package sweep import ( "errors" "fmt" "math/rand" "sync" "sync/atomic" "time" "github.com/btcsuite/btcd/wire" "github.com/btcsuite/btcutil" "github.com/davecgh/go-spew/spew" "github.com/lightningnetwork/lnd/chainntnfs" "github.com/lightningnetwork/lnd/input" "github.com/lightningnetwork/lnd/lnwallet" ) var ( // ErrRemoteSpend is returned in case an output that we try to sweep is // confirmed in a tx of the remote party. ErrRemoteSpend = errors.New("remote party swept utxo") // ErrTooManyAttempts is returned in case sweeping an output has failed // for the configured max number of attempts. ErrTooManyAttempts = errors.New("sweep failed after max attempts") // DefaultMaxSweepAttempts specifies the default maximum number of times // an input is included in a publish attempt before giving up and // returning an error to the caller. DefaultMaxSweepAttempts = 10 ) // pendingInput is created when an input reaches the main loop for the first // time. It tracks all relevant state that is needed for sweeping. type pendingInput struct { // listeners is a list of channels over which the final outcome of the // sweep needs to be broadcasted. listeners []chan Result // input is the original struct that contains the input and sign // descriptor. input input.Input // ntfnRegCancel is populated with a function that cancels the chain // notifier spend registration. ntfnRegCancel func() // minPublishHeight indicates the minimum block height at which this // input may be (re)published. minPublishHeight int32 // publishAttempts records the number of attempts that have already been // made to sweep this tx. publishAttempts int } // UtxoSweeper is responsible for sweeping outputs back into the wallet type UtxoSweeper struct { started uint32 // To be used atomically. stopped uint32 // To be used atomically. cfg *UtxoSweeperConfig newInputs chan *sweepInputMessage spendChan chan *chainntnfs.SpendDetail pendingInputs map[wire.OutPoint]*pendingInput // timer is the channel that signals expiry of the sweep batch timer. timer <-chan time.Time testSpendChan chan wire.OutPoint currentOutputScript []byte relayFeePerKW lnwallet.SatPerKWeight quit chan struct{} wg sync.WaitGroup } // 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) // FeeEstimator is used when crafting sweep transactions to estimate // the necessary fee relative to the expected size of the sweep // transaction. FeeEstimator lnwallet.FeeEstimator // PublishTransaction facilitates the process of broadcasting a signed // transaction to the appropriate network. PublishTransaction func(*wire.MsgTx) error // NewBatchTimer creates a channel that will be sent on when a certain // time window has passed. During this time window, new inputs can still // be added to the sweep tx that is about to be generated. NewBatchTimer func() <-chan time.Time // Notifier is an instance of a chain notifier we'll use to watch for // certain on-chain events. Notifier chainntnfs.ChainNotifier // ChainIO is used to determine the current block height. ChainIO lnwallet.BlockChainIO // Store stores the published sweeper txes. Store SweeperStore // Signer is used by the sweeper to generate valid witnesses at the // time the incubated outputs need to be spent. Signer input.Signer // SweepTxConfTarget assigns a confirmation target for sweep txes on // which the fee calculation will be based. SweepTxConfTarget uint32 // MaxInputsPerTx 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. MaxInputsPerTx int // MaxSweepAttempts specifies the maximum number of times an input is // included in a publish attempt before giving up and returning an error // to the caller. MaxSweepAttempts int // NextAttemptDeltaFunc returns given the number of already attempted // sweeps, how many blocks to wait before retrying to sweep. NextAttemptDeltaFunc func(int) int32 } // Result is the struct that is pushed through the result channel. Callers can // use this to be informed of the final sweep result. In case of a remote // spend, Err will be ErrRemoteSpend. type Result struct { // Err is the final result of the sweep. It is nil when the input is // swept successfully by us. ErrRemoteSpend is returned when another // party took the input. Err error // Tx is the transaction that spent the input. Tx *wire.MsgTx } // sweepInputMessage structs are used in the internal channel between the // SweepInput call and the sweeper main loop. type sweepInputMessage struct { input input.Input resultChan chan Result } // New returns a new Sweeper instance. func New(cfg *UtxoSweeperConfig) *UtxoSweeper { return &UtxoSweeper{ cfg: cfg, newInputs: make(chan *sweepInputMessage), spendChan: make(chan *chainntnfs.SpendDetail), quit: make(chan struct{}), pendingInputs: make(map[wire.OutPoint]*pendingInput), } } // Start starts the process of constructing and publish sweep txes. func (s *UtxoSweeper) Start() error { if !atomic.CompareAndSwapUint32(&s.started, 0, 1) { return nil } log.Tracef("Sweeper starting") // Retrieve last published tx from database. lastTx, err := s.cfg.Store.GetLastPublishedTx() if err != nil { return fmt.Errorf("get last published tx: %v", err) } // Republish in case the previous call crashed lnd. We don't care about // the return value, because inputs will be re-offered and retried // anyway. The only reason we republish here is to prevent the corner // case where lnd goes into a restart loop because of a crashing publish // tx where we keep deriving new output script. By publishing and // possibly crashing already now, we haven't derived a new output script // yet. if lastTx != nil { log.Debugf("Publishing last tx %v", lastTx.TxHash()) // Error can be ignored. Because we are starting up, there are // no pending inputs to update based on the publish result. err := s.cfg.PublishTransaction(lastTx) if err != nil && err != lnwallet.ErrDoubleSpend { log.Errorf("last tx publish: %v", err) } } // Retrieve relay fee for dust limit calculation. Assume that this will // not change from here on. s.relayFeePerKW = s.cfg.FeeEstimator.RelayFeePerKW() // Register for block epochs to retry sweeping every block. bestHash, bestHeight, err := s.cfg.ChainIO.GetBestBlock() if err != nil { return fmt.Errorf("get best block: %v", err) } log.Debugf("Best height: %v", bestHeight) blockEpochs, err := s.cfg.Notifier.RegisterBlockEpochNtfn( &chainntnfs.BlockEpoch{ Height: bestHeight, Hash: bestHash, }, ) if err != nil { return fmt.Errorf("register block epoch ntfn: %v", err) } // Start sweeper main loop. s.wg.Add(1) go func() { defer blockEpochs.Cancel() defer s.wg.Done() err := s.collector(blockEpochs.Epochs, bestHeight) if err != nil { log.Errorf("sweeper stopped: %v", err) } }() return nil } // Stop stops sweeper from listening to block epochs and constructing sweep // txes. func (s *UtxoSweeper) Stop() error { if !atomic.CompareAndSwapUint32(&s.stopped, 0, 1) { return nil } log.Debugf("Sweeper shutting down") close(s.quit) s.wg.Wait() log.Debugf("Sweeper shut down") return nil } // SweepInput sweeps inputs back into the wallet. The inputs will be batched and // swept after the batch time window ends. // // NOTE: Extreme care needs to be taken that input isn't changed externally. // Because it is an interface and we don't know what is exactly behind it, we // cannot make a local copy in sweeper. func (s *UtxoSweeper) SweepInput(input input.Input) (chan Result, error) { if input == nil || input.OutPoint() == nil || input.SignDesc() == nil { return nil, errors.New("nil input received") } log.Infof("Sweep request received: out_point=%v, witness_type=%v, "+ "time_lock=%v, size=%v", input.OutPoint(), input.WitnessType(), input.BlocksToMaturity(), btcutil.Amount(input.SignDesc().Output.Value)) sweeperInput := &sweepInputMessage{ input: input, resultChan: make(chan Result, 1), } // Deliver input to main event loop. select { case s.newInputs <- sweeperInput: case <-s.quit: return nil, fmt.Errorf("sweeper shutting down") } return sweeperInput.resultChan, nil } // collector is the sweeper main loop. It processes new inputs, spend // notifications and counts down to publication of the sweep tx. func (s *UtxoSweeper) collector(blockEpochs <-chan *chainntnfs.BlockEpoch, bestHeight int32) error { for { select { // A new inputs is offered to the sweeper. We check to see if we // are already trying to sweep this input and if not, set up a // listener for spend and schedule a sweep. case input := <-s.newInputs: outpoint := *input.input.OutPoint() pendInput, pending := s.pendingInputs[outpoint] if pending { log.Debugf("Already pending input %v received", outpoint) // Add additional result channel to signal // spend of this input. pendInput.listeners = append( pendInput.listeners, input.resultChan, ) continue } // Create a new pendingInput and initialize the // listeners slice with the passed in result channel. If // this input is offered for sweep again, the result // channel will be appended to this slice. pendInput = &pendingInput{ listeners: []chan Result{input.resultChan}, input: input.input, minPublishHeight: bestHeight, } s.pendingInputs[outpoint] = pendInput // Start watching for spend of this input, either by us // or the remote party. cancel, err := s.waitForSpend( outpoint, input.input.SignDesc().Output.PkScript, input.input.HeightHint(), ) if err != nil { err := fmt.Errorf("wait for spend: %v", err) s.signalAndRemove(&outpoint, Result{Err: err}) continue } pendInput.ntfnRegCancel = cancel // Check to see if with this new input a sweep tx can be // formed. if err := s.scheduleSweep(bestHeight); err != nil { log.Errorf("schedule sweep: %v", err) } // A spend of one of our inputs is detected. Signal sweep // results to the caller(s). case spend := <-s.spendChan: // For testing purposes. if s.testSpendChan != nil { s.testSpendChan <- *spend.SpentOutPoint } // Query store to find out if we every published this // tx. spendHash := *spend.SpenderTxHash isOurTx, err := s.cfg.Store.IsOurTx(spendHash) if err != nil { log.Errorf("cannot determine if tx %v "+ "is ours: %v", spendHash, err, ) continue } log.Debugf("Detected spend related to in flight inputs "+ "(is_ours=%v): %v", newLogClosure(func() string { return spew.Sdump(spend.SpendingTx) }), isOurTx, ) // Signal sweep results for inputs in this confirmed // tx. for _, txIn := range spend.SpendingTx.TxIn { outpoint := txIn.PreviousOutPoint // Check if this input is known to us. It could // probably be unknown if we canceled the // registration, deleted from pendingInputs but // the ntfn was in-flight already. Or this could // be not one of our inputs. _, ok := s.pendingInputs[outpoint] if !ok { continue } // Return either a nil or a remote spend result. var err error if !isOurTx { err = ErrRemoteSpend } // Signal result channels. s.signalAndRemove(&outpoint, Result{ Tx: spend.SpendingTx, Err: err, }) } // Now that an input of ours is spent, we can try to // resweep the remaining inputs. if err := s.scheduleSweep(bestHeight); err != nil { log.Errorf("schedule sweep: %v", err) } // The timer expires and we are going to (re)sweep. case <-s.timer: log.Debugf("Sweep timer expired") // Set timer to nil so we know that a new timer needs to // be started when new inputs arrive. s.timer = nil // Retrieve fee estimate for input filtering and final // tx fee calculation. satPerKW, err := s.cfg.FeeEstimator.EstimateFeePerKW( s.cfg.SweepTxConfTarget, ) if err != nil { log.Errorf("estimate fee: %v", err) continue } // Examine pending inputs and try to construct lists of // inputs. inputLists, err := s.getInputLists(bestHeight, satPerKW) if err != nil { log.Errorf("get input lists: %v", err) continue } // Sweep selected inputs. for _, inputs := range inputLists { err := s.sweep(inputs, satPerKW, bestHeight) if err != nil { log.Errorf("sweep: %v", err) } } // A new block comes in. Things may have changed, so we retry a // sweep. case epoch, ok := <-blockEpochs: if !ok { return nil } bestHeight = epoch.Height log.Debugf("New blocks: height=%v, sha=%v", epoch.Height, epoch.Hash) if err := s.scheduleSweep(bestHeight); err != nil { log.Errorf("schedule sweep: %v", err) } case <-s.quit: return nil } } } // scheduleSweep starts the sweep timer to create an opportunity for more inputs // to be added. func (s *UtxoSweeper) scheduleSweep(currentHeight int32) error { // The timer is already ticking, no action needed for the sweep to // happen. if s.timer != nil { log.Debugf("Timer still ticking") return nil } // Retrieve fee estimate for input filtering and final tx fee // calculation. satPerKW, err := s.cfg.FeeEstimator.EstimateFeePerKW( s.cfg.SweepTxConfTarget, ) if err != nil { return fmt.Errorf("estimate fee: %v", err) } // Examine pending inputs and try to construct lists of inputs. inputLists, err := s.getInputLists(currentHeight, satPerKW) if err != nil { return fmt.Errorf("get input lists: %v", err) } log.Infof("Sweep candidates at height=%v, yield %v distinct txns", currentHeight, len(inputLists)) // If there are no input sets, there is nothing sweepable and we can // return without starting the timer. if len(inputLists) == 0 { return nil } // Start sweep timer to create opportunity for more inputs to be added // before a tx is constructed. s.timer = s.cfg.NewBatchTimer() log.Debugf("Sweep timer started") return nil } // signalAndRemove notifies the listeners of the final result of the input // sweep. It cancels any pending spend notification and removes the input from // the list of pending inputs. When this function returns, the sweeper has // completely forgotten about the input. func (s *UtxoSweeper) signalAndRemove(outpoint *wire.OutPoint, result Result) { pendInput := s.pendingInputs[*outpoint] listeners := pendInput.listeners if result.Err == nil { log.Debugf("Dispatching sweep success for %v to %v listeners", outpoint, len(listeners), ) } else { log.Debugf("Dispatching sweep error for %v to %v listeners: %v", outpoint, len(listeners), result.Err, ) } // Signal all listeners. Channel is buffered. Because we only send once // on every channel, it should never block. for _, resultChan := range listeners { resultChan <- result } // Cancel spend notification with chain notifier. This is not necessary // in case of a success, except for that a reorg could still happen. if pendInput.ntfnRegCancel != nil { log.Debugf("Canceling spend ntfn for %v", outpoint) pendInput.ntfnRegCancel() } // Inputs are no longer pending after result has been sent. delete(s.pendingInputs, *outpoint) } // getInputLists goes through all pending inputs and constructs sweep lists, // each up to the configured maximum number of inputs. Negative yield inputs are // skipped. Transactions with an output below the dust limit are not published. // Those inputs remain pending and will be bundled with future inputs if // possible. func (s *UtxoSweeper) getInputLists(currentHeight int32, satPerKW lnwallet.SatPerKWeight) ([]inputSet, error) { // Filter for inputs that need to be swept. Create two lists: all // sweepable inputs and a list containing only the new, never tried // inputs. // // We want to create as large a tx as possible, so we return a final set // list that starts with sets created from all inputs. However, there is // a chance that those txes will not publish, because they already // contain inputs that failed before. Therefore we also add sets // consisting of only new inputs to the list, to make sure that new // inputs are given a good, isolated chance of being published. var newInputs, retryInputs []input.Input for _, input := range s.pendingInputs { // Skip inputs that have a minimum publish height that is not // yet reached. if input.minPublishHeight > currentHeight { continue } // Add input to the either one of the lists. if input.publishAttempts == 0 { newInputs = append(newInputs, input.input) } else { retryInputs = append(retryInputs, input.input) } } // If there is anything to retry, combine it with the new inputs and // form input sets. var allSets []inputSet if len(retryInputs) > 0 { var err error allSets, err = generateInputPartitionings( append(retryInputs, newInputs...), s.relayFeePerKW, satPerKW, s.cfg.MaxInputsPerTx, ) if err != nil { return nil, fmt.Errorf("input partitionings: %v", err) } } // Create sets for just the new inputs. newSets, err := generateInputPartitionings( newInputs, s.relayFeePerKW, satPerKW, s.cfg.MaxInputsPerTx, ) if err != nil { return nil, fmt.Errorf("input partitionings: %v", err) } log.Debugf("Sweep candidates at height=%v: total_num_pending=%v, "+ "total_num_new=%v", currentHeight, len(allSets), len(newSets)) // Append the new sets at the end of the list, because those tx likely // have a higher fee per input. return append(allSets, newSets...), nil } // sweep takes a set of preselected inputs, creates a sweep tx and publishes the // tx. The output address is only marked as used if the publish succeeds. func (s *UtxoSweeper) sweep(inputs inputSet, satPerKW lnwallet.SatPerKWeight, currentHeight int32) error { var err error // Generate output script if no unused script available. if s.currentOutputScript == nil { s.currentOutputScript, err = s.cfg.GenSweepScript() if err != nil { return fmt.Errorf("gen sweep script: %v", err) } } // Create sweep tx. tx, err := createSweepTx( inputs, s.currentOutputScript, uint32(currentHeight), satPerKW, s.cfg.Signer, ) if err != nil { return fmt.Errorf("create sweep tx: %v", err) } // Add tx before publication, so that we will always know that a spend // by this tx is ours. Otherwise if the publish doesn't return, but did // publish, we loose track of this tx. Even republication on startup // doesn't prevent this, because that call returns a double spend error // then and would also not add the hash to the store. err = s.cfg.Store.NotifyPublishTx(tx) if err != nil { return fmt.Errorf("notify publish tx: %v", err) } // Publish sweep tx. log.Debugf("Publishing sweep tx %v, num_inputs=%v, height=%v", tx.TxHash(), len(tx.TxIn), currentHeight) log.Tracef("Sweep tx at height=%v: %v", currentHeight, newLogClosure(func() string { return spew.Sdump(tx) }), ) err = s.cfg.PublishTransaction(tx) // In case of an unexpected error, don't try to recover. if err != nil && err != lnwallet.ErrDoubleSpend { return fmt.Errorf("publish tx: %v", err) } // Keep outputScript in case of an error, so that it can be reused for // the next tx and causes no address inflation. if err == nil { s.currentOutputScript = nil } // Reschedule sweep. for _, input := range tx.TxIn { pi, ok := s.pendingInputs[input.PreviousOutPoint] if !ok { // It can be that the input has been removed because it // exceed the maximum number of attempts in a previous // input set. continue } // Record another publish attempt. pi.publishAttempts++ // We don't care what the result of the publish call was. Even // if it is published successfully, it can still be that it // needs to be retried. Call NextAttemptDeltaFunc to calculate // when to resweep this input. nextAttemptDelta := s.cfg.NextAttemptDeltaFunc( pi.publishAttempts, ) pi.minPublishHeight = currentHeight + nextAttemptDelta log.Debugf("Rescheduling input %v after %v attempts at "+ "height %v (delta %v)", input.PreviousOutPoint, pi.publishAttempts, pi.minPublishHeight, nextAttemptDelta) if pi.publishAttempts >= s.cfg.MaxSweepAttempts { // Signal result channels sweep result. s.signalAndRemove(&input.PreviousOutPoint, Result{ Err: ErrTooManyAttempts, }) } } return nil } // waitForSpend registers a spend notification with the chain notifier. It // returns a cancel function that can be used to cancel the registration. func (s *UtxoSweeper) waitForSpend(outpoint wire.OutPoint, script []byte, heightHint uint32) (func(), error) { log.Debugf("Wait for spend of %v", outpoint) spendEvent, err := s.cfg.Notifier.RegisterSpendNtfn( &outpoint, script, heightHint, ) if err != nil { return nil, fmt.Errorf("register spend ntfn: %v", err) } s.wg.Add(1) go func() { defer s.wg.Done() select { case spend, ok := <-spendEvent.Spend: if !ok { log.Debugf("Spend ntfn for %v canceled", outpoint) return } log.Debugf("Delivering spend ntfn for %v", outpoint) select { case s.spendChan <- spend: log.Debugf("Delivered spend ntfn for %v", outpoint) case <-s.quit: } case <-s.quit: } }() return spendEvent.Cancel, nil } // 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.Input, feePref FeePreference, currentBlockHeight uint32) (*wire.MsgTx, error) { feePerKw, err := DetermineFeePerKw(s.cfg.FeeEstimator, feePref) if err != nil { return nil, err } // Generate the receiving script to which the funds will be swept. pkScript, err := s.cfg.GenSweepScript() if err != nil { return nil, err } return createSweepTx( inputs, pkScript, currentBlockHeight, feePerKw, s.cfg.Signer, ) } // DefaultNextAttemptDeltaFunc is the default calculation for next sweep attempt // scheduling. It implements exponential back-off with some randomness. This is // to prevent a stuck tx (for example because fee is too low and can't be bumped // in btcd) from blocking all other retried inputs in the same tx. func DefaultNextAttemptDeltaFunc(attempts int) int32 { return 1 + rand.Int31n(1<