sweepbatcher: add greedy batch selection algorithm

If custom fee rates are used, the algorithm is tried. It selects a batch for the
sweep using the greedy algorithm, which minimizes costs, and adds the sweep to
the batch. If it fails, old algorithm is used, trying to add to any batch, or
starting a new batch.

sweepbatcher/greedy_batch_selection.go

@@ -0,0 +1,290 @@
+package sweepbatcher
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"github.com/btcsuite/btcd/btcutil"
+	"github.com/btcsuite/btcd/txscript"
+	sweeppkg "github.com/lightninglabs/loop/sweep"
+	"github.com/lightningnetwork/lnd/input"
+	"github.com/lightningnetwork/lnd/lntypes"
+	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
+)
+
+// greedyAddSweep selects a batch for the sweep using the greedy algorithm,
+// which minimizes costs, and adds the sweep to the batch.
+func (b *Batcher) greedyAddSweep(ctx context.Context, sweep *sweep) error {
+	if b.customFeeRate == nil {
+		return errors.New("greedy batch selection algorithm requires " +
+			"setting custom fee rate provider")
+	}
+
+	// Collect weight and fee rate info about the sweep and new batch.
+	sweepEntity, newBatchEntity, err := estimateSweepWeight(sweep)
+	if err != nil {
+		return fmt.Errorf("failed to estimate tx weight for "+
+			"sweep %x: %w", sweep.swapHash[:6], err)
+	}
+
+	// Collect weight and fee rate info about existing batches.
+	batches := make([]entity, 0, len(b.batches))
+	for _, batch := range b.batches {
+		batchEntity, err := estimateBatchWeight(batch)
+		if err != nil {
+			return fmt.Errorf("failed to estimate tx weight for "+
+				"batch %d: %w", batch.id, err)
+		}
+		batches = append(batches, *batchEntity)
+	}
+
+	// Run the algorithm. Get batchId of the best batch, or specialId if the
+	// best option is to create new batch.
+	batchId, err := selectBatch(batches, *sweepEntity, *newBatchEntity)
+	if err != nil {
+		return fmt.Errorf("batch selection algorithm failed for sweep "+
+			"%x: %w", sweep.swapHash[:6], err)
+	}
+
+	// If the best option is to start new batch, do it.
+	if batchId == specialId {
+		return b.launchNewBatch(ctx, sweep)
+	}
+
+	// Locate the batch to add the sweep to.
+	batch, has := b.batches[batchId]
+	if !has {
+		return fmt.Errorf("batch selection algorithm returned "+
+			"batch id %d which doesn't exist, for sweep %x",
+			batchId, sweep.swapHash[:6])
+	}
+
+	// Add the sweep to the batch.
+	accepted, err := batch.addSweep(ctx, sweep)
+	if err != nil {
+		return fmt.Errorf("batch selection algorithm returned "+
+			"batch id %d for sweep %x, but adding failed: %w",
+			batchId, sweep.swapHash[:6], err)
+	}
+	if !accepted {
+		return fmt.Errorf("batch selection algorithm returned "+
+			"batch id %d for sweep %x, but acceptance failed",
+			batchId, sweep.swapHash[:6])
+	}
+
+	return nil
+}
+
+// estimateSweepWeight returns sweep's input weight and one sweep batch weight.
+func estimateSweepWeight(s *sweep) (*entity, *entity, error) {
+	// Create a fake batch with this sweep.
+	batch := &batch{
+		rbfCache: rbfCache{
+			FeeRate: s.minFeeRate,
+		},
+		sweeps: map[lntypes.Hash]sweep{
+			s.swapHash: *s,
+		},
+	}
+
+	// Estimate new batch.
+	newBatch, err := estimateBatchWeight(batch)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// Add the same sweep again to measure weight increments.
+	swapHash2 := s.swapHash
+	swapHash2[0]++
+	batch.sweeps[swapHash2] = *s
+
+	// Estimate weight of a batch with two sweeps.
+	batch2, err := estimateBatchWeight(batch)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// Create entity for sweep.
+	sweepEntity := &entity{
+		FeeRate:        s.minFeeRate,
+		NonCoopHint:    s.nonCoopHint,
+		IsExternalAddr: s.isExternalAddr,
+
+		// Calculate sweep weight as a difference.
+		CoopWeight:    batch2.CoopWeight - newBatch.CoopWeight,
+		NonCoopWeight: batch2.NonCoopWeight - newBatch.NonCoopWeight,
+	}
+
+	return sweepEntity, newBatch, nil
+}
+
+// estimateBatchWeight estimates batch weight.
+func estimateBatchWeight(batch *batch) (*entity, error) {
+	// Make sure the batch is not empty.
+	if len(batch.sweeps) == 0 {
+		return nil, errors.New("empty batch")
+	}
+
+	// Make sure fee rate is valid.
+	if batch.rbfCache.FeeRate < chainfee.AbsoluteFeePerKwFloor {
+		return nil, fmt.Errorf("feeRate is too low: %v",
+			batch.rbfCache.FeeRate)
+	}
+
+	// Find if the batch has at least one non-cooperative sweep.
+	hasNonCoop := false
+	for _, sweep := range batch.sweeps {
+		if sweep.nonCoopHint {
+			hasNonCoop = true
+		}
+	}
+
+	// Find some sweep of the batch. It is used if there is just one sweep.
+	var theSweep sweep
+	for _, sweep := range batch.sweeps {
+		theSweep = sweep
+		break
+	}
+
+	// Find sweep destination address (type) for weight estimations.
+	var destAddr btcutil.Address
+	if theSweep.isExternalAddr {
+		if theSweep.destAddr == nil {
+			return nil, errors.New("isExternalAddr=true, but " +
+				"destAddr is nil")
+		}
+		destAddr = theSweep.destAddr
+	} else {
+		// Assume it is taproot by default.
+		destAddr = (*btcutil.AddressTaproot)(nil)
+	}
+
+	// Make two estimators: for coop and non-coop cases.
+	var coopWeight, nonCoopWeight input.TxWeightEstimator
+
+	// Add output weight to the estimator.
+	err := sweeppkg.AddOutputEstimate(&coopWeight, destAddr)
+	if err != nil {
+		return nil, fmt.Errorf("sweep.AddOutputEstimate: %w", err)
+	}
+	err = sweeppkg.AddOutputEstimate(&nonCoopWeight, destAddr)
+	if err != nil {
+		return nil, fmt.Errorf("sweep.AddOutputEstimate: %w", err)
+	}
+
+	// Add inputs.
+	for _, sweep := range batch.sweeps {
+		// TODO: it should be txscript.SigHashDefault.
+		coopWeight.AddTaprootKeySpendInput(txscript.SigHashAll)
+
+		err = sweep.htlcSuccessEstimator(&nonCoopWeight)
+		if err != nil {
+			return nil, fmt.Errorf("sweep.htlcSuccessEstimator "+
+				"failed: %w", err)
+		}
+	}
+
+	return &entity{
+		BatchId:        batch.id,
+		FeeRate:        batch.rbfCache.FeeRate,
+		CoopWeight:     coopWeight.Weight(),
+		NonCoopWeight:  nonCoopWeight.Weight(),
+		NonCoopHint:    hasNonCoop,
+		IsExternalAddr: theSweep.isExternalAddr,
+	}, nil
+}
+
+// specialId is the value that indicates a new batch. It is returned by
+// selectBatch if the most cost-efficient action is new batch creation.
+const specialId = -1
+
+// entity is either batch of sweep and it holds data important for selection.
+type entity struct {
+	BatchId        int32
+	FeeRate        chainfee.SatPerKWeight
+	CoopWeight     lntypes.WeightUnit
+	NonCoopWeight  lntypes.WeightUnit
+	NonCoopHint    bool
+	IsExternalAddr bool
+}
+
+// fee returns fee of onchain transaction representing this entity.
+func (e entity) fee() btcutil.Amount {
+	var weight lntypes.WeightUnit
+	if e.NonCoopHint {
+		weight = e.NonCoopWeight
+	} else {
+		weight = e.CoopWeight
+	}
+
+	return e.FeeRate.FeeForWeight(weight)
+}
+
+// combine returns new entity, combining properties.
+func (e1 entity) combine(e2 entity) entity {
+	// The fee rate is max of two fee rates.
+	feeRate := e1.FeeRate
+	if feeRate < e2.FeeRate {
+		feeRate = e2.FeeRate
+	}
+
+	return entity{
+		FeeRate:        feeRate,
+		CoopWeight:     e1.CoopWeight + e2.CoopWeight,
+		NonCoopWeight:  e1.NonCoopWeight + e2.NonCoopWeight,
+		NonCoopHint:    e1.NonCoopHint || e2.NonCoopHint,
+		IsExternalAddr: e1.IsExternalAddr || e2.IsExternalAddr,
+	}
+}
+
+// selectBatch returns index of the batch adding to which minimizes costs.
+// For each batch its fee rate and two weight are provided: weight in case of
+// cooperative spending and weight in case non-cooperative spending (using
+// preimages instead of taproot key spend). Also a hint is provided to signal
+// if the batch has to use non-cooperative spending path. The same data is also
+// provided to the sweep for which we are selecting a batch to add. In case of
+// the sweep weights are weight deltas resulted from adding the sweep. Finally,
+// the same data is provided for new batch having this sweep only. The algorithm
+// compares costs of adding the sweep to each existing batch, and costs of new
+// batch creation for this sweep and returns BatchId of the winning batch. If
+// the best option is to create a new batch, it returns specialId. Each entity
+// has also IsExternalAddr flag. There is a rule that sweeps having flag
+// IsExternalAddr must go in individual batches. Cooperative spending is only
+// available if all the sweeps support cooperative spending path.
+func selectBatch(batches []entity, sweep, oneSweepBatch entity) (int32, error) {
+	// Track the best batch to add a sweep to. The default case is new batch
+	// creation with this sweep only in it. The cost is its full fee.
+	bestBatchId := int32(specialId)
+	bestCost := oneSweepBatch.fee()
+
+	// Try to add the sweep to every batch, calculate the costs and
+	// find the batch adding to which results in minimum costs.
+	for _, batch := range batches {
+		// If either the batch or the sweep has IsExternalAddr flag,
+		// the sweep can't be added to the batch, so skip.
+		if batch.IsExternalAddr || sweep.IsExternalAddr {
+			continue
+		}
+
+		// Add the sweep to the batch virtually.
+		combinedBatch := batch.combine(sweep)
+
+		// The cost is fee increase.
+		cost := combinedBatch.fee() - batch.fee()
+
+		// The cost must be positive, because we added a sweep.
+		if cost <= 0 {
+			return 0, fmt.Errorf("got non-positive cost of adding "+
+				"sweep to batch %d: %d", batch.BatchId, cost)
+		}
+
+		// Track the best batch, according to the costs.
+		if bestCost > cost {
+			bestBatchId = batch.BatchId
+			bestCost = cost
+		}
+	}
+
+	return bestBatchId, nil
+}