psbt: Indicate which psbt inputs were signed

lnwallet/btcwallet/psbt.go

@@ -98,12 +98,16 @@ func (b *BtcWallet) FundPsbt(packet *psbt.Packet, minConfs int32,
 // perform any other tasks (such as coin selection, UTXO locking or
 // input/output/fee value validation, PSBT finalization). Any input that is
 // incomplete will be skipped.
-func (b *BtcWallet) SignPsbt(packet *psbt.Packet) error {
+func (b *BtcWallet) SignPsbt(packet *psbt.Packet) ([]uint32, error) {
+	// In signedInputs we return the indices of psbt inputs that were signed
+	// by our wallet. This way the caller can check if any inputs were signed.
+	var signedInputs []uint32
+
 	// Let's check that this is actually something we can and want to sign.
 	// We need at least one input and one output.
 	err := psbt.VerifyInputOutputLen(packet, true, true)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	// Go through each input that doesn't have final witness data attached
@@ -173,7 +177,7 @@ func (b *BtcWallet) SignPsbt(packet *psbt.Packet) error {
 		// information we need?
 		signMethod, err := validateSigningMethod(in)
 		if err != nil {
-			return err
+			return nil, err
 		}
 
 		switch signMethod {
@@ -211,11 +215,11 @@ func (b *BtcWallet) SignPsbt(packet *psbt.Packet) error {
 				"PSBT signing: %v", signMethod)
 		}
 		if err != nil {
-			return err
+			return nil, err
 		}
+		signedInputs = append(signedInputs, uint32(idx))
 	}
-
-	return nil
+	return signedInputs, nil
 }
 
 // validateSigningMethod attempts to detect the signing method that is required

lnwallet/btcwallet/psbt_test.go

@@ -318,9 +318,13 @@ func TestSignPsbt(t *testing.T) {
 
 		// Let the wallet do its job. We expect to be the only signer
 		// for this PSBT, so we'll be able to finalize it later.
-		err = w.SignPsbt(packet)
+		signedInputs, err := w.SignPsbt(packet)
 		require.NoError(t, err)
 
+		// We expect one signed input at index 0.
+		require.Len(t, signedInputs, 1)
+		require.EqualValues(t, 0, signedInputs[0])
+
 		// If the witness stack needs to be assembled, give the caller
 		// the option to do that now.
 		tc.inputType.finalize(t, packet)

lnwallet/interface.go

@@ -428,7 +428,7 @@ type WalletController interface {
 	// does not perform any other tasks (such as coin selection, UTXO
 	// locking or input/output/fee value validation, PSBT finalization). Any
 	// input that is incomplete will be skipped.
-	SignPsbt(packet *psbt.Packet) error
+	SignPsbt(packet *psbt.Packet) ([]uint32, error)
 
 	// FinalizePsbt expects a partial transaction with all inputs and
 	// outputs fully declared and tries to sign all inputs that belong to

lnwallet/rpcwallet/rpcwallet.go

@@ -188,13 +188,13 @@ func (r *RPCKeyRing) SendOutputs(outputs []*wire.TxOut,
 // perform any other tasks (such as coin selection, UTXO locking or
 // input/output/fee value validation, PSBT finalization). Any input that is
 // incomplete will be skipped.
-func (r *RPCKeyRing) SignPsbt(packet *psbt.Packet) error {
+func (r *RPCKeyRing) SignPsbt(packet *psbt.Packet) ([]uint32, error) {
 	ctxt, cancel := context.WithTimeout(context.Background(), r.rpcTimeout)
 	defer cancel()
 
 	var buf bytes.Buffer
 	if err := packet.Serialize(&buf); err != nil {
-		return fmt.Errorf("error serializing PSBT: %v", err)
+		return nil, fmt.Errorf("error serializing PSBT: %v", err)
 	}
 
 	resp, err := r.walletClient.SignPsbt(ctxt, &walletrpc.SignPsbtRequest{
@@ -202,7 +202,7 @@ func (r *RPCKeyRing) SignPsbt(packet *psbt.Packet) error {
 	})
 	if err != nil {
 		considerShutdown(err)
-		return fmt.Errorf("error signing PSBT in remote signer "+
+		return nil, fmt.Errorf("error signing PSBT in remote signer "+
 			"instance: %v", err)
 	}
 
@@ -210,7 +210,7 @@ func (r *RPCKeyRing) SignPsbt(packet *psbt.Packet) error {
 		bytes.NewReader(resp.SignedPsbt), false,
 	)
 	if err != nil {
-		return fmt.Errorf("error parsing signed PSBT: %v", err)
+		return nil, fmt.Errorf("error parsing signed PSBT: %v", err)
 	}
 
 	// The caller expects the packet to be modified instead of a new
@@ -221,7 +221,7 @@ func (r *RPCKeyRing) SignPsbt(packet *psbt.Packet) error {
 	packet.Outputs = signedPacket.Outputs
 	packet.Unknowns = signedPacket.Unknowns
 
-	return nil
+	return resp.SignedInputs, nil
 }
 
 // FinalizePsbt expects a partial transaction with all inputs and outputs fully