swap: add locking conditions to HtlcScript interface

Use of the Script() function is problematic when we introduce taproot
because our script will vary depending whether we use keyspend or a
tapleaf spend path (and on the tapleaf spent). This has not previously
been a problem for segwitv0 scripts, because they contain all of the
logical branches for each of our spend conditions in a single script.

This commit prepares for removal of the Script() function by moving
our address/pkScript/sigScript generation (which need Script()) into
each script's implementation of the HtlcScript interface so that
they have access to the script directly.

Author: carlaKC

swap/htlc.go

@@ -65,6 +65,11 @@ type HtlcScript interface {
 	// Script returns the htlc script.
 	Script() []byte
 
+	// lockingConditions return the address, pkScript and sigScript (if
+	// required) for a htlc script.
+	lockingConditions(HtlcOutputType, *chaincfg.Params) (btcutil.Address,
+		[]byte, []byte, error)
+
 	// MaxSuccessWitnessSize returns the maximum witness size for the
 	// success case witness.
 	MaxSuccessWitnessSize() int
@@ -190,14 +195,36 @@ func NewHtlc(version ScriptVersion, cltvExpiry int32,
 		return nil, err
 	}
 
-	var pkScript, sigScript []byte
-	var address btcutil.Address
+	address, pkScript, sigScript, err := htlc.lockingConditions(
+		outputType, chainParams,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("could not get address: %w", err)
+	}
+
+	return &Htlc{
+		HtlcScript:  htlc,
+		Hash:        hash,
+		Version:     version,
+		PkScript:    pkScript,
+		OutputType:  outputType,
+		ChainParams: chainParams,
+		Address:     address,
+		SigScript:   sigScript,
+	}, nil
+}
+
+// segwitV0LockingConditions provides the address, pkScript and sigScript (if
+// required) for the segwit v0 script and output type provided.
+func segwitV0LockingConditions(outputType HtlcOutputType,
+	chainParams *chaincfg.Params, script []byte) (btcutil.Address,
+	[]byte, []byte, error) {
 
 	switch outputType {
 	case HtlcNP2WSH:
-		p2wshPkScript, err := input.WitnessScriptHash(htlc.Script())
+		p2wshPkScript, err := input.WitnessScriptHash(script)
 		if err != nil {
-			return nil, err
+			return nil, nil, nil, err
 		}
 
 		// Generate p2sh script for p2wsh (nested).
@@ -210,78 +237,54 @@ func NewHtlc(version ScriptVersion, cltvExpiry int32,
 		builder.AddData(hash160)
 		builder.AddOp(txscript.OP_EQUAL)
 
-		pkScript, err = builder.Script()
+		pkScript, err := builder.Script()
 		if err != nil {
-			return nil, err
+			return nil, nil, nil, err
 		}
 
 		// Generate a valid sigScript that will allow us to spend the
 		// p2sh output. The sigScript will contain only a single push of
 		// the p2wsh witness program corresponding to the matching
 		// public key of this address.
-		sigScript, err = txscript.NewScriptBuilder().
+		sigScript, err := txscript.NewScriptBuilder().
 			AddData(p2wshPkScript).
 			Script()
 		if err != nil {
-			return nil, err
+			return nil, nil, nil, err
 		}
 
-		address, err = btcutil.NewAddressScriptHash(
+		address, err := btcutil.NewAddressScriptHash(
 			p2wshPkScript, chainParams,
 		)
 		if err != nil {
-			return nil, err
+			return nil, nil, nil, err
 		}
 
+		return address, pkScript, sigScript, nil
+
 	case HtlcP2WSH:
-		pkScript, err = input.WitnessScriptHash(htlc.Script())
+		pkScript, err := input.WitnessScriptHash(script)
 		if err != nil {
-			return nil, err
+			return nil, nil, nil, err
 		}
 
-		address, err = btcutil.NewAddressWitnessScriptHash(
+		address, err := btcutil.NewAddressWitnessScriptHash(
 			pkScript[2:],
 			chainParams,
 		)
 		if err != nil {
-			return nil, err
-		}
-
-	case HtlcP2TR:
-		// Confirm we have a v3 htlc.
-		trHtlc, ok := htlc.(*HtlcScriptV3)
-		if !ok {
-			return nil, ErrInvalidOutputSelected
-		}
-
-		// Generate a tapscript address from our HTLC's taptree.
-		address, err = btcutil.NewAddressTaproot(
-			schnorr.SerializePubKey(trHtlc.TaprootKey), chainParams,
-		)
-		if err != nil {
-			return nil, err
+			return nil, nil, nil, err
 		}
 
-		// Generate locking script.
-		pkScript, err = txscript.PayToAddrScript(address)
-		if err != nil {
-			return nil, err
-		}
+		// Pay to witness script hash (segwit v0) does not need a
+		// sigScript (we provide it in the witness instead), so we
+		// return nil for our sigScript.
+		return address, pkScript, nil, nil
 
 	default:
-		return nil, errors.New("unknown output type")
+		return nil, nil, nil, fmt.Errorf("unexpected output type: %d",
+			outputType)
 	}
-
-	return &Htlc{
-		HtlcScript:  htlc,
-		Hash:        hash,
-		Version:     version,
-		PkScript:    pkScript,
-		OutputType:  outputType,
-		ChainParams: chainParams,
-		Address:     address,
-		SigScript:   sigScript,
-	}, nil
 }
 
 // GenSuccessWitness returns the success script to spend this htlc with
@@ -491,6 +494,14 @@ func (h *HtlcScriptV1) SuccessSequence() uint32 {
 	return 0
 }
 
+// lockingConditions return the address, pkScript and sigScript (if
+// required) for a htlc script.
+func (h *HtlcScriptV1) lockingConditions(htlcOutputType HtlcOutputType,
+	params *chaincfg.Params) (btcutil.Address, []byte, []byte, error) {
+
+	return segwitV0LockingConditions(htlcOutputType, params, h.script)
+}
+
 // HtlcScriptV2 encapsulates the htlc v2 script.
 type HtlcScriptV2 struct {
 	script    []byte
@@ -625,6 +636,14 @@ func (h *HtlcScriptV2) SuccessSequence() uint32 {
 	return 1
 }
 
+// lockingConditions return the address, pkScript and sigScript (if
+// required) for a htlc script.
+func (h *HtlcScriptV2) lockingConditions(htlcOutputType HtlcOutputType,
+	params *chaincfg.Params) (btcutil.Address, []byte, []byte, error) {
+
+	return segwitV0LockingConditions(htlcOutputType, params, h.script)
+}
+
 // HtlcScriptV3 encapsulates the htlc v3 script.
 type HtlcScriptV3 struct {
 	// The final locking script for the timeout path which is available to
@@ -861,3 +880,34 @@ func (h *HtlcScriptV3) MaxTimeoutWitnessSize() int {
 func (h *HtlcScriptV3) SuccessSequence() uint32 {
 	return 1
 }
+
+// lockingConditions return the address, pkScript and sigScript (if required)
+// for a htlc script.
+func (h *HtlcScriptV3) lockingConditions(outputType HtlcOutputType,
+	chainParams *chaincfg.Params) (btcutil.Address, []byte, []byte, error) {
+
+	// HtlcV3 can only have taproot output type, because we utilize
+	// tapscript claim paths.
+	if outputType != HtlcP2TR {
+		return nil, nil, nil, fmt.Errorf("htlc v3 only supports P2TR "+
+			"outputs, got: %v", outputType)
+	}
+
+	// Generate a tapscript address from our tree.
+	address, err := btcutil.NewAddressTaproot(
+		schnorr.SerializePubKey(h.TaprootKey), chainParams,
+	)
+	if err != nil {
+		return nil, nil, nil, err
+	}
+
+	// Generate locking script.
+	pkScript, err := txscript.PayToAddrScript(address)
+	if err != nil {
+		return nil, nil, nil, err
+	}
+
+	// Taproot (segwit v1) does not need a sigScript (we provide it in the
+	// witness instead), so we return nil for our sigScript.
+	return address, pkScript, nil, nil
+}