lnd.xprv/sweep/txgenerator.go

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package sweep
import (
"fmt"
"sort"
"strings"
"github.com/btcsuite/btcd/blockchain"
"github.com/btcsuite/btcd/txscript"
"github.com/btcsuite/btcd/wire"
"github.com/btcsuite/btcutil"
"github.com/btcsuite/btcwallet/wallet/txrules"
"github.com/lightningnetwork/lnd/input"
"github.com/lightningnetwork/lnd/lnwallet"
)
var (
// DefaultMaxInputsPerTx specifies the default maximum number of inputs
// allowed in a single sweep tx. If more need to be swept, multiple txes
// are created and published.
DefaultMaxInputsPerTx = 100
)
// inputSet is a set of inputs that can be used as the basis to generate a tx
// on.
type inputSet []input.Input
// generateInputPartitionings goes through all given inputs and constructs sets
// of inputs that can be used to generate a sensible transaction. Each set
// contains up to the configured maximum number of inputs. Negative yield
// inputs are skipped. No input sets with a total value after fees below the
// dust limit are returned.
func generateInputPartitionings(sweepableInputs []input.Input,
relayFeePerKW, feePerKW lnwallet.SatPerKWeight,
maxInputsPerTx int) ([]inputSet, error) {
// Calculate dust limit based on the P2WPKH output script of the sweep
// txes.
dustLimit := txrules.GetDustThreshold(
input.P2WPKHSize,
btcutil.Amount(relayFeePerKW.FeePerKVByte()),
)
// Sort input by yield. We will start constructing input sets starting
// with the highest yield inputs. This is to prevent the construction
// of a set with an output below the dust limit, causing the sweep
// process to stop, while there are still higher value inputs
// available. It also allows us to stop evaluating more inputs when the
// first input in this ordering is encountered with a negative yield.
//
// Yield is calculated as the difference between value and added fee
// for this input. The fee calculation excludes fee components that are
// common to all inputs, as those wouldn't influence the order. The
// single component that is differentiating is witness size.
//
// For witness size, the upper limit is taken. The actual size depends
// on the signature length, which is not known yet at this point.
yields := make(map[wire.OutPoint]int64)
for _, input := range sweepableInputs {
size, _, err := input.WitnessType().SizeUpperBound()
if err != nil {
return nil, fmt.Errorf(
"failed adding input weight: %v", err)
}
yields[*input.OutPoint()] = input.SignDesc().Output.Value -
int64(feePerKW.FeeForWeight(int64(size)))
}
sort.Slice(sweepableInputs, func(i, j int) bool {
return yields[*sweepableInputs[i].OutPoint()] >
yields[*sweepableInputs[j].OutPoint()]
})
// Select blocks of inputs up to the configured maximum number.
var sets []inputSet
for len(sweepableInputs) > 0 {
// Get the maximum number of inputs from sweepableInputs that
// we can use to create a positive yielding set from.
count, outputValue := getPositiveYieldInputs(
sweepableInputs, maxInputsPerTx, feePerKW,
)
// If there are no positive yield inputs left, we can stop
// here.
if count == 0 {
return sets, nil
}
// If the output value of this block of inputs does not reach
// the dust limit, stop sweeping. Because of the sorting,
// continuing with the remaining inputs will only lead to sets
// with a even lower output value.
if outputValue < dustLimit {
log.Debugf("Set value %v below dust limit of %v",
outputValue, dustLimit)
return sets, nil
}
log.Infof("Candidate sweep set of size=%v, has yield=%v",
count, outputValue)
sets = append(sets, sweepableInputs[:count])
sweepableInputs = sweepableInputs[count:]
}
return sets, nil
}
// getPositiveYieldInputs returns the maximum of a number n for which holds
// that the inputs [0,n) of sweepableInputs have a positive yield.
// Additionally, the total values of these inputs minus the fee is returned.
//
// TODO(roasbeef): Consider including some negative yield inputs too to clean
// up the utxo set even if it costs us some fees up front. In the spirit of
// minimizing any negative externalities we cause for the Bitcoin system as a
// whole.
func getPositiveYieldInputs(sweepableInputs []input.Input, maxInputs int,
feePerKW lnwallet.SatPerKWeight) (int, btcutil.Amount) {
var weightEstimate input.TxWeightEstimator
// Add the sweep tx output to the weight estimate.
weightEstimate.AddP2WKHOutput()
var total, outputValue btcutil.Amount
for idx, input := range sweepableInputs {
// Can ignore error, because it has already been checked when
// calculating the yields.
size, isNestedP2SH, _ := input.WitnessType().SizeUpperBound()
// Keep a running weight estimate of the input set.
if isNestedP2SH {
weightEstimate.AddNestedP2WSHInput(size)
} else {
weightEstimate.AddWitnessInput(size)
}
newTotal := total + btcutil.Amount(input.SignDesc().Output.Value)
weight := weightEstimate.Weight()
fee := feePerKW.FeeForWeight(int64(weight))
// Calculate the output value if the current input would be
// added to the set.
newOutputValue := newTotal - fee
// If adding this input makes the total output value of the set
// decrease, this is a negative yield input. It shouldn't be
// added to the set. We return the current index as the number
// of inputs, so the current input is being excluded.
if newOutputValue <= outputValue {
return idx, outputValue
}
// Update running values.
total = newTotal
outputValue = newOutputValue
// Stop if max inputs is reached.
if idx == maxInputs-1 {
return maxInputs, outputValue
}
}
// We could add all inputs to the set, so return them all.
return len(sweepableInputs), outputValue
}
// createSweepTx builds a signed tx spending the inputs to a the output script.
func createSweepTx(inputs []input.Input, outputPkScript []byte,
currentBlockHeight uint32, feePerKw lnwallet.SatPerKWeight,
signer input.Signer) (*wire.MsgTx, error) {
inputs, txWeight := getWeightEstimate(inputs)
log.Infof("Creating sweep transaction for %v inputs (%s) "+
"using %v sat/kw", len(inputs), inputTypeSummary(inputs),
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: outputPkScript,
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 input script
// function to generate the final witness required for spending.
addInputScript := func(idx int, tso input.Input) error {
inputScript, err := tso.CraftInputScript(
signer, sweepTx, hashCache, idx,
)
if err != nil {
return err
}
sweepTx.TxIn[idx].Witness = inputScript.Witness
if len(inputScript.SigScript) != 0 {
sweepTx.TxIn[idx].SignatureScript = inputScript.SigScript
}
return nil
}
// Finally we'll attach a valid input script to each csv and cltv input
// within the sweeping transaction.
for i, input := range inputs {
if err := addInputScript(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 getWeightEstimate(inputs []input.Input) ([]input.Input, int64) {
// 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 input.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.Input
for i := range inputs {
inp := inputs[i]
wt := inp.WitnessType()
err := wt.AddWeightEstimation(&weightEstimate)
if err != nil {
log.Warn(err)
// Skip inputs for which no weight estimate can be
// given.
continue
}
sweepInputs = append(sweepInputs, inp)
}
return sweepInputs, int64(weightEstimate.Weight())
}
// inputSummary returns a string containing a human readable summary about the
// witness types of a list of inputs.
func inputTypeSummary(inputs []input.Input) string {
// Count each input by the string representation of its witness type.
// We also keep track of the keys so we can later sort by them to get
// a stable output.
counts := make(map[string]uint32)
keys := make([]string, 0, len(inputs))
for _, i := range inputs {
key := i.WitnessType().String()
_, ok := counts[key]
if !ok {
counts[key] = 0
keys = append(keys, key)
}
counts[key]++
}
sort.Strings(keys)
// Return a nice string representation of the counts by comma joining a
// slice.
var parts []string
for _, witnessType := range keys {
part := fmt.Sprintf("%d %s", counts[witnessType], witnessType)
parts = append(parts, part)
}
return strings.Join(parts, ", ")
}