psbt: implement PSBTv2 (BIP-370) support

psbt/creator.go

@@ -18,9 +18,8 @@ const MinTxVersion = 1
 // input) and transaction version (must be 1 of 2) must be specified here. Note
 // that the default nSequence value is wire.MaxTxInSequenceNum.  Referencing
 // the PSBT BIP, this function serves the roles of the Creator.
-func New(inputs []*wire.OutPoint,
-	outputs []*wire.TxOut, version int32, nLockTime uint32,
-	nSequences []uint32) (*Packet, error) {
+func New(inputs []*wire.OutPoint, outputs []*wire.TxOut, version int32,
+	nLockTime uint32, nSequences []uint32) (*Packet, error) {
 
 	// Create the new struct; the input and output lists will be empty, the
 	// unsignedTx object must be constructed and serialized, and that
@@ -50,6 +49,9 @@ func New(inputs []*wire.OutPoint,
 	// two lists, and each one must be of length matching the unsigned
 	// transaction; the unknown list can be nil.
 	pInputs := make([]PInput, len(unsignedTx.TxIn))
+	for i := range pInputs {
+		pInputs[i].Sequence = nSequences[i]
+	}
 	pOutputs := make([]POutput, len(unsignedTx.TxOut))
 
 	// This new Psbt is "raw" and contains no key-value fields, so sanity
@@ -61,3 +63,68 @@ func New(inputs []*wire.OutPoint,
 		Unknowns:   nil,
 	}, nil
 }
+
+// NewV2 creates a new PSBTv2 Packet.
+func NewV2(txVersion int32, fallbackLocktime *uint32,
+	txModifiable *uint8) (*Packet, error) {
+
+	if txVersion < 2 {
+		return nil, ErrV2TxVersionBelowTwo
+	}
+
+	// For V2, UnsignedTx must be nil and TxVersion is explicitly required.
+	return &Packet{
+		Version:          PsbtVersion2,
+		TxVersion:        txVersion,
+		FallbackLocktime: fallbackLocktime,
+		TxModifiable:     txModifiable,
+		Inputs:           []PInput{},
+		Outputs:          []POutput{},
+		XPubs:            nil,
+		Unknowns:         nil,
+	}, nil
+}
+
+// AddInputV2 appends a new PInput to a Version 2 PSBT, incrementing the
+// internal count. It returns an error if the PSBT is not Version 2. As the
+// Constructor, it enforces the PSBT_GLOBAL_TX_MODIFIABLE Inputs-Modifiable flag
+// (Bit 0) per BIP 370.
+func (p *Packet) AddInputV2(input PInput) error {
+	switch p.Version {
+	case PsbtVersion2:
+		// Valid, continue to add the input.
+
+	default:
+		return ErrUnsupportedDynamicAdd
+	}
+
+	// Check the TxModifiable rule before adding
+	if p.TxModifiable != nil && *p.TxModifiable&FlagInputsModifiable == 0 {
+		return ErrInputsNotModifiable
+	}
+	p.Inputs = append(p.Inputs, input)
+	p.InputCount = uint32(len(p.Inputs))
+	return nil
+}
+
+// AddOutputV2 appends a new POutput to a Version 2 PSBT, incrementing the
+// internal count. It returns an error if the PSBT is not Version 2.
+// As the Constructor, it enforces the PSBT_GLOBAL_TX_MODIFIABLE
+// Outputs-Modifiable flag (Bit 1) per BIP 370.
+func (p *Packet) AddOutputV2(output POutput) error {
+	switch p.Version {
+	case PsbtVersion2:
+		// Valid, continue to add the output.
+
+	default:
+		return ErrUnsupportedDynamicAdd
+	}
+
+	// Check the TxModifiable rule before adding
+	if p.TxModifiable != nil && *p.TxModifiable&FlagOutputsModifiable == 0 {
+		return ErrOutputsNotModifiable
+	}
+	p.Outputs = append(p.Outputs, output)
+	p.OutputCount = uint32(len(p.Outputs))
+	return nil
+}

psbt/extractor.go

@@ -27,9 +27,13 @@ func Extract(p *Packet) (*wire.MsgTx, error) {
 		return nil, ErrIncompletePSBT
 	}
 
-	// First, we'll make a copy of the underlying unsigned transaction (the
-	// initial template) so we don't mutate it during our activates below.
-	finalTx := p.UnsignedTx.Copy()
+	// First, we'll get a fresh copy of the underlying unsigned transaction
+	// (the initial template) so we don't mutate it during our activates
+	// below.
+	finalTx, err := p.GetUnsignedTx()
+	if err != nil {
+		return nil, err
+	}
 
 	// For each input, we'll now populate any relevant witness and
 	// sigScript data.

psbt/finalizer.go

@@ -83,7 +83,10 @@ func isFinalizableWitnessInput(pInput *PInput) bool {
 			if pInput.WitnessScript == nil {
 				return false
 			}
-		} else if txscript.IsPayToWitnessPubKeyHash(pInput.RedeemScript) {
+		} else if txscript.IsPayToWitnessPubKeyHash(
+			pInput.RedeemScript,
+		) {
+
 			if pInput.WitnessScript != nil {
 				return false
 			}
@@ -111,8 +114,15 @@ func isFinalizableLegacyInput(p *Packet, pInput *PInput, inIndex int) bool {
 
 	// Otherwise, we'll verify that we only have a RedeemScript if the prev
 	// output script is P2SH.
-	outIndex := p.UnsignedTx.TxIn[inIndex].PreviousOutPoint.Index
-	if txscript.IsPayToScriptHash(pInput.NonWitnessUtxo.TxOut[outIndex].PkScript) {
+	if p.Version == PsbtVersion0 && p.UnsignedTx == nil {
+		return false
+	}
+	outIndex := p.outIndex(inIndex)
+
+	if txscript.IsPayToScriptHash(
+		pInput.NonWitnessUtxo.TxOut[outIndex].PkScript,
+	) {
+
 		if pInput.RedeemScript == nil {
 			return false
 		}
@@ -186,7 +196,7 @@ func MaybeFinalize(p *Packet, inIndex int) (bool, error) {
 // MaybeFinalizeAll attempts to finalize all inputs of the psbt.Packet that are
 // not already finalized, and returns an error if it fails to do so.
 func MaybeFinalizeAll(p *Packet) error {
-	for i := range p.UnsignedTx.TxIn {
+	for i := range p.Inputs {
 		success, err := MaybeFinalize(p, i)
 		if err != nil || !success {
 			return err
@@ -351,6 +361,9 @@ func finalizeNonWitnessInput(p *Packet, inIndex int) error {
 	newInput := NewPsbtInput(pInput.NonWitnessUtxo, nil)
 	newInput.FinalScriptSig = sigScript
 
+	// Preserve required PSBTv2 fields and unknowns as mandated by BIP-370
+	newInput.CopyInputFields(&pInput)
+
 	// Overwrite the entry in the input list at the correct index. Note
 	// that this removes all the other entries in the list for this input
 	// index.
@@ -493,6 +506,9 @@ func finalizeWitnessInput(p *Packet, inIndex int) error {
 
 	newInput.FinalScriptWitness = serializedWitness
 
+	// Preserve required PSBTv2 fields and unknowns as mandated by BIP-370
+	newInput.CopyInputFields(&pInput)
+
 	// Finally, we overwrite the entry in the input list at the correct
 	// index.
 	p.Inputs[inIndex] = *newInput
@@ -556,7 +572,10 @@ func finalizeTaprootInput(p *Packet, inIndex int) error {
 		for idx, scriptSpendSig := range pInput.TaprootScriptSpendSig {
 			// Make sure that if there are indeed multiple
 			// signatures, they all reference the same leaf hash.
-			if !bytes.Equal(scriptSpendSig.LeafHash, targetLeafHash) {
+			if !bytes.Equal(
+				scriptSpendSig.LeafHash, targetLeafHash,
+			) {
+
 				return fmt.Errorf("script spend signature %d "+
 					"references different target leaf "+
 					"hash than first signature; only one "+
@@ -590,6 +609,9 @@ func finalizeTaprootInput(p *Packet, inIndex int) error {
 	newInput := NewPsbtInput(nil, pInput.WitnessUtxo)
 	newInput.FinalScriptWitness = serializedWitness
 
+	// Preserve required PSBTv2 fields and unknowns as mandated by BIP-370
+	newInput.CopyInputFields(pInput)
+
 	// Finally, we overwrite the entry in the input list at the correct
 	// index.
 	p.Inputs[inIndex] = *newInput