input: add anchor witness type

input/size.go

@@ -1,8 +1,12 @@
 package input
 
 import (
+	"math/big"
+
 	"github.com/btcsuite/btcd/blockchain"
+	"github.com/btcsuite/btcd/btcec"
 	"github.com/btcsuite/btcd/wire"
+	"github.com/lightningnetwork/lnd/keychain"
 )
 
 const (
@@ -406,6 +410,47 @@ const (
 	OfferedHtlcPenaltyWitnessSize = 1 + 1 + 73 + 1 + 33 + 1 + OfferedHtlcScriptSize
 )
 
+// dummySigner is a fake signer used for size (upper bound) calculations.
+type dummySigner struct {
+	Signer
+}
+
+// SignOutputRaw generates a signature for the passed transaction according to
+// the data within the passed SignDescriptor.
+func (s *dummySigner) SignOutputRaw(tx *wire.MsgTx, signDesc *SignDescriptor) (
+	[]byte, error) {
+
+	// Always return worst-case signature length, excluding the one byte
+	// sighash flag.
+	return make([]byte, 73-1), nil
+}
+
+var (
+	// dummyPubKey is a pubkey used in script size calculation.
+	dummyPubKey = btcec.PublicKey{
+		X: &big.Int{},
+		Y: &big.Int{},
+	}
+
+	// dummyAnchorScript is a script used for size calculation.
+	dummyAnchorScript, _ = CommitScriptAnchor(&dummyPubKey)
+
+	// dummyAnchorWitness is a witness used for size calculation.
+	dummyAnchorWitness, _ = CommitSpendAnchor(
+		&dummySigner{},
+		&SignDescriptor{
+			KeyDesc: keychain.KeyDescriptor{
+				PubKey: &dummyPubKey,
+			},
+			WitnessScript: dummyAnchorScript,
+		},
+		nil,
+	)
+
+	// AnchorWitnessSize 116 bytes
+	AnchorWitnessSize = dummyAnchorWitness.SerializeSize()
+)
+
 // EstimateCommitTxWeight estimate commitment transaction weight depending on
 // the precalculated weight of base transaction, witness data, which is needed
 // for paying for funding tx, and htlc weight multiplied by their count.

input/size_test.go

@@ -280,3 +280,11 @@ func TestTxWeightEstimator(t *testing.T) {
 		}
 	}
 }
+
+// TestSizes guards calculated constants to make sure their values remain
+// unchanged.
+func TestSizes(t *testing.T) {
+	if input.AnchorWitnessSize != 116 {
+		t.Fatal("unexpected anchor witness size")
+	}
+}

input/witnessgen.go

@@ -125,6 +125,10 @@ const (
 	// output on the counterparty's commitment transaction after a
 	// confirmation.
 	CommitmentToRemoteConfirmed StandardWitnessType = 13
+
+	// CommitmentAnchor is a witness that allows us to spend our anchor on
+	// the commitment transaction.
+	CommitmentAnchor StandardWitnessType = 14
 )
 
 // String returns a human readable version of the target WitnessType.
@@ -138,6 +142,9 @@ func (wt StandardWitnessType) String() string {
 	case CommitmentToRemoteConfirmed:
 		return "CommitmentToRemoteConfirmed"
 
+	case CommitmentAnchor:
+		return "CommitmentAnchor"
+
 	case CommitmentNoDelay:
 		return "CommitmentNoDelay"
 
@@ -218,6 +225,16 @@ func (wt StandardWitnessType) WitnessGenerator(signer Signer,
 				Witness: witness,
 			}, nil
 
+		case CommitmentAnchor:
+			witness, err := CommitSpendAnchor(signer, desc, tx)
+			if err != nil {
+				return nil, err
+			}
+
+			return &Script{
+				Witness: witness,
+			}, nil
+
 		case CommitmentNoDelay:
 			witness, err := CommitSpendNoDelay(signer, desc, tx, false)
 			if err != nil {
@@ -348,6 +365,10 @@ func (wt StandardWitnessType) SizeUpperBound() (int, bool, error) {
 	case CommitmentToRemoteConfirmed:
 		return ToRemoteConfirmedWitnessSize, false, nil
 
+	// Anchor output on the commitment transaction.
+	case CommitmentAnchor:
+		return AnchorWitnessSize, false, nil
+
 	// Outgoing second layer HTLC's that have confirmed within the
 	// chain, and the output they produced is now mature enough to
 	// sweep.