input: add anchor witness type

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.