more comments

contracts/fee_currencies.go

@@ -55,6 +55,7 @@ func ConvertGoldToCurrency(exchangeRates map[common.Address]*big.Rat, feeCurrenc
 	return new(big.Int).Div(new(big.Int).Mul(goldAmount, exchangeRate.Num()), exchangeRate.Denom()), nil
 }
 
+// Debits transaction fees from the transaction sender and stores them in the temporary address
 func DebitFees(evm *vm.EVM, address common.Address, amount *big.Int, feeCurrency *common.Address) error {
 	if amount.Cmp(big.NewInt(0)) == 0 {
 		return nil
@@ -77,6 +78,11 @@ func DebitFees(evm *vm.EVM, address common.Address, amount *big.Int, feeCurrency
 	return err
 }
 
+// Credits fees to the respective parties
+// - the base fee goes to the fee handler
+// - the transaction tip goes to the miner
+// - the l1 data fee goes the the data fee receiver, is the node runs in rollup mode
+// - remaining funds are refunded to the transaction sender
 func CreditFees(
 	evm *vm.EVM,
 	feeCurrency *common.Address,
@@ -96,17 +102,22 @@ func CreditFees(
 		return fmt.Errorf("base fee: %w", err)
 	}
 
-	if tipReceiver != common.ZeroAddress && feeTip.Cmp(common.Big0) == 1 {
-		leftoverGas, err = transferErc20(evm, abi, caller, feeCurrency, &tipReceiver, feeTip, leftoverGas)
-		if err != nil {
-			return fmt.Errorf("fee tip: %w", err)
+	// If the tip is non-zero, but the coinbase of the miner is not set, send the tip back to the tx sender
+	if feeTip.Cmp(common.Big0) == 1 {
+		if tipReceiver != common.ZeroAddress {
+			leftoverGas, err = transferErc20(evm, abi, caller, feeCurrency, &tipReceiver, feeTip, leftoverGas)
+			if err != nil {
+				return fmt.Errorf("fee tip: %w", err)
+			}
+		} else {
+			// If the coinbase is the zero address, instead refund the tip to the transaction sender
+			refund.Add(refund, feeTip)
 		}
-	} else {
-		// If the coinbase is the zero address, instead refund the tip to the transaction sender
-		refund.Add(refund, feeTip)
 	}
 
-	if l1DataFee != nil {
+	// The data fee is `nil` when the node is not running in rollup mode
+	// If the data fee is non-zero, but the data fee receiver is not set, send the tip back to the tx sender
+	if l1DataFee != nil && l1DataFee.Cmp(common.Big0) == 1 {
 		if l1DataFeeReceiver != common.ZeroAddress {
 			leftoverGas, err = transferErc20(evm, abi, caller, feeCurrency, &l1DataFeeReceiver, l1DataFee, leftoverGas)
 			if err != nil {
@@ -116,6 +127,7 @@ func CreditFees(
 			refund.Add(refund, l1DataFee)
 		}
 	}
+
 	if refund.Cmp(common.Big0) == 1 {
 		leftoverGas, err = transferErc20(evm, abi, caller, feeCurrency, &txSender, refund, leftoverGas)
 		if err != nil {