wallet: return change index from FundPsbt

To make it easy to show the user what change output was created (if any)
during the funding process, we return its index (or -1 if no change
output was created).

wallet/psbt.go

@@ -19,7 +19,9 @@ import (
 
 // FundPsbt creates a fully populated PSBT packet that contains enough inputs to
 // fund the outputs specified in the passed in packet with the specified fee
-// rate. If there is change left, a change output from the wallet is added.
+// rate. If there is change left, a change output from the wallet is added and
+// the index of the change output is returned. Otherwise no additional output
+// is created and the index -1 is returned.
 //
 // NOTE: If the packet doesn't contain any inputs, coin selection is performed
 // automatically. If the packet does contain any inputs, it is assumed that full
@@ -32,13 +34,13 @@ import (
 // selected/validated inputs by this method. It is in the caller's
 // responsibility to lock the inputs before handing the partial transaction out.
 func (w *Wallet) FundPsbt(packet *psbt.Packet, account uint32,
-	feeSatPerKB btcutil.Amount) error {
+	feeSatPerKB btcutil.Amount) (int32, error) {
 
 	// Make sure the packet is well formed. We only require there to be at
 	// least one output but not necessarily any inputs.
 	err := psbt.VerifyInputOutputLen(packet, false, true)
 	if err != nil {
-		return err
+		return 0, err
 	}
 
 	txOut := packet.UnsignedTx.TxOut
@@ -53,7 +55,7 @@ func (w *Wallet) FundPsbt(packet *psbt.Packet, account uint32,
 		// dust.
 		err := txrules.CheckOutput(output, txrules.DefaultRelayFeePerKb)
 		if err != nil {
-			return err
+			return 0, err
 		}
 	}
 
@@ -108,7 +110,8 @@ func (w *Wallet) FundPsbt(packet *psbt.Packet, account uint32,
 			false,
 		)
 		if err != nil {
-			return fmt.Errorf("error creating funding TX: %v", err)
+			return 0, fmt.Errorf("error creating funding TX: %v",
+				err)
 		}
 
 		// Copy over the inputs now then collect all UTXO information
@@ -118,7 +121,7 @@ func (w *Wallet) FundPsbt(packet *psbt.Packet, account uint32,
 		packet.UnsignedTx.TxIn = tx.Tx.TxIn
 		err = addInputInfo(tx.Tx.TxIn)
 		if err != nil {
-			return err
+			return 0, err
 		}
 
 	// If there are inputs, we need to check if they're sufficient and add
@@ -127,7 +130,7 @@ func (w *Wallet) FundPsbt(packet *psbt.Packet, account uint32,
 		// Make sure all inputs provided are actually ours.
 		err = addInputInfo(txIn)
 		if err != nil {
-			return err
+			return 0, err
 		}
 
 		// We can leverage the fee calculation of the txauthor package
@@ -147,7 +150,7 @@ func (w *Wallet) FundPsbt(packet *psbt.Packet, account uint32,
 		// a new change addresse into the database.
 		dbtx, err := w.db.BeginReadWriteTx()
 		if err != nil {
-			return err
+			return 0, err
 		}
 		_, changeSource := w.addrMgrWithChangeSource(dbtx, account)
 
@@ -159,24 +162,25 @@ func (w *Wallet) FundPsbt(packet *psbt.Packet, account uint32,
 		)
 		if err != nil {
 			_ = dbtx.Rollback()
-			return fmt.Errorf("fee estimation not successful: %v",
-				err)
+			return 0, fmt.Errorf("fee estimation not successful: "+
+				"%v", err)
 		}
 
 		// The transaction could be created, let's commit the DB TX to
 		// store the change address (if one was created).
 		err = dbtx.Commit()
 		if err != nil {
-			return fmt.Errorf("could not add change address to "+
+			return 0, fmt.Errorf("could not add change address to "+
 				"database: %v", err)
 		}
 	}
 
 	// If there is a change output, we need to copy it over to the PSBT now.
+	var changeTxOut *wire.TxOut
 	if tx.ChangeIndex >= 0 {
+		changeTxOut = tx.Tx.TxOut[tx.ChangeIndex]
 		packet.UnsignedTx.TxOut = append(
-			packet.UnsignedTx.TxOut,
-			tx.Tx.TxOut[tx.ChangeIndex],
+			packet.UnsignedTx.TxOut, changeTxOut,
 		)
 		packet.Outputs = append(packet.Outputs, psbt.POutput{})
 	}
@@ -186,10 +190,22 @@ func (w *Wallet) FundPsbt(packet *psbt.Packet, account uint32,
 	// partial inputs and outputs accordingly.
 	err = psbt.InPlaceSort(packet)
 	if err != nil {
-		return fmt.Errorf("could not sort PSBT: %v", err)
+		return 0, fmt.Errorf("could not sort PSBT: %v", err)
 	}
 
-	return nil
+	// The change output index might have changed after the sorting. We need
+	// to find our index again.
+	changeIndex := int32(-1)
+	if changeTxOut != nil {
+		for idx, txOut := range packet.UnsignedTx.TxOut {
+			if psbt.TxOutsEqual(changeTxOut, txOut) {
+				changeIndex = int32(idx)
+				break
+			}
+		}
+	}
+
+	return changeIndex, nil
 }
 
 // FinalizePsbt expects a partial transaction with all inputs and outputs fully

wallet/psbt_test.go

@@ -142,7 +142,9 @@ func TestFundPsbt(t *testing.T) {
 	for _, tc := range testCases {
 		tc := tc
 		t.Run(tc.name, func(t *testing.T) {
-			err := w.FundPsbt(tc.packet, 0, tc.feeRateSatPerKB)
+			changeIndex, err := w.FundPsbt(
+				tc.packet, 0, tc.feeRateSatPerKB,
+			)
 
 			// Make sure the error is what we expected.
 			if err == nil && tc.expectedErr != "" {
@@ -258,9 +260,10 @@ func TestFundPsbt(t *testing.T) {
 			}
 			p2wkhIndex := -1
 			p2wshIndex := -1
-			changeIndex := -1
+			totalOut := int64(0)
 			for idx, txOut := range txOuts {
 				script := txOut.PkScript
+				totalOut += txOut.Value
 
 				switch {
 				case bytes.Equal(script, testScriptP2WKH):
@@ -269,10 +272,12 @@ func TestFundPsbt(t *testing.T) {
 				case bytes.Equal(script, testScriptP2WSH):
 					p2wshIndex = idx
 
-				default:
-					changeIndex = idx
 				}
 			}
+			totalIn := int64(0)
+			for _, txIn := range packet.Inputs {
+				totalIn += txIn.WitnessUtxo.Value
+			}
 
 			// All outputs must be found.
 			if p2wkhIndex < 0 || p2wshIndex < 0 || changeIndex < 0 {