package sweep import ( "fmt" "math" "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" "github.com/lightningnetwork/lnd/lnwallet/chainfee" ) // addConstraints defines the constraints to apply when adding an input. type addConstraints uint8 const ( // constraintsRegular is for regular input sweeps that should have a positive // yield. constraintsRegular addConstraints = iota // constraintsWallet is for wallet inputs that are only added to bring up the tx // output value. constraintsWallet // constraintsForce is for inputs that should be swept even with a negative // yield at the set fee rate. constraintsForce ) type txInputSetState struct { // feeRate is the fee rate to use for the sweep transaction. feeRate chainfee.SatPerKWeight // inputTotal is the total value of all inputs. inputTotal btcutil.Amount // requiredOutput is the sum of the outputs committed to by the inputs. requiredOutput btcutil.Amount // changeOutput is the value of the change output. This will be what is // left over after subtracting the requiredOutput and the tx fee from // the inputTotal. // // NOTE: This might be below the dust limit, or even negative since it // is the change remaining in csse we pay the fee for a change output. changeOutput btcutil.Amount // inputs is the set of tx inputs. inputs []input.Input // walletInputTotal is the total value of inputs coming from the wallet. walletInputTotal btcutil.Amount // force indicates that this set must be swept even if the total yield // is negative. force bool } // weightEstimate is the (worst case) tx weight with the current set of // inputs. It takes a parameter whether to add a change output or not. func (t *txInputSetState) weightEstimate(change bool) *weightEstimator { weightEstimate := newWeightEstimator(t.feeRate) for _, i := range t.inputs { // Can ignore error, because it has already been checked when // calculating the yields. _ = weightEstimate.add(i) r := i.RequiredTxOut() if r != nil { weightEstimate.addOutput(r) } } // Add a change output to the weight estimate if requested. if change { weightEstimate.addP2WKHOutput() } return weightEstimate } // totalOutput is the total amount left for us after paying fees. // // NOTE: This might be dust. func (t *txInputSetState) totalOutput() btcutil.Amount { return t.requiredOutput + t.changeOutput } func (t *txInputSetState) clone() txInputSetState { s := txInputSetState{ feeRate: t.feeRate, inputTotal: t.inputTotal, changeOutput: t.changeOutput, requiredOutput: t.requiredOutput, walletInputTotal: t.walletInputTotal, force: t.force, inputs: make([]input.Input, len(t.inputs)), } copy(s.inputs, t.inputs) return s } // txInputSet is an object that accumulates tx inputs and keeps running counters // on various properties of the tx. type txInputSet struct { txInputSetState // dustLimit is the minimum output value of the tx. dustLimit btcutil.Amount // maxInputs is the maximum number of inputs that will be accepted in // the set. maxInputs int // wallet contains wallet functionality required by the input set to // retrieve utxos. wallet Wallet } func dustLimit(relayFee chainfee.SatPerKWeight) btcutil.Amount { return txrules.GetDustThreshold( input.P2WPKHSize, btcutil.Amount(relayFee.FeePerKVByte()), ) } // newTxInputSet constructs a new, empty input set. func newTxInputSet(wallet Wallet, feePerKW, relayFee chainfee.SatPerKWeight, maxInputs int) *txInputSet { dustLimit := dustLimit(relayFee) state := txInputSetState{ feeRate: feePerKW, } b := txInputSet{ dustLimit: dustLimit, maxInputs: maxInputs, wallet: wallet, txInputSetState: state, } return &b } // enoughInput returns true if we've accumulated enough inputs to pay the fees // and have at least one output that meets the dust limit. func (t *txInputSet) enoughInput() bool { // If we have a change output above dust, then we certainly have enough // inputs to the transaction. if t.changeOutput >= t.dustLimit { return true } // We did not have enough input for a change output. Check if we have // enough input to pay the fees for a transaction with no change // output. fee := t.weightEstimate(false).fee() if t.inputTotal < t.requiredOutput+fee { return false } // We could pay the fees, but we still need at least one output to be // above the dust limit for the tx to be valid (we assume that these // required outputs only get added if they are above dust) for _, inp := range t.inputs { if inp.RequiredTxOut() != nil { return true } } return false } // add adds a new input to the set. It returns a bool indicating whether the // input was added to the set. An input is rejected if it decreases the tx // output value after paying fees. func (t *txInputSet) addToState(inp input.Input, constraints addConstraints) *txInputSetState { // Stop if max inputs is reached. Do not count additional wallet inputs, // because we don't know in advance how many we may need. if constraints != constraintsWallet && len(t.inputs) >= t.maxInputs { return nil } // If the input comes with a required tx out that is below dust, we // won't add it. reqOut := inp.RequiredTxOut() if reqOut != nil && btcutil.Amount(reqOut.Value) < t.dustLimit { return nil } // Clone the current set state. s := t.clone() // Add the new input. s.inputs = append(s.inputs, inp) // Add the value of the new input. value := btcutil.Amount(inp.SignDesc().Output.Value) s.inputTotal += value // Recalculate the tx fee. fee := s.weightEstimate(true).fee() // Calculate the new output value. if reqOut != nil { s.requiredOutput += btcutil.Amount(reqOut.Value) } s.changeOutput = s.inputTotal - s.requiredOutput - fee // Calculate the yield of this input from the change in total tx output // value. inputYield := s.totalOutput() - t.totalOutput() switch constraints { // Don't sweep inputs that cost us more to sweep than they give us. case constraintsRegular: if inputYield <= 0 { return nil } // For force adds, no further constraints apply. case constraintsForce: s.force = true // We are attaching a wallet input to raise the tx output value above // the dust limit. case constraintsWallet: // Skip this wallet input if adding it would lower the output // value. if inputYield <= 0 { return nil } // Calculate the total value that we spend in this tx from the // wallet if we'd add this wallet input. s.walletInputTotal += value // In any case, we don't want to lose money by sweeping. If we // don't get more out of the tx then we put in ourselves, do not // add this wallet input. If there is at least one force sweep // in the set, this does no longer apply. // // We should only add wallet inputs to get the tx output value // above the dust limit, otherwise we'd only burn into fees. // This is guarded by tryAddWalletInputsIfNeeded. // // TODO(joostjager): Possibly require a max ratio between the // value of the wallet input and what we get out of this // transaction. To prevent attaching and locking a big utxo for // very little benefit. if !s.force && s.walletInputTotal >= s.totalOutput() { log.Debugf("Rejecting wallet input of %v, because it "+ "would make a negative yielding transaction "+ "(%v)", value, s.totalOutput()-s.walletInputTotal) return nil } } return &s } // add adds a new input to the set. It returns a bool indicating whether the // input was added to the set. An input is rejected if it decreases the tx // output value after paying fees. func (t *txInputSet) add(input input.Input, constraints addConstraints) bool { newState := t.addToState(input, constraints) if newState == nil { return false } t.txInputSetState = *newState return true } // addPositiveYieldInputs adds sweepableInputs that have a positive yield to the // input set. This function assumes that the list of inputs is sorted descending // by yield. // // 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 (t *txInputSet) addPositiveYieldInputs(sweepableInputs []txInput) { for _, input := range sweepableInputs { // Apply relaxed constraints for force sweeps. constraints := constraintsRegular if input.parameters().Force { constraints = constraintsForce } // Try to add the input to the transaction. If that doesn't // succeed because it wouldn't increase the output value, // return. Assuming inputs are sorted by yield, any further // inputs wouldn't increase the output value either. if !t.add(input, constraints) { return } } // We managed to add all inputs to the set. } // tryAddWalletInputsIfNeeded retrieves utxos from the wallet and tries adding as // many as required to bring the tx output value above the given minimum. func (t *txInputSet) tryAddWalletInputsIfNeeded() error { // If we've already have enough to pay the transaction fees and have at // least one output materialize, no action is needed. if t.enoughInput() { return nil } // Retrieve wallet utxos. Only consider confirmed utxos to prevent // problems around RBF rules for unconfirmed inputs. utxos, err := t.wallet.ListUnspentWitness(1, math.MaxInt32) if err != nil { return err } for _, utxo := range utxos { input, err := createWalletTxInput(utxo) if err != nil { return err } // If the wallet input isn't positively-yielding at this fee // rate, skip it. if !t.add(input, constraintsWallet) { continue } // Return if we've reached the minimum output amount. if t.enoughInput() { return nil } } // We were not able to reach the minimum output amount. return nil } // createWalletTxInput converts a wallet utxo into an object that can be added // to the other inputs to sweep. func createWalletTxInput(utxo *lnwallet.Utxo) (input.Input, error) { var witnessType input.WitnessType switch utxo.AddressType { case lnwallet.WitnessPubKey: witnessType = input.WitnessKeyHash case lnwallet.NestedWitnessPubKey: witnessType = input.NestedWitnessKeyHash default: return nil, fmt.Errorf("unknown address type %v", utxo.AddressType) } signDesc := &input.SignDescriptor{ Output: &wire.TxOut{ PkScript: utxo.PkScript, Value: int64(utxo.Value), }, HashType: txscript.SigHashAll, } // A height hint doesn't need to be set, because we don't monitor these // inputs for spend. heightHint := uint32(0) return input.NewBaseInput( &utxo.OutPoint, witnessType, signDesc, heightHint, ), nil }