lnd.xprv/lnwallet/wallet.go
2016-12-13 11:01:57 -08:00

1472 lines
51 KiB
Go

package lnwallet
import (
"fmt"
"net"
"sync"
"sync/atomic"
"github.com/btcsuite/fastsha256"
"github.com/davecgh/go-spew/spew"
"github.com/lightningnetwork/lnd/chainntnfs"
"github.com/lightningnetwork/lnd/channeldb"
"github.com/lightningnetwork/lnd/elkrem"
"github.com/roasbeef/btcd/chaincfg"
"github.com/roasbeef/btcutil/hdkeychain"
"github.com/roasbeef/btcd/btcec"
"github.com/roasbeef/btcd/txscript"
"github.com/roasbeef/btcd/wire"
"github.com/roasbeef/btcutil"
"github.com/roasbeef/btcutil/txsort"
)
const (
// The size of the buffered queue of requests to the wallet from the
// outside word.
msgBufferSize = 100
// elkremRootIndex is the top level HD key index from which secrets
// used to generate elkrem roots should be derived from.
elkremRootIndex = hdkeychain.HardenedKeyStart + 1
// identityKeyIndex is the top level HD key index which is used to
// generate/rotate identity keys.
//
// TODO(roasbeef): should instead be child to make room for future
// rotations, etc.
identityKeyIndex = hdkeychain.HardenedKeyStart + 2
commitFee = 5000
)
var (
// Namespace bucket keys.
lightningNamespaceKey = []byte("ln-wallet")
waddrmgrNamespaceKey = []byte("waddrmgr")
wtxmgrNamespaceKey = []byte("wtxmgr")
)
// 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 outputs to create funding transaction,"+
" need %v only have %v available", e.amountAvailable,
e.amountSelected)
}
// initFundingReserveReq 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
// after a timeout period in order to avoid "exhaustion" attacks.
//
// TODO(roasbeef): zombie reservation sweeper goroutine.
type initFundingReserveMsg struct {
// The ID of the remote node we would like to open a channel with.
nodeID *btcec.PublicKey
// The IP address plus port that we used to either establish or accept
// the connection which led to the negotiation of this funding
// workflow.
nodeAddr *net.TCPAddr
// The number of confirmations required before the channel is considered
// open.
numConfs uint16
// The amount of funds requested for this channel.
fundingAmount btcutil.Amount
// The total capacity of the channel which includes the amount of funds
// the remote party contributes (if any).
capacity btcutil.Amount
// The minimum accepted satoshis/KB fee for the funding transaction. In
// order to ensure timely confirmation, it is recomened that this fee
// should be generous, paying some multiple of the accepted base fee
// rate of the network.
// TODO(roasbeef): integrate fee estimation project...
minFeeRate btcutil.Amount
// ourDustLimit is the threshold below which no HTLC output should be
// generated for our commitment transaction; ie. HTLCs below
// this amount are not enforceable onchain from our point of view.
ourDustLimit btcutil.Amount
// The delay on the "pay-to-self" output(s) of the commitment transaction.
csvDelay uint32
// A channel in which all errors will be sent accross. Will be nil if
// this initial set is succesful.
// NOTE: In order to avoid deadlocks, this channel MUST be buffered.
err chan error
// A ChannelReservation with our contributions filled in will be sent
// accross this channel in the case of a succesfully 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 []*InputScript
// This should be 1/2 of the signatures needed to succesfully spend our
// version of the commitment transaction.
theirCommitmentSig []byte
// 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
// revokeKey is the revocation public key derived by the remote node to
// be used within the initial version of the commitment transaction we
// construct for them.
revokeKey *btcec.PublicKey
// theirCommitmentSig are the 1/2 of the signatures needed to
// succesfully spend our version of the commitment transaction.
theirCommitmentSig []byte
// obsfucator is the bytes to be used to obsfucate the state hints on
// the commitment transaction.
obsfucator [StateHintSize]byte
// NOTE: In order to avoid deadlocks, this channel MUST be buffered.
err chan error
}
// channelOpenMsg is the final message sent to finalize a single funder channel
// workflow to which we are the responder to. This message is sent once the
// remote peer deems the channel open, meaning it has reached a sufficient
// number of confirmations in the blockchain.
type channelOpenMsg struct {
pendingFundingID uint64
// TODO(roasbeef): move verification up to upper layer, yeh?
spvProof []byte
// 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 embedds 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 independant 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 dependant
// on btcd's websockets notifications as even 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 inot btcwallet.
type LightningWallet struct {
// 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
// A wrapper around a namespace within boltdb reserved for ln-based
// wallet meta-data. See the 'channeldb' package for further
// information.
ChannelDB *channeldb.DB
// Used by in order to obtain notifications about funding transaction
// reaching a specified confirmation depth, and to catch
// counterparty's broadcasting revoked commitment states.
chainNotifier chainntnfs.ChainNotifier
// wallet is the the core wallet, all non Lightning Network specific
// interaction is proxied to the internal wallet.
WalletController
// Signer is the wallet's current Signer implementation. This Signer is
// used to generate signature for all inputs to potential funding
// transactions, as well as for spends from the funding transaction to
// update the commitment state.
Signer Signer
// ChainIO is an instance of the BlockChainIO interface. ChainIO is
// used to lookup the existence of outputs within the UTXO set.
ChainIO BlockChainIO
// rootKey is the root HD key derived from a WalletController private
// key. This rootKey is used to derive all LN specific secrets.
rootKey *hdkeychain.ExtendedKey
// 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
nextFundingID uint64
limboMtx sync.RWMutex
// TODO(roasbeef): zombie garbage collection routine to solve
// lost-object/starvation problem/attack.
// 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{}
netParams *chaincfg.Params
started int32
shutdown int32
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.
//
// NOTE: The passed channeldb, and ChainNotifier should already be fully
// initialized/started before being passed as a function arugment.
func NewLightningWallet(cdb *channeldb.DB, notifier chainntnfs.ChainNotifier,
wallet WalletController, signer Signer, bio BlockChainIO,
netParams *chaincfg.Params) (*LightningWallet, error) {
// TODO(roasbeef): need a another wallet level config
// Fetch the root derivation key from the wallet's HD chain. We'll use
// this to generate specific Lightning related secrets on the fly.
rootKey, err := wallet.FetchRootKey()
if err != nil {
return nil, err
}
// TODO(roasbeef): always re-derive on the fly?
rootKeyRaw := rootKey.Serialize()
rootMasterKey, err := hdkeychain.NewMaster(rootKeyRaw, netParams)
if err != nil {
return nil, err
}
return &LightningWallet{
rootKey: rootMasterKey,
chainNotifier: notifier,
Signer: signer,
WalletController: wallet,
ChainIO: bio,
ChannelDB: cdb,
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
}
// LockOutpoints 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 trakcs 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-cancalled) reservations.
func (l *LightningWallet) ActiveReservations() []*ChannelReservation {
reservations := make([]*ChannelReservation, 0, len(l.fundingLimbo))
for _, reservation := range l.fundingLimbo {
reservations = append(reservations, reservation)
}
return reservations
}
// GetIdentitykey returns the identity private key of the wallet.
// TODO(roasbeef): should be moved elsewhere
func (l *LightningWallet) GetIdentitykey() (*btcec.PrivateKey, error) {
identityKey, err := l.rootKey.Child(identityKeyIndex)
if err != nil {
return nil, err
}
return identityKey.ECPrivKey()
}
// requestHandler is the primary goroutine(s) responsible for handling, and
// dispatching relies 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 *channelOpenMsg:
l.handleChannelOpen(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
// counter party to build the funding transaction, and both versions of the
// commitment transaction. Otherwise, an error occurred 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 records for our version of the
// commitment transaction is valid.
func (l *LightningWallet) InitChannelReservation(capacity,
ourFundAmt btcutil.Amount, theirID *btcec.PublicKey,
theirAddr *net.TCPAddr, numConfs uint16,
csvDelay uint32, ourDustLimit btcutil.Amount) (*ChannelReservation, error) {
errChan := make(chan error, 1)
respChan := make(chan *ChannelReservation, 1)
l.msgChan <- &initFundingReserveMsg{
capacity: capacity,
numConfs: numConfs,
fundingAmount: ourFundAmt,
csvDelay: csvDelay,
ourDustLimit: ourDustLimit,
nodeID: theirID,
err: errChan,
resp: respChan,
}
return <-respChan, <-errChan
}
// 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.fundingAmount+req.capacity == 0 {
req.err <- fmt.Errorf("cannot have channel with zero " +
"satoshis funded")
req.resp <- nil
return
}
id := atomic.AddUint64(&l.nextFundingID, 1)
totalCapacity := req.capacity + commitFee
reservation := NewChannelReservation(totalCapacity, req.fundingAmount,
req.minFeeRate, l, id, req.numConfs)
// Grab the mutex on the ChannelReservation to ensure thread-safety
reservation.Lock()
defer reservation.Unlock()
reservation.nodeAddr = req.nodeAddr
reservation.ourContribution.CsvDelay = req.csvDelay
reservation.partialState.IdentityPub = req.nodeID
reservation.partialState.LocalCsvDelay = req.csvDelay
reservation.partialState.OurDustLimit = req.ourDustLimit
ourContribution := reservation.ourContribution
// If we're on the receiving end of a single funder channel then we
// don't need to perform any coin selection. Otherwise, attempt to
// obtain enough coins to meet the required funding amount.
if req.fundingAmount != 0 {
// TODO(roasbeef): consult model for proper fee rate on funding
// tx
feeRate := uint64(10)
amt := req.fundingAmount + commitFee
err := l.selectCoinsAndChange(feeRate, amt, ourContribution)
if err != nil {
req.err <- err
req.resp <- nil
return
}
}
// Grab two fresh keys from our HD chain, one will be used for the
// multi-sig funding transaction, and the other for the commitment
// transaction.
multiSigKey, err := l.NewRawKey()
if err != nil {
req.err <- err
req.resp <- nil
return
}
commitKey, err := l.NewRawKey()
if err != nil {
req.err <- err
req.resp <- nil
return
}
reservation.partialState.OurMultiSigKey = multiSigKey
ourContribution.MultiSigKey = multiSigKey
reservation.partialState.OurCommitKey = commitKey
ourContribution.CommitKey = commitKey
// Generate a fresh address to be used in the case of a cooperative
// channel close.
deliveryAddress, err := l.NewAddress(WitnessPubKey, false)
if err != nil {
req.err <- err
req.resp <- nil
return
}
deliveryScript, err := txscript.PayToAddrScript(deliveryAddress)
if err != nil {
req.err <- err
req.resp <- nil
return
}
reservation.partialState.OurDeliveryScript = deliveryScript
ourContribution.DeliveryAddress = deliveryAddress
// 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 canceled.
req.resp <- reservation
req.err <- 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, "unkown funding state" or something
req.err <- fmt.Errorf("attempted to cancel non-existant funding state")
return
}
// Grab the mutex on the ChannelReservation to ensure thead-safety
pendingReservation.Lock()
defer pendingReservation.Unlock()
// Mark all previously locked outpoints as usuable 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 unsed keys?
// TODO(roasbeef): Is it possible to mark the unused change also as
// available?
delete(l.fundingLimbo, req.pendingFundingID)
req.err <- 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-existant funding state")
return
}
// Grab the mutex on the ChannelReservation to ensure thead-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()
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.partialState.OurMultiSigKey
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 := GenFundingPkScript(ourKey.SerializeCompressed(),
theirKey.SerializeCompressed(), channelCapacity)
if err != nil {
req.err <- err
return
}
pendingReservation.partialState.FundingWitnessScript = witnessScript
// 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([]*InputScript, 0, len(ourContribution.Inputs))
signDesc := 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.Signer.ComputeInputScript(fundingTx, &signDesc)
if err != nil {
req.err <- err
return
}
txIn.SignatureScript = inputScript.ScriptSig
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.TxSha()
_, multiSigIndex := FindScriptOutputIndex(fundingTx, multiSigOut.PkScript)
fundingOutpoint := wire.NewOutPoint(&fundingTxID, multiSigIndex)
pendingReservation.partialState.FundingOutpoint = fundingOutpoint
// Initialize an empty sha-chain for them, tracking the current pending
// revocation hash (we don't yet know the pre-image so we can't add it
// to the chain).
e := &elkrem.ElkremReceiver{}
pendingReservation.partialState.RemoteElkrem = e
pendingReservation.partialState.TheirCurrentRevocation = theirContribution.RevocationKey
masterElkremRoot, err := l.deriveMasterElkremRoot()
if err != nil {
req.err <- err
return
}
// Now that we have their commitment key, we can create the revocation
// key for the first version of our commitment transaction. To do so,
// we'll first create our elkrem root, then grab the first pre-iamge
// from it.
elkremRoot := deriveElkremRoot(masterElkremRoot, ourKey, theirKey)
elkremSender := elkrem.NewElkremSender(elkremRoot)
pendingReservation.partialState.LocalElkrem = elkremSender
firstPreimage, err := elkremSender.AtIndex(0)
if err != nil {
req.err <- err
return
}
theirCommitKey := theirContribution.CommitKey
ourRevokeKey := DeriveRevocationPubkey(theirCommitKey, firstPreimage[:])
// Create the txIn to our commitment transaction; required to construct
// the commitment transactions.
fundingTxIn := wire.NewTxIn(wire.NewOutPoint(&fundingTxID, multiSigIndex), nil, nil)
// With the funding tx complete, create both commitment transactions.
// TODO(roasbeef): much cleanup + de-duplication
ourBalance := ourContribution.FundingAmount
theirBalance := theirContribution.FundingAmount
ourCommitKey := ourContribution.CommitKey
ourCommitTx, err := CreateCommitTx(fundingTxIn, ourCommitKey, theirCommitKey,
ourRevokeKey, ourContribution.CsvDelay,
ourBalance, theirBalance)
if err != nil {
req.err <- err
return
}
theirCommitTx, err := CreateCommitTx(fundingTxIn, theirCommitKey, ourCommitKey,
theirContribution.RevocationKey, theirContribution.CsvDelay,
theirBalance, ourBalance)
if err != nil {
req.err <- err
return
}
// With both commitment transactions constructed, generate the state
// obsfucator then use it to encode the current state number withi both
// commitment transactions.
// TODO(roasbeef): define obsfucator scheme for dual funder
var stateObsfucator [StateHintSize]byte
if pendingReservation.partialState.IsInitiator {
stateObsfucator, err = deriveStateHintObsfucator(elkremSender)
if err != nil {
req.err <- err
return
}
}
err = initStateHints(ourCommitTx, theirCommitTx, stateObsfucator)
if err != nil {
req.err <- err
return
}
// Sort both transactions according to the agreed upon cannonical
// ordering. This lets us skip sending the entire transaction over,
// instead we'll just send signatures.
txsort.InPlaceSort(ourCommitTx)
txsort.InPlaceSort(theirCommitTx)
deliveryScript, err := txscript.PayToAddrScript(theirContribution.DeliveryAddress)
if err != nil {
req.err <- err
return
}
// Record newly available information witin the open channel state.
pendingReservation.partialState.RemoteCsvDelay = theirContribution.CsvDelay
pendingReservation.partialState.TheirDeliveryScript = deliveryScript
pendingReservation.partialState.ChanID = fundingOutpoint
pendingReservation.partialState.TheirCommitKey = theirCommitKey
pendingReservation.partialState.TheirMultiSigKey = theirContribution.MultiSigKey
pendingReservation.partialState.OurCommitTx = ourCommitTx
pendingReservation.partialState.StateHintObsfucator = stateObsfucator
pendingReservation.ourContribution.RevocationKey = ourRevokeKey
// Generate a signature for their version of the initial commitment
// transaction.
signDesc = SignDescriptor{
WitnessScript: witnessScript,
PubKey: ourKey,
Output: multiSigOut,
HashType: txscript.SigHashAll,
SigHashes: txscript.NewTxSigHashes(theirCommitTx),
InputIndex: 0,
}
sigTheirCommit, err := l.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-existant funding state")
return
}
// Grab the mutex on the ChannelReservation to ensure thead-safety
pendingReservation.Lock()
defer pendingReservation.Unlock()
// 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
// Additionally, we can now also record the redeem script of the
// funding transaction.
// TODO(roasbeef): switch to proper pubkey derivation
ourKey := pendingReservation.partialState.OurMultiSigKey
theirKey := theirContribution.MultiSigKey
channelCapacity := int64(pendingReservation.partialState.Capacity)
witnessScript, _, err := GenFundingPkScript(ourKey.SerializeCompressed(),
theirKey.SerializeCompressed(), channelCapacity)
if err != nil {
req.err <- err
return
}
pendingReservation.partialState.FundingWitnessScript = witnessScript
masterElkremRoot, err := l.deriveMasterElkremRoot()
if err != nil {
req.err <- err
return
}
// Now that we know their commitment key, we can create the revocation
// key for our version of the initial commitment transaction.
elkremRoot := deriveElkremRoot(masterElkremRoot, ourKey, theirKey)
elkremSender := elkrem.NewElkremSender(elkremRoot)
firstPreimage, err := elkremSender.AtIndex(0)
if err != nil {
req.err <- err
return
}
pendingReservation.partialState.LocalElkrem = elkremSender
theirCommitKey := theirContribution.CommitKey
ourRevokeKey := DeriveRevocationPubkey(theirCommitKey, firstPreimage[:])
// Initialize an empty sha-chain for them, tracking the current pending
// revocation hash (we don't yet know the pre-image so we can't add it
// to the chain).
remoteElkrem := &elkrem.ElkremReceiver{}
pendingReservation.partialState.RemoteElkrem = remoteElkrem
// Record the counterpaty's remaining contributions to the channel,
// converting their delivery address into a public key script.
deliveryScript, err := txscript.PayToAddrScript(theirContribution.DeliveryAddress)
if err != nil {
req.err <- err
return
}
pendingReservation.partialState.RemoteCsvDelay = theirContribution.CsvDelay
pendingReservation.partialState.TheirDeliveryScript = deliveryScript
pendingReservation.partialState.TheirCommitKey = theirContribution.CommitKey
pendingReservation.partialState.TheirMultiSigKey = theirContribution.MultiSigKey
pendingReservation.ourContribution.RevocationKey = ourRevokeKey
req.err <- nil
return
}
// 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-existant funding state")
return
}
// Grab the mutex on the ChannelReservation to ensure thead-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].ScriptSig
// Fetch the alleged previous output along with the
// pkscript referenced by this input.
prevOut := txin.PreviousOutPoint
output, err := l.ChainIO.GetUtxo(&prevOut.Hash, prevOut.Index)
if output == nil {
msg.err <- fmt.Errorf("input to funding tx does not exist: %v", err)
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 {
// TODO(roasbeef): cancel at this stage if invalid sigs?
msg.err <- fmt.Errorf("cannot create script engine: %s", err)
return
}
if err = vm.Execute(); err != nil {
msg.err <- fmt.Errorf("cannot validate transaction: %s", err)
return
}
sigIndex++
}
}
// At this point, we can also record and verify their signature for our
// commitment transaction.
res.theirCommitmentSig = msg.theirCommitmentSig
commitTx := res.partialState.OurCommitTx
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 := res.partialState.FundingWitnessScript
// Next, create the spending scriptSig, and then verify that the script
// is complete, allowing us to spend from the funding transaction.
theirCommitSig := msg.theirCommitmentSig
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 <- fmt.Errorf("counterparty's commitment signature is invalid: %v", err)
return
}
// Verify that we've received a valid signature from the remote party
// for our version of the commitment transaction.
sig, err := btcec.ParseSignature(theirCommitSig, btcec.S256())
if err != nil {
msg.err <- err
return
} else if !sig.Verify(sigHash, theirKey) {
msg.err <- fmt.Errorf("counterparty's commitment signature is invalid")
return
}
res.partialState.OurCommitSig = theirCommitSig
// Funding complete, this entry can be removed from limbo.
l.limboMtx.Lock()
delete(l.fundingLimbo, res.reservationID)
l.limboMtx.Unlock()
walletLog.Infof("Broadcasting funding tx for ChannelPoint(%v): %v",
res.partialState.FundingOutpoint, spew.Sdump(fundingTx))
// Broacast the finalized funding transaction to the network.
if err := l.PublishTransaction(fundingTx); err != nil {
msg.err <- err
return
}
// Add the complete funding transaction to the DB, in it's open bucket
// which will be used for the lifetime of this channel.
// TODO(roasbeef): revisit faul-tolerance of this flow
nodeAddr := res.nodeAddr
if err := res.partialState.FullSyncWithAddr(nodeAddr); err != nil {
msg.err <- err
return
}
// Create a goroutine to watch the chain so we can open the channel once
// the funding tx has enough confirmations.
go l.openChannelAfterConfirmations(res)
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-existant funding state")
return
}
// Grab the mutex on the ChannelReservation to ensure thead-safety
pendingReservation.Lock()
defer pendingReservation.Unlock()
pendingReservation.partialState.FundingOutpoint = req.fundingOutpoint
pendingReservation.partialState.TheirCurrentRevocation = req.revokeKey
pendingReservation.partialState.ChanID = req.fundingOutpoint
pendingReservation.partialState.StateHintObsfucator = req.obsfucator
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.
ourCommitKey := pendingReservation.ourContribution.CommitKey
theirCommitKey := pendingReservation.theirContribution.CommitKey
ourBalance := pendingReservation.ourContribution.FundingAmount
theirBalance := pendingReservation.theirContribution.FundingAmount
ourCommitTx, err := CreateCommitTx(fundingTxIn, ourCommitKey, theirCommitKey,
pendingReservation.ourContribution.RevocationKey,
pendingReservation.ourContribution.CsvDelay, ourBalance, theirBalance)
if err != nil {
req.err <- err
return
}
theirCommitTx, err := CreateCommitTx(fundingTxIn, theirCommitKey, ourCommitKey,
req.revokeKey, pendingReservation.theirContribution.CsvDelay,
theirBalance, ourBalance)
if err != nil {
req.err <- err
return
}
// With both commitment transactions constructed, generate the state
// obsfucator then use it to encode the current state number within
// both commitment transactions.
err = initStateHints(ourCommitTx, theirCommitTx, req.obsfucator)
if err != nil {
req.err <- err
return
}
// Sort both transactions according to the agreed upon cannonical
// ordering. This ensures that both parties sign the same sighash
// without further synchronization.
txsort.InPlaceSort(ourCommitTx)
pendingReservation.partialState.OurCommitTx = ourCommitTx
txsort.InPlaceSort(theirCommitTx)
witnessScript := pendingReservation.partialState.FundingWitnessScript
channelValue := int64(pendingReservation.partialState.Capacity)
hashCache := txscript.NewTxSigHashes(ourCommitTx)
theirKey := pendingReservation.theirContribution.MultiSigKey
ourKey := pendingReservation.partialState.OurMultiSigKey
sigHash, err := txscript.CalcWitnessSigHash(witnessScript, hashCache,
txscript.SigHashAll, ourCommitTx, 0, channelValue)
if err != nil {
req.err <- err
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
return
} else if !sig.Verify(sigHash, theirKey) {
req.err <- fmt.Errorf("counterparty's commitment signature is invalid")
return
}
pendingReservation.partialState.OurCommitSig = 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 := witnessScriptHash(witnessScript)
if err != nil {
req.err <- err
return
}
signDesc := SignDescriptor{
WitnessScript: witnessScript,
PubKey: ourKey,
Output: &wire.TxOut{
PkScript: p2wsh,
Value: channelValue,
},
HashType: txscript.SigHashAll,
SigHashes: txscript.NewTxSigHashes(theirCommitTx),
InputIndex: 0,
}
sigTheirCommit, err := l.Signer.SignOutputRaw(theirCommitTx, &signDesc)
if err != nil {
req.err <- err
return
}
pendingReservation.ourCommitmentSig = sigTheirCommit
req.err <- nil
}
// handleChannelOpen completes a single funder reservation to which we are the
// responder. This method saves the channel state to disk, finally "opening"
// the channel by sending it over to the caller of the reservation via the
// channel dispatch channel.
func (l *LightningWallet) handleChannelOpen(req *channelOpenMsg) {
l.limboMtx.RLock()
res, ok := l.fundingLimbo[req.pendingFundingID]
l.limboMtx.RUnlock()
if !ok {
req.err <- fmt.Errorf("attempted to update non-existant " +
"funding state")
res.chanOpen <- nil
return
}
// Grab the mutex on the ChannelReservation to ensure thead-safety
res.Lock()
defer res.Unlock()
// Funding complete, this entry can be removed from limbo.
l.limboMtx.Lock()
delete(l.fundingLimbo, res.reservationID)
l.limboMtx.Unlock()
// Add the complete funding transaction to the DB, in it's open bucket
// which will be used for the lifetime of this channel.
if err := res.partialState.FullSyncWithAddr(res.nodeAddr); err != nil {
req.err <- err
res.chanOpen <- nil
return
}
// Finally, create and officially open the payment channel!
// TODO(roasbeef): CreationTime once tx is 'open'
channel, _ := NewLightningChannel(l.Signer, l.ChainIO, l.chainNotifier,
res.partialState)
res.chanOpen <- channel
req.err <- nil
}
// openChannelAfterConfirmations creates, and opens a payment channel after
// the funding transaction created within the passed channel reservation
// obtains the specified number of confirmations.
func (l *LightningWallet) openChannelAfterConfirmations(res *ChannelReservation) {
// Register with the ChainNotifier for a notification once the funding
// transaction reaches `numConfs` confirmations.
txid := res.fundingTx.TxSha()
numConfs := uint32(res.numConfsToOpen)
confNtfn, _ := l.chainNotifier.RegisterConfirmationsNtfn(&txid, numConfs)
walletLog.Infof("Waiting for funding tx (txid: %v) to reach %v confirmations",
txid, numConfs)
// Wait until the specified number of confirmations has been reached,
// or the wallet signals a shutdown.
out:
select {
case _, ok := <-confNtfn.Confirmed:
// Reading a falsey value for the second parameter indicates that
// the notifier is in the process of shutting down. Therefore, we
// don't count this as the signal that the funding transaction has
// been confirmed.
if !ok {
res.chanOpen <- nil
return
}
break out
case <-l.quit:
res.chanOpen <- nil
return
}
// Finally, create and officially open the payment channel!
// TODO(roasbeef): CreationTime once tx is 'open'
channel, _ := NewLightningChannel(l.Signer, l.ChainIO, l.chainNotifier,
res.partialState)
res.chanOpen <- channel
}
// selectCoinsAndChange performs coin selection in order to obtain witness
// outputs which sum to at least 'numCoins' amount of satoshis. If coin
// selection is successful/possible, then the selected coins are available
// within the passed contribution's inputs. If necessary, a change address will
// also be generated.
// TODO(roasbeef): remove hardcoded fees and req'd confs for outputs.
func (l *LightningWallet) selectCoinsAndChange(feeRate uint64, amt btcutil.Amount,
contribution *ChannelContribution) 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()
// Find all unlocked unspent witness outputs with greater than 1
// confirmation.
// TODO(roasbeef): make num confs a configuration paramter
coins, err := l.ListUnspentWitness(1)
if err != nil {
return err
}
// Perform coin selection over our available, unlocked unspent outputs
// in order to find enough coins to meet the funding amount
// requirements.
selectedCoins, changeAmt, err := coinSelect(feeRate, amt, coins)
if err != nil {
return err
}
// 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.
contribution.Inputs = make([]*wire.TxIn, len(selectedCoins))
for i, coin := range selectedCoins {
l.lockedOutPoints[*coin] = struct{}{}
l.LockOutpoint(*coin)
// Empty sig script, we'll actually sign if this reservation is
// queued up to be completed (the other side accepts).
contribution.Inputs[i] = wire.NewTxIn(coin, nil, nil)
}
// Record any change output(s) generated as a result of the coin
// selection.
if changeAmt != 0 {
changeAddr, err := l.NewAddress(WitnessPubKey, true)
if err != nil {
return err
}
changeScript, err := txscript.PayToAddrScript(changeAddr)
if err != nil {
return err
}
contribution.ChangeOutputs = make([]*wire.TxOut, 1)
contribution.ChangeOutputs[0] = &wire.TxOut{
Value: int64(changeAmt),
PkScript: changeScript,
}
}
return nil
}
// deriveMasterElkremRoot derives the private key which serves as the master
// elkrem root. This master secret is used as the secret input to a HKDF to
// generate elkrem secrets based on random, but public data.
func (l *LightningWallet) deriveMasterElkremRoot() (*btcec.PrivateKey, error) {
masterElkremRoot, err := l.rootKey.Child(elkremRootIndex)
if err != nil {
return nil, err
}
return masterElkremRoot.ECPrivKey()
}
// deriveStateHintObsfucator derives the bytes to be used for obsfucatating the
// state hints from the elkerem root to be used for a new channel. The
// obsfucator is generated by performing an additional sha256 hash of the first
// child derived from the elkrem root. The leading 4 bytes are used for the
// obsfucator.
func deriveStateHintObsfucator(elkremRoot *elkrem.ElkremSender) ([StateHintSize]byte, error) {
var obsfucator [StateHintSize]byte
firstChild, err := elkremRoot.AtIndex(0)
if err != nil {
return obsfucator, err
}
grandChild := fastsha256.Sum256(firstChild[:])
copy(obsfucator[:], grandChild[:])
return obsfucator, nil
}
// initStateHints properly sets the obsfucated state ints ob both commitment
// transactions using the passed obsfucator.
func initStateHints(commit1, commit2 *wire.MsgTx,
obsfucator [StateHintSize]byte) error {
if err := SetStateNumHint(commit1, 0, obsfucator); err != nil {
return err
}
if err := SetStateNumHint(commit2, 0, obsfucator); 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 suceed to to insuffcient
// 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, []*wire.OutPoint, error) {
var (
selectedUtxos []*wire.OutPoint
satSelected btcutil.Amount
)
i := 0
for satSelected < amt {
// If we're about to go past the number of available coins,
// then exit with an error.
if i > len(coins)-1 {
return 0, nil, &ErrInsufficientFunds{amt, satSelected}
}
// Otherwise, collect this new coin as it may be used for final
// coin selection.
coin := coins[i]
utxo := &wire.OutPoint{
Hash: coin.Hash,
Index: coin.Index,
}
selectedUtxos = append(selectedUtxos, utxo)
satSelected += coin.Value
i++
}
return satSelected, selectedUtxos, nil
}
// coinSelect attemps to select a sufficient amount of coins, including a
// change output to fund amt satoshis, adhearing to the specified fee rate. The
// specified fee rate should be expressed in sat/byte for coin selection to
// function properly.
func coinSelect(feeRate uint64, amt btcutil.Amount,
coins []*Utxo) ([]*wire.OutPoint, btcutil.Amount, error) {
const (
// txOverhead is the overhead of a transaction residing within
// the version number and lock time.
txOverhead = 8
// p2wkhSpendSize an estimate of the number of bytes it takes
// to spend a p2wkh output.
//
// (p2wkh witness) + txid + index + varint script size + sequence
// TODO(roasbeef): div by 3 due to witness size?
p2wkhSpendSize = (1 + 73 + 1 + 33) + 32 + 4 + 1 + 4
// p2wkhOutputSize is an estimate of the size of a regualr
// p2wkh output.
//
// 8 (output) + 1 (var int script) + 22 (p2wkh output)
p2wkhOutputSize = 8 + 1 + 22
// p2wkhOutputSize is an estimate of the p2wsh funding uotput.
p2wshOutputSize = 8 + 1 + 34
)
var estimatedSize int
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
}
// Based on the selected coins, estimate the size of the final
// fully signed transaction.
estimatedSize = ((len(selectedUtxos) * p2wkhSpendSize) +
p2wshOutputSize + txOverhead)
// The difference bteween the selected amount and the amount
// requested will be used to pay fees, and generate a change
// output with the remaining.
overShootAmt := totalSat - amtNeeded
// 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.
requiredFee := btcutil.Amount(uint64(estimatedSize) * feeRate)
if overShootAmt < requiredFee {
amtNeeded += requiredFee
continue
}
// If the fee is sufficient, then calculate the size of the change output.
changeAmt := overShootAmt - requiredFee
return selectedUtxos, changeAmt, nil
}
}