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sweeper.go
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sweeper.go
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package sweep
import (
"github.com/btcsuite/btcd/blockchain"
"github.com/btcsuite/btcd/txscript"
"github.com/btcsuite/btcd/wire"
"github.com/btcsuite/btcutil"
"github.com/lightningnetwork/lnd/lnwallet"
)
// UtxoSweeper provides the functionality to generate sweep txes. The plan is
// to extend UtxoSweeper in the future to also manage the actual sweeping
// process by itself.
type UtxoSweeper struct {
cfg *UtxoSweeperConfig
}
// UtxoSweeperConfig contains dependencies of UtxoSweeper.
type UtxoSweeperConfig struct {
// GenSweepScript generates a P2WKH script belonging to the wallet
// where funds can be swept.
GenSweepScript func() ([]byte, error)
// Estimator is used when crafting sweep transactions to estimate the
// necessary fee relative to the expected size of the sweep
// transaction.
Estimator lnwallet.FeeEstimator
// Signer is used by the sweeper to generate valid witnesses at the
// time the incubated outputs need to be spent.
Signer lnwallet.Signer
}
// New returns a new UtxoSweeper instance.
func New(cfg *UtxoSweeperConfig) *UtxoSweeper {
return &UtxoSweeper{
cfg: cfg,
}
}
// CreateSweepTx accepts a list of inputs and signs and generates a txn that
// spends from them. This method also makes an accurate fee estimate before
// generating the required witnesses.
//
// The created transaction has a single output sending all the funds back to
// the source wallet, after accounting for the fee estimate.
//
// The value of currentBlockHeight argument will be set as the tx locktime.
// This function assumes that all CLTV inputs will be unlocked after
// currentBlockHeight. Reasons not to use the maximum of all actual CLTV expiry
// values of the inputs:
//
// - Make handling re-orgs easier.
// - Thwart future possible fee sniping attempts.
// - Make us blend in with the bitcoind wallet.
func (s *UtxoSweeper) CreateSweepTx(inputs []Input, confTarget uint32,
currentBlockHeight uint32) (*wire.MsgTx, error) {
// Generate the receiving script to which the funds will be swept.
pkScript, err := s.cfg.GenSweepScript()
if err != nil {
return nil, err
}
// Using the txn weight estimate, compute the required txn fee.
feePerKw, err := s.cfg.Estimator.EstimateFeePerKW(confTarget)
if err != nil {
return nil, err
}
inputs, txWeight, csvCount, cltvCount := s.getWeightEstimate(inputs)
log.Infof("Creating sweep transaction for %v inputs (%v CSV, %v CLTV) "+
"using %v sat/kw", len(inputs), csvCount, cltvCount,
int64(feePerKw))
txFee := feePerKw.FeeForWeight(txWeight)
// Sum up the total value contained in the inputs.
var totalSum btcutil.Amount
for _, o := range inputs {
totalSum += btcutil.Amount(o.SignDesc().Output.Value)
}
// Sweep as much possible, after subtracting txn fees.
sweepAmt := int64(totalSum - txFee)
// Create the sweep transaction that we will be building. We use
// version 2 as it is required for CSV. The txn will sweep the amount
// after fees to the pkscript generated above.
sweepTx := wire.NewMsgTx(2)
sweepTx.AddTxOut(&wire.TxOut{
PkScript: pkScript,
Value: sweepAmt,
})
sweepTx.LockTime = currentBlockHeight
// Add all inputs to the sweep transaction. Ensure that for each
// csvInput, we set the sequence number properly.
for _, input := range inputs {
sweepTx.AddTxIn(&wire.TxIn{
PreviousOutPoint: *input.OutPoint(),
Sequence: input.BlocksToMaturity(),
})
}
// Before signing the transaction, check to ensure that it meets some
// basic validity requirements.
//
// TODO(conner): add more control to sanity checks, allowing us to
// delay spending "problem" outputs, e.g. possibly batching with other
// classes if fees are too low.
btx := btcutil.NewTx(sweepTx)
if err := blockchain.CheckTransactionSanity(btx); err != nil {
return nil, err
}
hashCache := txscript.NewTxSigHashes(sweepTx)
// With all the inputs in place, use each output's unique witness
// function to generate the final witness required for spending.
addWitness := func(idx int, tso Input) error {
witness, err := tso.BuildWitness(
s.cfg.Signer, sweepTx, hashCache, idx,
)
if err != nil {
return err
}
sweepTx.TxIn[idx].Witness = witness
return nil
}
// Finally we'll attach a valid witness to each csv and cltv input
// within the sweeping transaction.
for i, input := range inputs {
if err := addWitness(i, input); err != nil {
return nil, err
}
}
return sweepTx, nil
}
// getWeightEstimate returns a weight estimate for the given inputs.
// Additionally, it returns counts for the number of csv and cltv inputs.
func (s *UtxoSweeper) getWeightEstimate(inputs []Input) ([]Input, int64, int, int) {
// We initialize a weight estimator so we can accurately asses the
// amount of fees we need to pay for this sweep transaction.
//
// TODO(roasbeef): can be more intelligent about buffering outputs to
// be more efficient on-chain.
var weightEstimate lnwallet.TxWeightEstimator
// Our sweep transaction will pay to a single segwit p2wkh address,
// ensure it contributes to our weight estimate.
weightEstimate.AddP2WKHOutput()
// For each output, use its witness type to determine the estimate
// weight of its witness, and add it to the proper set of spendable
// outputs.
var (
sweepInputs []Input
csvCount, cltvCount int
)
for i := range inputs {
input := inputs[i]
switch input.WitnessType() {
// Outputs on a remote commitment transaction that pay directly
// to us.
case lnwallet.CommitmentNoDelay:
weightEstimate.AddP2WKHInput()
sweepInputs = append(sweepInputs, input)
// Outputs on a past commitment transaction that pay directly
// to us.
case lnwallet.CommitmentTimeLock:
weightEstimate.AddWitnessInput(
lnwallet.ToLocalTimeoutWitnessSize,
)
sweepInputs = append(sweepInputs, input)
csvCount++
// Outgoing second layer HTLC's that have confirmed within the
// chain, and the output they produced is now mature enough to
// sweep.
case lnwallet.HtlcOfferedTimeoutSecondLevel:
weightEstimate.AddWitnessInput(
lnwallet.ToLocalTimeoutWitnessSize,
)
sweepInputs = append(sweepInputs, input)
csvCount++
// Incoming second layer HTLC's that have confirmed within the
// chain, and the output they produced is now mature enough to
// sweep.
case lnwallet.HtlcAcceptedSuccessSecondLevel:
weightEstimate.AddWitnessInput(
lnwallet.ToLocalTimeoutWitnessSize,
)
sweepInputs = append(sweepInputs, input)
csvCount++
// An HTLC on the commitment transaction of the remote party,
// that has had its absolute timelock expire.
case lnwallet.HtlcOfferedRemoteTimeout:
weightEstimate.AddWitnessInput(
lnwallet.AcceptedHtlcTimeoutWitnessSize,
)
sweepInputs = append(sweepInputs, input)
cltvCount++
// An HTLC on the commitment transaction of the remote party,
// that can be swept with the preimage.
case lnwallet.HtlcAcceptedRemoteSuccess:
weightEstimate.AddWitnessInput(
lnwallet.OfferedHtlcSuccessWitnessSize,
)
sweepInputs = append(sweepInputs, input)
default:
log.Warnf("kindergarten output in nursery store "+
"contains unexpected witness type: %v",
input.WitnessType())
}
}
txWeight := int64(weightEstimate.Weight())
return sweepInputs, txWeight, csvCount, cltvCount
}