wallet: include BIP 32 derivation paths for inputs in PSBTs

Watch-only accounts are usually backed by an external hardware signer,
some of which require derivation paths to be populated for each relevant
input to sign.

wallet/psbt.go

@@ -70,7 +70,7 @@ func (w *Wallet) FundPsbt(packet *psbt.Packet, account uint32,
 	addInputInfo := func(inputs []*wire.TxIn) error {
 		packet.Inputs = make([]psbt.PInput, len(inputs))
 		for idx, in := range inputs {
-			tx, utxo, _, err := w.FetchInputInfo(
+			tx, utxo, derivationPath, _, err := w.FetchInputInfo(
 				&in.PreviousOutPoint,
 			)
 			if err != nil {
@@ -91,6 +91,11 @@ func (w *Wallet) FundPsbt(packet *psbt.Packet, account uint32,
 			}
 			packet.Inputs[idx].SighashType = txscript.SigHashAll
 
+			// Include the derivation path for each input.
+			packet.Inputs[idx].Bip32Derivation = []*psbt.Bip32Derivation{
+				derivationPath,
+			}
+
 			// We don't want to include the witness or any script
 			// just yet.
 			packet.UnsignedTx.TxIn[idx].Witness = wire.TxWitness{}
@@ -227,6 +232,8 @@ func (w *Wallet) FundPsbt(packet *psbt.Packet, account uint32,
 //
 // NOTE: This method does NOT publish the transaction after it's been finalized
 // successfully.
+//
+// TODO: require account and check if watch only to avoid signing.
 func (w *Wallet) FinalizePsbt(packet *psbt.Packet) error {
 	// Let's check that this is actually something we can and want to sign.
 	// We need at least one input and one output.
@@ -259,7 +266,7 @@ func (w *Wallet) FinalizePsbt(packet *psbt.Packet) error {
 		// We can only sign this input if it's ours, so we try to map it
 		// to a coin we own. If we can't, then we'll continue as it
 		// isn't our input.
-		fullTx, txOut, _, err := w.FetchInputInfo(
+		fullTx, txOut, _, _, err := w.FetchInputInfo(
 			&txIn.PreviousOutPoint,
 		)
 		if err != nil {

wallet/utxos.go

@@ -11,6 +11,8 @@ import (
 
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
+	"github.com/btcsuite/btcutil/hdkeychain"
+	"github.com/btcsuite/btcutil/psbt"
 	"github.com/btcsuite/btcwallet/waddrmgr"
 	"github.com/btcsuite/btcwallet/walletdb"
 )
@@ -105,15 +107,15 @@ func (w *Wallet) UnspentOutputs(policy OutputSelectionPolicy) ([]*TransactionOut
 // full transaction, the target txout and the number of confirmations are
 // returned. Otherwise, a non-nil error value of ErrNotMine is returned instead.
 func (w *Wallet) FetchInputInfo(prevOut *wire.OutPoint) (*wire.MsgTx,
-	*wire.TxOut, int64, error) {
+	*wire.TxOut, *psbt.Bip32Derivation, int64, error) {
 
 	// We manually look up the output within the tx store.
 	txid := &prevOut.Hash
 	txDetail, err := UnstableAPI(w).TxDetails(txid)
 	if err != nil {
-		return nil, nil, 0, err
+		return nil, nil, nil, 0, err
 	} else if txDetail == nil {
-		return nil, nil, 0, ErrNotMine
+		return nil, nil, nil, 0, ErrNotMine
 	}
 
 	// With the output retrieved, we'll make an additional check to ensure
@@ -122,19 +124,25 @@ func (w *Wallet) FetchInputInfo(prevOut *wire.OutPoint) (*wire.MsgTx,
 	// like in the event of us being the sender of the transaction.
 	numOutputs := uint32(len(txDetail.TxRecord.MsgTx.TxOut))
 	if prevOut.Index >= numOutputs {
-		return nil, nil, 0, fmt.Errorf("invalid output index %v for "+
+		return nil, nil, nil, 0, fmt.Errorf("invalid output index %v for "+
 			"transaction with %v outputs", prevOut.Index,
 			numOutputs)
 	}
 	pkScript := txDetail.TxRecord.MsgTx.TxOut[prevOut.Index].PkScript
-	if _, err := w.fetchOutputAddr(pkScript); err != nil {
-		return nil, nil, 0, err
+	addr, err := w.fetchOutputAddr(pkScript)
+	if err != nil {
+		return nil, nil, nil, 0, err
 	}
+	pubKeyAddr, ok := addr.(waddrmgr.ManagedPubKeyAddress)
+	if !ok {
+		return nil, nil, nil, 0, err
+	}
+	keyScope, derivationPath, _ := pubKeyAddr.DerivationInfo()
 
 	// Determine the number of confirmations the output currently has.
 	_, currentHeight, err := w.chainClient.GetBestBlock()
 	if err != nil {
-		return nil, nil, 0, fmt.Errorf("unable to retrieve current "+
+		return nil, nil, nil, 0, fmt.Errorf("unable to retrieve current "+
 			"height: %v", err)
 	}
 	confs := int64(0)
@@ -143,9 +151,19 @@ func (w *Wallet) FetchInputInfo(prevOut *wire.OutPoint) (*wire.MsgTx,
 	}
 
 	return &txDetail.TxRecord.MsgTx, &wire.TxOut{
-		Value:    txDetail.TxRecord.MsgTx.TxOut[prevOut.Index].Value,
-		PkScript: pkScript,
-	}, confs, nil
+			Value:    txDetail.TxRecord.MsgTx.TxOut[prevOut.Index].Value,
+			PkScript: pkScript,
+		}, &psbt.Bip32Derivation{
+			PubKey:               pubKeyAddr.PubKey().SerializeCompressed(),
+			MasterKeyFingerprint: 0, // TODO
+			Bip32Path: []uint32{
+				keyScope.Purpose + hdkeychain.HardenedKeyStart,
+				keyScope.Coin + hdkeychain.HardenedKeyStart,
+				derivationPath.Account,
+				derivationPath.Branch,
+				derivationPath.Index,
+			},
+		}, confs, nil
 }
 
 // fetchOutputAddr attempts to fetch the managed address corresponding to the

wallet/utxos_test.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
+	"github.com/btcsuite/btcutil/hdkeychain"
 	"github.com/btcsuite/btcwallet/waddrmgr"
 )
 
@@ -43,7 +44,7 @@ func TestFetchInputInfo(t *testing.T) {
 		Hash:  incomingTx.TxHash(),
 		Index: 0,
 	}
-	tx, out, confirmations, err := w.FetchInputInfo(prevOut)
+	tx, out, derivationPath, confirmations, err := w.FetchInputInfo(prevOut)
 	if err != nil {
 		t.Fatalf("error fetching input info: %v", err)
 	}
@@ -54,6 +55,32 @@ func TestFetchInputInfo(t *testing.T) {
 		t.Fatalf("unexpected TX out, got %v wanted %v",
 			tx.TxOut[prevOut.Index].PkScript, utxOut)
 	}
+	if len(derivationPath.Bip32Path) != 5 {
+		t.Fatalf("expected derivation path of length %v, got %v", 3,
+			len(derivationPath.Bip32Path))
+	}
+	if derivationPath.Bip32Path[0] != waddrmgr.KeyScopeBIP0084.Purpose {
+		t.Fatalf("expected purpose %v, got %v",
+			waddrmgr.KeyScopeBIP0084.Purpose,
+			derivationPath.Bip32Path[0])
+	}
+	if derivationPath.Bip32Path[1] != waddrmgr.KeyScopeBIP0084.Coin {
+		t.Fatalf("expected coin type %v, got %v",
+			waddrmgr.KeyScopeBIP0084.Coin,
+			derivationPath.Bip32Path[1])
+	}
+	if derivationPath.Bip32Path[2] != hdkeychain.HardenedKeyStart {
+		t.Fatalf("expected account %v, got %v",
+			hdkeychain.HardenedKeyStart, derivationPath.Bip32Path[2])
+	}
+	if derivationPath.Bip32Path[3] != 0 {
+		t.Fatalf("expected branch %v, got %v", 0,
+			derivationPath.Bip32Path[3])
+	}
+	if derivationPath.Bip32Path[4] != 0 {
+		t.Fatalf("expected index %v, got %v", 0,
+			derivationPath.Bip32Path[4])
+	}
 	if confirmations != int64(0-testBlockHeight) {
 		t.Fatalf("unexpected number of confirmations, got %d wanted %d",
 			confirmations, 0-testBlockHeight)