tapfreighter+tapscript: replace weight estimator

In this commit, we remove the amount of tap-specific code handling fee
estimation by replacing EstimateWeight() with EstimateFee(). This is a
thin wrapper around txsizes.EstimateVirttualSize(), and is very similar
to the implementation in txauthor/NewUnsignedTransaction.

Author: jharveyb

go.mod

@@ -10,6 +10,8 @@ require (
 	github.com/btcsuite/btcd/chaincfg/chainhash v1.0.2
 	github.com/btcsuite/btclog v0.0.0-20170628155309-84c8d2346e9f
 	github.com/btcsuite/btcwallet v0.16.10-0.20231017144732-e3ff37491e9c
+	github.com/btcsuite/btcwallet/wallet/txrules v1.2.0
+	github.com/btcsuite/btcwallet/wallet/txsizes v1.2.3
 	github.com/caddyserver/certmagic v0.17.2
 	github.com/davecgh/go-spew v1.1.1
 	github.com/decred/dcrd/dcrec/secp256k1/v4 v4.0.1
@@ -60,8 +62,6 @@ require (
 	github.com/andybalholm/brotli v1.0.3 // indirect
 	github.com/beorn7/perks v1.0.1 // indirect
 	github.com/btcsuite/btcwallet/wallet/txauthor v1.3.2 // indirect
-	github.com/btcsuite/btcwallet/wallet/txrules v1.2.0 // indirect
-	github.com/btcsuite/btcwallet/wallet/txsizes v1.2.3 // indirect
 	github.com/btcsuite/btcwallet/walletdb v1.4.0 // indirect
 	github.com/btcsuite/btcwallet/wtxmgr v1.5.0 // indirect
 	github.com/btcsuite/go-socks v0.0.0-20170105172521-4720035b7bfd // indirect

tapfreighter/wallet.go

@@ -18,6 +18,7 @@ import (
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
+	"github.com/btcsuite/btcwallet/wallet/txrules"
 	"github.com/davecgh/go-spew/spew"
 	"github.com/lightninglabs/taproot-assets/address"
 	"github.com/lightninglabs/taproot-assets/asset"
@@ -28,7 +29,6 @@ import (
 	"github.com/lightninglabs/taproot-assets/tapgarden"
 	"github.com/lightninglabs/taproot-assets/tappsbt"
 	"github.com/lightninglabs/taproot-assets/tapscript"
-	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 )
@@ -1654,61 +1654,30 @@ func addAnchorPsbtInputs(btcPkt *psbt.Packet, vPkt *tappsbt.VPacket,
 	}
 
 	// Now that we've added an extra input, we'll want to re-calculate the
-	// total weight of the transaction, so we can ensure we're paying
-	// enough in fees.
-	var (
-		weightEstimator     input.TxWeightEstimator
-		inputAmt, outputAmt int64
-	)
-	for _, pIn := range btcPkt.Inputs {
-		inputAmt += pIn.WitnessUtxo.Value
-
-		inputPkScript := pIn.WitnessUtxo.PkScript
-		switch {
-		case txscript.IsPayToWitnessPubKeyHash(inputPkScript):
-			weightEstimator.AddP2WKHInput()
-
-		case txscript.IsPayToScriptHash(inputPkScript):
-			weightEstimator.AddNestedP2WKHInput()
-
-		case txscript.IsPayToTaproot(inputPkScript):
-			weightEstimator.AddTaprootKeySpendInput(
-				txscript.SigHashDefault,
-			)
-		default:
-			return fmt.Errorf("unknown pkScript: %x",
-				inputPkScript)
+	// total vsize of the transaction and the necessary fee at the desired
+	// fee rate.
+	inputScripts := make([][]byte, 0, len(btcPkt.Inputs))
+	for inputIdx, input := range btcPkt.Inputs {
+		if input.WitnessUtxo == nil {
+			return fmt.Errorf("PSBT input %d doesn't specify "+
+				"witness UTXO, which is not supported",
+				inputIdx)
 		}
-	}
-	for _, txOut := range btcPkt.UnsignedTx.TxOut {
-		outputAmt += txOut.Value
 
-		addrType, _, _, err := txscript.ExtractPkScriptAddrs(
-			txOut.PkScript, params,
-		)
-		if err != nil {
-			return err
+		if len(input.WitnessUtxo.PkScript) == 0 {
+			return fmt.Errorf("input %d on psbt missing "+
+				"pkscript", inputIdx)
 		}
 
-		switch addrType {
-		case txscript.WitnessV0PubKeyHashTy:
-			weightEstimator.AddP2WKHOutput()
-
-		case txscript.WitnessV0ScriptHashTy:
-			weightEstimator.AddP2WSHOutput()
-
-		case txscript.WitnessV1TaprootTy:
-			weightEstimator.AddP2TROutput()
-		default:
-			return fmt.Errorf("unknown pkscript: %x",
-				txOut.PkScript)
-		}
+		inputScripts = append(inputScripts, input.WitnessUtxo.PkScript)
 	}
 
-	// With this, we can now calculate the total fee we need to pay. We'll
-	// also make sure to round up the required fee to the floor.
-	totalWeight := int64(weightEstimator.Weight())
-	requiredFee := feeRate.FeeForWeight(totalWeight)
+	estimatedSize, requiredFee := tapscript.EstimateFee(
+		inputScripts, btcPkt.UnsignedTx.TxOut, feeRate,
+	)
+	log.Infof("Estimated TX vsize: %d", estimatedSize)
+	log.Infof("TX required fee before change adjustment: %d at feerate "+
+		"%d sat/vB", requiredFee, feeRate.FeePerKVByte()/1000)
 
 	// Given the current fee (which doesn't account for our input) and the
 	// total fee we want to pay, we'll adjust the wallet's change output
@@ -1729,8 +1698,8 @@ func addAnchorPsbtInputs(btcPkt *psbt.Packet, vPkt *tappsbt.VPacket,
 	// The fee may exceed the total value of the change output, which means
 	// this spend is impossible with the given inputs and fee rate.
 	if changeValue-feeDelta < 0 {
-		return fmt.Errorf("fee of %d sats exceeds change amount of %d"+
-			"sats", requiredFee, changeValue)
+		return fmt.Errorf("fee exceeds change amount: (fee=%d, "+
+			"change=%d) ", requiredFee, changeValue)
 	}
 
 	btcPkt.UnsignedTx.TxOut[lastIdx].Value -= feeDelta

tapscript/util.go

@@ -3,9 +3,13 @@ package tapscript
 import (
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/btcec/v2/schnorr"
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/txscript"
+	"github.com/btcsuite/btcd/wire"
+	"github.com/btcsuite/btcwallet/wallet/txsizes"
 	"github.com/lightninglabs/taproot-assets/commitment"
+	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 )
 
 // PayToAddrScript constructs a P2TR script that embeds a Taproot Asset
@@ -31,3 +35,40 @@ func PayToTaprootScript(taprootKey *btcec.PublicKey) ([]byte, error) {
 		AddData(schnorr.SerializePubKey(taprootKey)).
 		Script()
 }
+
+// EstimateFee provides a worst-case fee and vsize estimate for a transaction
+// built from the given inputs and outputs. This mirrors the fee estimation
+// implemented in btcwallet/wallet/txauthor/author.go:NewUnsignedTransaction()
+// EstimateFee assumes that a change output (or a dummy output for change) is
+// included in the set of given outputs.
+func EstimateFee(inputScripts [][]byte, outputs []*wire.TxOut,
+	feeRate chainfee.SatPerKWeight) (int, btcutil.Amount) {
+
+	// Count the types of input scripts.
+	var nested, p2wpkh, p2tr, p2pkh int
+	for _, pkScript := range inputScripts {
+		switch {
+		// If this is a p2sh output, we assume this is a
+		// nested P2WKH.
+		case txscript.IsPayToScriptHash(pkScript):
+			nested++
+		case txscript.IsPayToWitnessPubKeyHash(pkScript):
+			p2wpkh++
+		case txscript.IsPayToTaproot(pkScript):
+			p2tr++
+		default:
+			p2pkh++
+		}
+	}
+
+	// The change output is already in the set of given outputs, so we don't
+	// need to account for an additional output.
+	maxSignedSize := txsizes.EstimateVirtualSize(
+		p2pkh, p2tr, p2wpkh, nested, outputs, 0,
+	)
+	maxRequiredFee := feeRate.FeePerKVByte().FeeForVSize(
+		int64(maxSignedSize),
+	)
+
+	return maxSignedSize, maxRequiredFee
+}