input+sweep: let weightestimator take known TxOut

input/size.go

@@ -533,6 +533,14 @@ func (twe *TxWeightEstimator) AddNestedP2WSHInput(witnessSize int) *TxWeightEsti
 	return twe
 }
 
+// AddTxOutput adds a known TxOut to the weight estimator.
+func (twe *TxWeightEstimator) AddTxOutput(txOut *wire.TxOut) *TxWeightEstimator {
+	twe.outputSize += txOut.SerializeSize()
+	twe.outputCount++
+
+	return twe
+}
+
 // AddP2PKHOutput updates the weight estimate to account for an additional P2PKH
 // output.
 func (twe *TxWeightEstimator) AddP2PKHOutput() *TxWeightEstimator {

sweep/weight_estimator.go

@@ -2,6 +2,7 @@ package sweep
 
 import (
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
+	"github.com/btcsuite/btcd/wire"
 	"github.com/btcsuite/btcutil"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
@@ -92,6 +93,12 @@ func (w *weightEstimator) addP2WKHOutput() {
 	w.estimator.AddP2WKHOutput()
 }
 
+// addOutput updates the weight estimate to account for the known
+// output given.
+func (w *weightEstimator) addOutput(txOut *wire.TxOut) {
+	w.estimator.AddTxOutput(txOut)
+}
+
 // weight gets the estimated weight of the transaction.
 func (w *weightEstimator) weight() int {
 	return w.estimator.Weight()

sweep/weight_estimator_test.go

@@ -3,7 +3,10 @@ package sweep
 import (
 	"testing"
 
+	"github.com/btcsuite/btcd/chaincfg"
+	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
+	"github.com/btcsuite/btcutil"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 	"github.com/stretchr/testify/require"
@@ -77,3 +80,32 @@ func TestWeightEstimator(t *testing.T) {
 
 	require.Equal(t, expectedFee, w.fee())
 }
+
+// TestWeightEstimatorAddOutput tests that adding the raw P2WKH output to the
+// estimator yield the same result as an estimated add.
+func TestWeightEstimatorAddOutput(t *testing.T) {
+	testFeeRate := chainfee.SatPerKWeight(20000)
+
+	p2wkhAddr, err := btcutil.NewAddressWitnessPubKeyHash(
+		make([]byte, 20), &chaincfg.MainNetParams,
+	)
+	require.NoError(t, err)
+
+	p2wkhScript, err := txscript.PayToAddrScript(p2wkhAddr)
+	require.NoError(t, err)
+
+	// Create two estimators, add the raw P2WKH out to one.
+	txOut := &wire.TxOut{
+		PkScript: p2wkhScript,
+		Value:    10000,
+	}
+
+	w1 := newWeightEstimator(testFeeRate)
+	w1.addOutput(txOut)
+
+	w2 := newWeightEstimator(testFeeRate)
+	w2.addP2WKHOutput()
+
+	// Estimate hhould be the same.
+	require.Equal(t, w1.weight(), w2.weight())
+}