = pendingReservation.theirContribution.toChanConfig() err = chanState.SyncPending(pendingReservation.nodeAddr, uint32(bestHeight)) if err != nil { req.err <- err req.completeChan <- nil return } req.completeChan <- chanState req.err <- nil l.limboMtx.Lock() delete(l.fundingLimbo, req.pendingFundingID) l.limboMtx.Unlock() } // WithCoinSelectLock will execute the passed function closure in a // synchronized manner preventing any coin selection operations from proceeding // while the closure if executing. This can be seen as the ability to execute a // function closure under an exclusive coin selection lock. func (l *LightningWallet) WithCoinSelectLock(f func() error) error { l.coinSelectMtx.Lock() defer l.coinSelectMtx.Unlock() return f() } // coinSelection holds the result from selectCoinsAndChange. type coinSelection struct { coins []*wire.TxIn change []*wire.TxOut fundingAmt btcutil.Amount unlockCoins func() } // selectCoinsAndChange performs coin selection in order to obtain witness // outputs which sum to at least 'amt' amount of satoshis. If necessary, // a change address will also be generated. If coin selection is // successful/possible, then the selected coins and change outputs are // returned, and the value of the resulting funding output. This method locks // the selected outputs, and a function closure to unlock them in case of an // error is returned. func (l *LightningWallet) selectCoinsAndChange(feeRate SatPerKWeight, amt btcutil.Amount, minConfs int32, subtractFees bool) ( *coinSelection, error) { // We hold the coin select mutex while querying for outputs, and // performing coin selection in order to avoid inadvertent double // spends across funding transactions. l.coinSelectMtx.Lock() defer l.coinSelectMtx.Unlock() walletLog.Infof("Performing funding tx coin selection using %v "+ "sat/kw as fee rate", int64(feeRate)) // Find all unlocked unspent witness outputs that satisfy the minimum // number of confirmations required. coins, err := l.ListUnspentWitness(minConfs, math.MaxInt32) if err != nil { return nil, err } var ( selectedCoins []*Utxo fundingAmt btcutil.Amount changeAmt btcutil.Amount ) // Perform coin selection over our available, unlocked unspent outputs // in order to find enough coins to meet the funding amount // requirements. switch { // In case this request want the fees subtracted from the local amount, // we'll call the specialized method for that. This ensures that we // won't deduct more that the specified balance from our wallet. case subtractFees: dustLimit := l.Cfg.DefaultConstraints.DustLimit selectedCoins, fundingAmt, changeAmt, err = coinSelectSubtractFees( feeRate, amt, dustLimit, coins, ) if err != nil { return nil, err } // Ótherwise do a normal coin selection where we target a given funding // amount. default: fundingAmt = amt selectedCoins, changeAmt, err = coinSelect(feeRate, amt, coins) if err != nil { return nil, err } } // Record any change output(s) generated as a result of the coin // selection, but only if the addition of the output won't lead to the // creation of dust. var changeOutputs []*wire.TxOut if changeAmt != 0 && changeAmt > DefaultDustLimit() { changeAddr, err := l.NewAddress(WitnessPubKey, true) if err != nil { return nil, err } changeScript, err := txscript.PayToAddrScript(changeAddr) if err != nil { return nil, err } changeOutputs = make([]*wire.TxOut, 1) changeOutputs[0] = &wire.TxOut{ Value: int64(changeAmt), PkScript: changeScript, } } // Lock the selected coins. These coins are now "reserved", this // prevents concurrent funding requests from referring to and this // double-spending the same set of coins. inputs := make([]*wire.TxIn, len(selectedCoins)) for i, coin := range selectedCoins { outpoint := &coin.OutPoint l.lockedOutPoints[*outpoint] = struct{}{} l.LockOutpoint(*outpoint) // Empty sig script, we'll actually sign if this reservation is // queued up to be completed (the other side accepts). inputs[i] = wire.NewTxIn(outpoint, nil, nil) } unlock := func() { l.coinSelectMtx.Lock() defer l.coinSelectMtx.Unlock() for _, coin := range selectedCoins { outpoint := &coin.OutPoint delete(l.lockedOutPoints, *outpoint) l.UnlockOutpoint(*outpoint) } } return &coinSelection{ coins: inputs, change: changeOutputs, fundingAmt: fundingAmt, unlockCoins: unlock, }, nil } // DeriveStateHintObfuscator derives the bytes to be used for obfuscating the // state hints from the root to be used for a new channel. The obfuscator is // generated via the following computation: // // * sha256(initiatorKey || responderKey)[26:] // * where both keys are the multi-sig keys of the respective parties // // The first 6 bytes of the resulting hash are used as the state hint. func DeriveStateHintObfuscator(key1, key2 *btcec.PublicKey) [StateHintSize]byte { h := sha256.New() h.Write(key1.SerializeCompressed()) h.Write(key2.SerializeCompressed()) sha := h.Sum(nil) var obfuscator [StateHintSize]byte copy(obfuscator[:], sha[26:]) return obfuscator } // initStateHints properly sets the obfuscated state hints on both commitment // transactions using the passed obfuscator. func initStateHints(commit1, commit2 *wire.MsgTx, obfuscator [StateHintSize]byte) error { if err := SetStateNumHint(commit1, 0, obfuscator); err != nil { return err } if err := SetStateNumHint(commit2, 0, obfuscator); err != nil { return err } return nil } // selectInputs selects a slice of inputs necessary to meet the specified // selection amount. If input selection is unable to succeed due to insufficient // funds, a non-nil error is returned. Additionally, the total amount of the // selected coins are returned in order for the caller to properly handle // change+fees. func selectInputs(amt btcutil.Amount, coins []*Utxo) (btcutil.Amount, []*Utxo, error) { satSelected := btcutil.Amount(0) for i, coin := range coins { satSelected += coin.Value if satSelected >= amt { return satSelected, coins[:i+1], nil } } return 0, nil, &ErrInsufficientFunds{amt, satSelected} } // coinSelect attempts to select a sufficient amount of coins, including a // change output to fund amt satoshis, adhering to the specified fee rate. The // specified fee rate should be expressed in sat/kw for coin selection to // function properly. func coinSelect(feeRate SatPerKWeight, amt btcutil.Amount, coins []*Utxo) ([]*Utxo, btcutil.Amount, error) { amtNeeded := amt for { // First perform an initial round of coin selection to estimate // the required fee. totalSat, selectedUtxos, err := selectInputs(amtNeeded, coins) if err != nil { return nil, 0, err } var weightEstimate input.TxWeightEstimator for _, utxo := range selectedUtxos { switch utxo.AddressType { case WitnessPubKey: weightEstimate.AddP2WKHInput() case NestedWitnessPubKey: weightEstimate.AddNestedP2WKHInput() default: return nil, 0, fmt.Errorf("unsupported address type: %v", utxo.AddressType) } } // Channel funding multisig output is P2WSH. weightEstimate.AddP2WSHOutput() // Assume that change output is a P2WKH output. // // TODO: Handle wallets that generate non-witness change // addresses. // TODO(halseth): make coinSelect not estimate change output // for dust change. weightEstimate.AddP2WKHOutput() // The difference between the selected amount and the amount // requested will be used to pay fees, and generate a change // output with the remaining. overShootAmt := totalSat - amt // Based on the estimated size and fee rate, if the excess // amount isn't enough to pay fees, then increase the requested // coin amount by the estimate required fee, performing another // round of coin selection. totalWeight := int64(weightEstimate.Weight()) requiredFee := feeRate.FeeForWeight(totalWeight) if overShootAmt < requiredFee { amtNeeded = amt + requiredFee continue } // If the fee is sufficient, then calculate the size of the // change output. changeAmt := overShootAmt - requiredFee return selectedUtxos, changeAmt, nil } } // coinSelectSubtractFees attempts to select coins such that we'll spend up to // amt in total after fees, adhering to the specified fee rate. The selected // coins, the final output and change values are returned. func coinSelectSubtractFees(feeRate SatPerKWeight, amt, dustLimit btcutil.Amount, coins []*Utxo) ([]*Utxo, btcutil.Amount, btcutil.Amount, error) { // First perform an initial round of coin selection to estimate // the required fee. totalSat, selectedUtxos, err := selectInputs(amt, coins) if err != nil { return nil, 0, 0, err } var weightEstimate input.TxWeightEstimator for _, utxo := range selectedUtxos { switch utxo.AddressType { case WitnessPubKey: weightEstimate.AddP2WKHInput() case NestedWitnessPubKey: weightEstimate.AddNestedP2WKHInput() default: return nil, 0, 0, fmt.Errorf("unsupported "+ "address type: %v", utxo.AddressType) } } // Channel funding multisig output is P2WSH. weightEstimate.AddP2WSHOutput() // At this point we've got two possibilities, either create a // change output, or not. We'll first try without creating a // change output. // // Estimate the fee required for a transaction without a change // output. totalWeight := int64(weightEstimate.Weight()) requiredFee := feeRate.FeeForWeight(totalWeight) // For a transaction without a change output, we'll let everything go // to our multi-sig output after subtracting fees. outputAmt := totalSat - requiredFee changeAmt := btcutil.Amount(0) // If the the output is too small after subtracting the fee, the coin // selection cannot be performed with an amount this small. if outputAmt <= dustLimit { return nil, 0, 0, fmt.Errorf("output amount(%v) after "+ "subtracting fees(%v) below dust limit(%v)", outputAmt, requiredFee, dustLimit) } // We were able to create a transaction with no change from the // selected inputs. We'll remember the resulting values for // now, while we try to add a change output. Assume that change output // is a P2WKH output. weightEstimate.AddP2WKHOutput() // Now that we have added the change output, redo the fee // estimate. totalWeight = int64(weightEstimate.Weight()) requiredFee = feeRate.FeeForWeight(totalWeight) // For a transaction with a change output, everything we don't spend // will go to change. newChange := totalSat - amt newOutput := amt - requiredFee // If adding a change output leads to both outputs being above // the dust limit, we'll add the change output. Otherwise we'll // go with the no change tx we originally found. if newChange > dustLimit && newOutput > dustLimit { outputAmt = newOutput changeAmt = newChange } // Sanity check the resulting output values to make sure we // don't burn a great part to fees. totalOut := outputAmt + changeAmt fee := totalSat - totalOut // Fail if more than 20% goes to fees. // TODO(halseth): smarter fee limit. Make configurable or dynamic wrt // total funding size? if fee > totalOut/5 { return nil, 0, 0, fmt.Errorf("fee %v on total output"+ "value %v", fee, totalOut) } return selectedUtxos, outputAmt, changeAmt, nil }