lnwallet+sweep: extend the WitnessGenerator type to return an InputScript

In this commit, we extend the WitnessGenerator type to now return an
InputScript. This allows it to be more encompassing, as now callers can
expect a sigScript to be populated if the input being swept requires a
sigScript field.

Along the way, we've also renamed input.BuildWitness to
input.CraftInputScript.  We also take a step towards allowing the
sweeper to sweep transactions for n2pwkh outputs. We do so by modifying
the BuiltWitness method to instead return an InputScript. Additionally,
when populating inputs if a sigScript is present, it will now be
populated.

lnwallet/witnessgen.go

@@ -110,18 +110,22 @@ func (wt WitnessType) String() string {
 }
 
 // WitnessGenerator represents a function which is able to generate the final
-// witness for a particular public key script. This function acts as an
-// abstraction layer, hiding the details of the underlying script.
+// witness for a particular public key script. Additionally, if required, this
+// function will also return the sigScript for spending nested P2SH witness
+// outputs. This function acts as an abstraction layer, hiding the details of
+// the underlying script.
 type WitnessGenerator func(tx *wire.MsgTx, hc *txscript.TxSigHashes,
-	inputIndex int) ([][]byte, error)
+	inputIndex int) (*InputScript, error)
 
-// GenWitnessFunc will return a WitnessGenerator function that an output
-// uses to generate the witness for a sweep transaction.
+// GenWitnessFunc will return a WitnessGenerator function that an output uses
+// to generate the witness and optionally the sigScript for a sweep
+// transaction. The sigScript will be generated if the witness type warrants
+// one for spending, such as the NestedWitnessKeyHash witness type.
 func (wt WitnessType) GenWitnessFunc(signer Signer,
 	descriptor *SignDescriptor) WitnessGenerator {
 
 	return func(tx *wire.MsgTx, hc *txscript.TxSigHashes,
-		inputIndex int) ([][]byte, error) {
+		inputIndex int) (*InputScript, error) {
 
 		desc := descriptor
 		desc.SigHashes = hc
@@ -129,34 +133,98 @@ func (wt WitnessType) GenWitnessFunc(signer Signer,
 
 		switch wt {
 		case CommitmentTimeLock:
-			return CommitSpendTimeout(signer, desc, tx)
+			witness, err := CommitSpendTimeout(signer, desc, tx)
+			if err != nil {
+				return nil, err
+			}
+
+			return &InputScript{
+				Witness: witness,
+			}, nil
 
 		case CommitmentNoDelay:
-			return CommitSpendNoDelay(signer, desc, tx)
+			witness, err := CommitSpendNoDelay(signer, desc, tx)
+			if err != nil {
+				return nil, err
+			}
+
+			return &InputScript{
+				Witness: witness,
+			}, nil
 
 		case CommitmentRevoke:
-			return CommitSpendRevoke(signer, desc, tx)
+			witness, err := CommitSpendRevoke(signer, desc, tx)
+			if err != nil {
+				return nil, err
+			}
+
+			return &InputScript{
+				Witness: witness,
+			}, nil
 
 		case HtlcOfferedRevoke:
-			return ReceiverHtlcSpendRevoke(signer, desc, tx)
+			witness, err := ReceiverHtlcSpendRevoke(signer, desc, tx)
+			if err != nil {
+				return nil, err
+			}
+
+			return &InputScript{
+				Witness: witness,
+			}, nil
 
 		case HtlcAcceptedRevoke:
-			return SenderHtlcSpendRevoke(signer, desc, tx)
+			witness, err := SenderHtlcSpendRevoke(signer, desc, tx)
+			if err != nil {
+				return nil, err
+			}
+
+			return &InputScript{
+				Witness: witness,
+			}, nil
 
 		case HtlcOfferedTimeoutSecondLevel:
-			return HtlcSecondLevelSpend(signer, desc, tx)
+			witness, err := HtlcSecondLevelSpend(signer, desc, tx)
+			if err != nil {
+				return nil, err
+			}
+
+			return &InputScript{
+				Witness: witness,
+			}, nil
 
 		case HtlcAcceptedSuccessSecondLevel:
-			return HtlcSecondLevelSpend(signer, desc, tx)
+			witness, err := HtlcSecondLevelSpend(signer, desc, tx)
+			if err != nil {
+				return nil, err
+			}
+
+			return &InputScript{
+				Witness: witness,
+			}, nil
 
 		case HtlcOfferedRemoteTimeout:
 			// We pass in a value of -1 for the timeout, as we
 			// expect the caller to have already set the lock time
 			// value.
-			return receiverHtlcSpendTimeout(signer, desc, tx, -1)
+			witness, err := receiverHtlcSpendTimeout(signer, desc, tx, -1)
+			if err != nil {
+				return nil, err
+			}
+
+			return &InputScript{
+				Witness: witness,
+			}, nil
 
 		case HtlcSecondLevelRevoke:
-			return htlcSpendRevoke(signer, desc, tx)
+			witness, err := htlcSpendRevoke(signer, desc, tx)
+			if err != nil {
+				return nil, err
+			}
+
+			return &InputScript{
+				Witness: witness,
+			}, nil
+
 
 		default:
 			return nil, fmt.Errorf("unknown witness type: %v", wt)