fix(submitter): handle no change output

CHANGELOG.md

@@ -39,6 +39,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 
 ### Improvements
 
+* [#79](https://github.com/babylonlabs-io/vigilante/pull/79) handle no change output when building tx
+
 * [#77](https://github.com/babylonlabs-io/vigilante/pull/77) add arm64 static build
 
 * [#76](https://github.com/babylonlabs-io/vigilante/pull/76) add goreleaser 

submitter/relayer/relayer.go

@@ -487,7 +487,9 @@ func (rl *Relayer) ChainTwoTxAndSend(data1 []byte, data2 []byte) (*types.BtcTxIn
 func (rl *Relayer) buildTxWithData(data []byte, firstTx *wire.MsgTx) (*types.BtcTxInfo, error) {
 	tx := wire.NewMsgTx(wire.TxVersion)
 
-	if firstTx != nil {
+	isFirstTx := firstTx != nil
+
+	if isFirstTx {
 		txID := firstTx.TxHash()
 		outPoint := wire.NewOutPoint(&txID, changePosition)
 		txIn := wire.NewTxIn(outPoint, nil, nil)
@@ -514,33 +516,45 @@ func (rl *Relayer) buildTxWithData(data []byte, firstTx *wire.MsgTx) (*types.Btc
 		return nil, err
 	}
 
+	// we want to ensure that firstTx has change output, but for the second transaction we can ignore this
+	hasChange := len(rawTxResult.Transaction.TxOut) > changePosition
+	// fail so we retry, first tx must have change output
+	if isFirstTx && !hasChange {
+		return nil, fmt.Errorf("transaction doesn't have change output %s", rawTxResult.Transaction.TxID())
+	}
+
 	rl.logger.Debugf("Building a BTC tx using %s with data %x", rawTxResult.Transaction.TxID(), data)
 
-	_, addresses, _, err := txscript.ExtractPkScriptAddrs(
-		rawTxResult.Transaction.TxOut[changePosition].PkScript,
-		rl.GetNetParams(),
-	)
+	if hasChange {
+		_, addresses, _, err := txscript.ExtractPkScriptAddrs(
+			rawTxResult.Transaction.TxOut[changePosition].PkScript,
+			rl.GetNetParams(),
+		)
 
-	if err != nil {
-		return nil, err
-	}
+		if err != nil {
+			return nil, err
+		}
 
-	if len(addresses) == 0 {
-		return nil, errors.New("no change address found")
-	}
+		if len(addresses) == 0 {
+			return nil, errors.New("no change address found")
+		}
 
-	changeAddr := addresses[0]
-	rl.logger.Debugf("Got a change address %v", changeAddr.String())
+		rl.logger.Debugf("Got a change address %v", addresses[0].String())
+	}
 
 	txSize, err := calculateTxVirtualSize(rawTxResult.Transaction)
 	if err != nil {
 		return nil, err
 	}
 
-	changeAmount := btcutil.Amount(rawTxResult.Transaction.TxOut[changePosition].Value)
+	var changeAmount btcutil.Amount
+	if hasChange {
+		changeAmount = btcutil.Amount(rawTxResult.Transaction.TxOut[changePosition].Value)
+	}
+
 	minRelayFee := rl.calcMinRelayFee(txSize)
 
-	if changeAmount < minRelayFee {
+	if hasChange && changeAmount < minRelayFee {
 		return nil, fmt.Errorf("the value of the utxo is not sufficient for relaying the tx. Require: %v. Have: %v", minRelayFee, changeAmount)
 	}
 
@@ -550,7 +564,7 @@ func (rl *Relayer) buildTxWithData(data []byte, firstTx *wire.MsgTx) (*types.Btc
 		txFee = minRelayFee
 	}
 	// ensuring the tx fee is not higher than the utxo value
-	if changeAmount < txFee {
+	if hasChange && changeAmount < txFee {
 		return nil, fmt.Errorf("the value of the utxo is not sufficient for paying the calculated fee of the tx. Calculated: %v. Have: %v", txFee, changeAmount)
 	}
 
@@ -568,18 +582,17 @@ func (rl *Relayer) buildTxWithData(data []byte, firstTx *wire.MsgTx) (*types.Btc
 
 	change := changeAmount - txFee
 
-	if change < dustThreshold {
+	if hasChange && change < dustThreshold {
 		return nil, fmt.Errorf("change amount is %v less then dust treshold %v", change, dustThreshold)
 	}
 
 	rl.logger.Debugf("Successfully composed a BTC tx: tx fee: %v, output value: %v, tx size: %v, hex: %v",
 		txFee, changeAmount, txSize, hex.EncodeToString(signedTxBytes.Bytes()))
 
 	return &types.BtcTxInfo{
-		Tx:            tx,
-		ChangeAddress: changeAddr,
-		Size:          txSize,
-		Fee:           txFee,
+		Tx:   tx,
+		Size: txSize,
+		Fee:  txFee,
 	}, nil
 }
 

types/ckpt_info.go

@@ -18,9 +18,8 @@ type CheckpointInfo struct {
 
 // BtcTxInfo stores information of a BTC tx as part of a checkpoint
 type BtcTxInfo struct {
-	TxId          *chainhash.Hash
-	Tx            *wire.MsgTx
-	ChangeAddress btcutil.Address
-	Size          int64          // the size of the BTC tx
-	Fee           btcutil.Amount // tx fee cost by the BTC tx
+	TxId *chainhash.Hash
+	Tx   *wire.MsgTx
+	Size int64          // the size of the BTC tx
+	Fee  btcutil.Amount // tx fee cost by the BTC tx
 }