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 // ConfTarget specifies a target for the number of blocks until an // initial confirmation. ConfTarget uint32 } // 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, 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 } txWeight, csvCount, cltvCount := s.getWeightEstimate(inputs) // Using the txn weight estimate, compute the required txn fee. feePerKw, err := s.cfg.Estimator.EstimateFeePerKW(s.cfg.ConfTarget) if err != nil { return nil, err } 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) (int64, int, int) { // Create a transaction which sweeps all the newly mature outputs into // an output controlled by the wallet. // 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. csvCount := 0 cltvCount := 0 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() // Outputs on a past commitment transaction that pay directly // to us. case lnwallet.CommitmentTimeLock: weightEstimate.AddWitnessInput( lnwallet.ToLocalTimeoutWitnessSize, ) 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, ) 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, ) 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, ) 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, ) default: log.Warnf("kindergarten output in nursery store "+ "contains unexpected witness type: %v", input.WitnessType()) continue } } txWeight := int64(weightEstimate.Weight()) return txWeight, csvCount, cltvCount }