psbt: implement PSBTv2 (BIP-370) support

btcutil/psbt/creator.go

@@ -5,6 +5,8 @@
 package psbt
 
 import (
+	"errors"
+
 	"github.com/btcsuite/btcd/wire"
 )
 
@@ -50,6 +52,9 @@ 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 +66,62 @@ func New(inputs []*wire.OutPoint,
 		Unknowns:   nil,
 	}, nil
 }
+
+// NewV2 creates a new, empty Packet that is pre-configured to adhere to the
+// BIP-0370 PSBT Version 2 specification.
+func NewV2(txVersion uint32, fallbackLocktime uint32, txModifiable uint8) (*Packet, error) {
+
+	if txVersion < 2 {
+		return nil, errors.New("PSBTv2 requires a transaction version of at least 2")
+	}
+	return &Packet{
+		Version:          2,
+		TxVersion:        txVersion,
+		FallbackLocktime: fallbackLocktime,
+		TxModifiable:     txModifiable,
+		Inputs:           nil,
+		Outputs:          nil,
+		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.
+func (p *Packet) AddInputV2(input PInput) error {
+	if p.Version != 2 {
+		return errors.New("cannot dynamically add inputs to a non-v2 PSBT")
+	}
+	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.
+func (p *Packet) AddOutputV2(output POutput) error {
+	if p.Version != 2 {
+		return errors.New("cannot dynamically add outputs to a non-v2 PSBT")
+	}
+	p.Outputs = append(p.Outputs, output)
+	p.OutputCount = uint32(len(p.Outputs))
+	return nil
+}
+
+// AddInput adds a new input to a Version 2 PSBT using a standard wire.OutPoint.
+func (p *Packet) AddInput(outPoint wire.OutPoint, sequence uint32) error {
+	return p.AddInputV2(PInput{
+		PreviousTxid: outPoint.Hash[:],
+		OutputIndex:  outPoint.Index,
+		Sequence:     sequence,
+	})
+}
+
+// AddOutput adds a new output to a Version 2 PSBT using a standard amount and
+// script.
+func (p *Packet) AddOutput(amount int64, script []byte) error {
+	return p.AddOutputV2(POutput{
+		Amount: amount,
+		Script: script,
+	})
+}

btcutil/psbt/extractor.go

@@ -27,9 +27,12 @@ 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.

btcutil/psbt/finalizer.go

@@ -111,7 +111,17 @@ 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
+	var outIndex uint32
+	switch p.Version {
+	case 0:
+		if p.UnsignedTx == nil {
+			return false
+		}
+		outIndex = p.UnsignedTx.TxIn[inIndex].PreviousOutPoint.Index
+	default:
+		outIndex = pInput.OutputIndex
+	}
+
 	if txscript.IsPayToScriptHash(pInput.NonWitnessUtxo.TxOut[outIndex].PkScript) {
 		if pInput.RedeemScript == nil {
 			return false
@@ -186,7 +196,8 @@ 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 {
+	numInputs := len(p.Inputs)
+	for i := 0; i < numInputs; i++ {
 		success, err := MaybeFinalize(p, i)
 		if err != nil || !success {
 			return err
@@ -351,6 +362,9 @@ 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 +507,9 @@ 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
@@ -590,6 +607,9 @@ 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