lnd.xprv/lnwallet/wallet.go

1477 lines
50 KiB
Go

package lnwallet
import (
"bytes"
"crypto/sha256"
"fmt"
"net"
"sync"
"sync/atomic"
"github.com/davecgh/go-spew/spew"
"github.com/lightningnetwork/lnd/channeldb"
"github.com/lightningnetwork/lnd/lnwire"
"github.com/roasbeef/btcd/blockchain"
"github.com/roasbeef/btcd/chaincfg/chainhash"
"github.com/roasbeef/btcutil/hdkeychain"
"github.com/lightningnetwork/lnd/shachain"
"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
// revocationRootIndex is the top level HD key index from which secrets
// used to generate producer roots should be derived from.
revocationRootIndex = 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
)
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 witness 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 {
// 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
// fundingAmount is the amount of funds requested for this channel.
fundingAmount btcutil.Amount
// capacity is the total capacity of the channel which includes the
// amount of funds the remote party contributes (if any).
capacity 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
// fundingFeePerVSize is the fee rate in sat/vbyte to use for the
// initial funding transaction.
fundingFeePerVSize SatPerVByte
// 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
// 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 []*InputScript
// 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 {
// 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
// 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
// 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{}
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.
func NewLightningWallet(Cfg Config) (*LightningWallet, error) {
return &LightningWallet{
Cfg: Cfg,
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
}
// 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 := l.FetchRootKey()
if err != nil {
return err
}
// TODO(roasbeef): always re-derive on the fly?
rootKeyRaw := rootKey.Serialize()
l.rootKey, err = hdkeychain.NewMaster(rootKeyRaw, &l.Cfg.NetParams)
if 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
}
// 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 <-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 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, pushMSat lnwire.MilliSatoshi,
commitFeePerKw SatPerKWeight, fundingFeePerVSize SatPerVByte,
theirID *btcec.PublicKey, theirAddr net.Addr,
chainHash *chainhash.Hash, flags lnwire.FundingFlag) (*ChannelReservation, error) {
errChan := make(chan error, 1)
respChan := make(chan *ChannelReservation, 1)
l.msgChan <- &initFundingReserveMsg{
chainHash: chainHash,
nodeID: theirID,
nodeAddr: theirAddr,
fundingAmount: ourFundAmt,
capacity: capacity,
commitFeePerKw: commitFeePerKw,
fundingFeePerVSize: fundingFeePerVSize,
pushMSat: pushMSat,
flags: flags,
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 {
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
}
id := atomic.AddUint64(&l.nextFundingID, 1)
reservation, err := NewChannelReservation(req.capacity, req.fundingAmount,
req.commitFeePerKw, l, id, req.pushMSat,
l.Cfg.NetParams.GenesisHash, req.flags)
if err != nil {
req.err <- err
req.resp <- nil
return
}
// Grab the mutex on the ChannelReservation to ensure thread-safety
reservation.Lock()
defer reservation.Unlock()
reservation.nodeAddr = req.nodeAddr
reservation.partialState.IdentityPub = req.nodeID
// 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 {
// Coin selection is done on the basis of sat-per-vbyte, we'll
// use the passed sat/vbyte passed in to perform coin selection.
err := l.selectCoinsAndChange(
req.fundingFeePerVSize, req.fundingAmount,
reservation.ourContribution,
)
if err != nil {
req.err <- err
req.resp <- nil
return
}
}
// 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): special derivaiton?
reservation.ourContribution.MultiSigKey, err = l.NewRawKey()
if err != nil {
req.err <- err
req.resp <- nil
return
}
reservation.ourContribution.RevocationBasePoint, err = l.NewRawKey()
if err != nil {
req.err <- err
req.resp <- nil
return
}
reservation.ourContribution.HtlcBasePoint, err = l.NewRawKey()
if err != nil {
req.err <- err
req.resp <- nil
return
}
// TODO(roasbeef); allow for querying to extract key distinct from HD
// chain
// * allows for offline commitment keys
reservation.ourContribution.PaymentBasePoint, err = l.NewRawKey()
if err != nil {
req.err <- err
req.resp <- nil
return
}
reservation.ourContribution.DelayBasePoint, err = l.NewRawKey()
if err != nil {
req.err <- err
req.resp <- nil
return
}
// With the above keys created, we'll also need to initialization our
// initial revocation tree state. In order to do so in a deterministic
// manner (for recovery purposes), we'll use the current block hash
// along with the identity public key of the node we're creating the
// channel with. In the event of a recovery, given these two items and
// the initialize wallet HD seed, we can derive all of our revocation
// secrets.
masterElkremRoot, err := l.deriveMasterRevocationRoot()
if err != nil {
req.err <- err
req.resp <- nil
return
}
bestHash, _, err := l.Cfg.ChainIO.GetBestBlock()
if err != nil {
req.err <- err
req.resp <- nil
return
}
revocationRoot := DeriveRevocationRoot(masterElkremRoot, *bestHash,
req.nodeID)
// Once we have the root, we can then generate our shachain producer
// and from that generate the per-commitment point.
producer := shachain.NewRevocationProducer(revocationRoot)
firstPreimage, err := producer.AtIndex(0)
if err != nil {
req.err <- err
req.resp <- nil
return
}
reservation.ourContribution.FirstCommitmentPoint = ComputeCommitmentPoint(
firstPreimage[:],
)
reservation.partialState.RevocationProducer = producer
reservation.ourContribution.ChannelConstraints = l.Cfg.DefaultConstraints
// TODO(roasbeef): turn above into: initContributio()
// 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
}
// 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 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(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 := GenFundingPkScript(ourKey.SerializeCompressed(),
theirKey.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([]*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.Cfg.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.TxHash()
_, multiSigIndex := 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,
theirContribution.PaymentBasePoint,
)
} else {
ourSer := ourContribution.PaymentBasePoint.SerializeCompressed()
theirSer := theirContribution.PaymentBasePoint.SerializeCompressed()
switch bytes.Compare(ourSer, theirSer) {
case -1:
stateObfuscator = DeriveStateHintObfuscator(
ourContribution.PaymentBasePoint,
theirContribution.PaymentBasePoint,
)
default:
stateObfuscator = DeriveStateHintObfuscator(
theirContribution.PaymentBasePoint,
ourContribution.PaymentBasePoint,
)
}
}
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 = SignDescriptor{
WitnessScript: witnessScript,
PubKey: 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].ScriptSig
// Fetch the alleged previous output along with the
// pkscript referenced by this input.
// TODO(roasbeef): when dual funder pass actual height-hint
output, err := l.Cfg.ChainIO.GetUtxo(&txin.PreviousOutPoint, 0)
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 := GenFundingPkScript(ourKey.SerializeCompressed(),
theirKey.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) {
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()
// Add the complete funding transaction to the DB, in its open bucket
// which will be used for the lifetime of this channel.
// TODO(roasbeef):
// * attempt to retransmit funding transactions on re-start
nodeAddr := res.nodeAddr
err = res.partialState.SyncPending(nodeAddr, uint32(bestHeight))
if err != nil {
msg.err <- err
msg.completeChan <- nil
return
}
walletLog.Infof("Broadcasting funding tx for ChannelPoint(%v): %v",
res.partialState.FundingOutpoint, spew.Sdump(fundingTx))
// Broadcast the finalized funding transaction to the network.
if err := l.PublishTransaction(fundingTx); 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,
pendingReservation.ourContribution.PaymentBasePoint)
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 := GenFundingPkScript(ourKey.SerializeCompressed(),
theirKey.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) {
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 := witnessScriptHash(witnessScript)
if err != nil {
req.err <- err
req.completeChan <- nil
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.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()
}
// 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 SatPerVByte,
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()
walletLog.Infof("Performing funding tx coin selection using %v "+
"sat/vbyte as fee rate", int64(feeRate))
// Find all unlocked unspent witness outputs with greater than 1
// confirmation.
// TODO(roasbeef): make num confs a configuration parameter
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 {
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).
contribution.Inputs[i] = wire.NewTxIn(outpoint, nil, nil)
}
// 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.
if changeAmt != 0 && changeAmt > DefaultDustLimit() {
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
}
// deriveMasterRevocationRoot derives the private key which serves as the master
// producer root. This master secret is used as the secret input to a HKDF to
// generate revocation secrets based on random, but public data.
func (l *LightningWallet) deriveMasterRevocationRoot() (*btcec.PrivateKey, error) {
masterElkremRoot, err := l.rootKey.Child(revocationRootIndex)
if err != nil {
return nil, err
}
return masterElkremRoot.ECPrivKey()
}
// 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 to 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/vbyte for coin selection to
// function properly.
func coinSelect(feeRate SatPerVByte, 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 TxWeightEstimator
for _, utxo := range selectedUtxos {
switch utxo.AddressType {
case WitnessPubKey:
weightEstimate.AddP2WKHInput()
case NestedWitnessPubKey:
weightEstimate.AddNestedP2WKHInput()
case PubKeyHash:
weightEstimate.AddP2PKHInput()
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.
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.
requiredFee := feeRate.FeeForVSize(int64(weightEstimate.VSize()))
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
}
}