Review fixes: use max base fee as a refund backup

Author: MishkaRogachev

arbos/l2pricing/model.go

@@ -352,33 +352,21 @@ func (ps *L2PricingState) calcBaseFeeFromExponent(exponent arbmath.Bips) (*big.I
 }
 
 func (ps *L2PricingState) MultiDimensionalPriceForRefund(gasUsed multigas.MultiGas) (*big.Int, error) {
-	if ps.ArbosVersion < ArbosMultiGasConstraintsVersion {
-		baseFeeWei, err := ps.BaseFeeWei()
-		if err != nil {
-			return nil, err
-		}
-
-		return new(big.Int).Mul(
-			new(big.Int).SetUint64(gasUsed.SingleGas()),
-			baseFeeWei,
-		), nil
-	}
-
-	total := new(big.Int)
-	minBaseFeeWei, err := ps.MinBaseFeeWei()
+	// Base fee is max of per-resource-kind base fees
+	baseFeeWei, err := ps.BaseFeeWei()
 	if err != nil {
 		return nil, err
 	}
 
+	total := new(big.Int)
 	for kind := range multigas.ResourceKind(multigas.NumResourceKind) {
 		baseFee, err := ps.multiGasBaseFees.Get(kind)
 		if err != nil {
 			return nil, err
 		}
-		// Override zero with minimum base fee,
-		// it may happen if there is no constraint configured for this resource kind
-		if baseFee.Cmp(big.NewInt(0)) == 0 {
-			baseFee = minBaseFeeWei
+		// Force L1 calldata (and the unlikely zero-basefee case) to use the max base fee.
+		if kind == multigas.ResourceKindL1Calldata || baseFee.Cmp(big.NewInt(0)) == 0 {
+			baseFee = baseFeeWei
 		}
 
 		amount := gasUsed.Get(kind)

arbos/l2pricing/model_test.go

@@ -200,17 +200,10 @@ func TestMultiDimensionalPriceForRefund(t *testing.T) {
 	expectedPrice := minPrice.Mul(minPrice, singleGas)
 	Require(t, err)
 
-	// Should use base fee under ArbosMultiGasConstraintsVersion
-	price, err := pricing.MultiDimensionalPriceForRefund(multiGas)
-	Require(t, err)
-	if price.Cmp(expectedPrice) != 0 {
-		t.Errorf("Unexpected initial price: got %v, want %v", price, expectedPrice)
-	}
-
 	pricing.ArbosVersion = ArbosMultiGasConstraintsVersion
 
-	// Should still use base fee before any multi gas constraints are set
-	price, err = pricing.MultiDimensionalPriceForRefund(multiGas)
+	// Initial price check
+	price, err := pricing.MultiDimensionalPriceForRefund(multiGas)
 	Require(t, err)
 	if price.Cmp(expectedPrice) != 0 {
 		t.Errorf("Unexpected initial price: got %v, want %v", price, expectedPrice)

arbos/tx_processor.go

@@ -21,6 +21,7 @@ import (
 
 	"github.com/offchainlabs/nitro/arbos/arbosState"
 	"github.com/offchainlabs/nitro/arbos/l1pricing"
+	"github.com/offchainlabs/nitro/arbos/l2pricing"
 	"github.com/offchainlabs/nitro/arbos/retryables"
 	"github.com/offchainlabs/nitro/arbos/util"
 	"github.com/offchainlabs/nitro/util/arbmath"
@@ -534,8 +535,12 @@ func (p *TxProcessor) EndTxHook(gasLeft uint64, usedMultiGas multigas.MultiGas,
 	}
 	gasUsed := p.msg.GasLimit - gasLeft
 
-	multiDimensionalCost, err := p.state.L2PricingState().MultiDimensionalPriceForRefund(usedMultiGas)
-	p.state.Restrict(err)
+	var multiDimensionalCost *big.Int
+	var err error
+	if p.state.L2PricingState().ArbosVersion >= l2pricing.ArbosMultiGasConstraintsVersion {
+		multiDimensionalCost, err = p.state.L2PricingState().MultiDimensionalPriceForRefund(usedMultiGas)
+		p.state.Restrict(err)
+	}
 
 	if underlyingTx != nil && underlyingTx.Type() == types.ArbitrumRetryTxType {
 		inner, _ := underlyingTx.GetInner().(*types.ArbitrumRetryTx)
@@ -559,7 +564,7 @@ func (p *TxProcessor) EndTxHook(gasLeft uint64, usedMultiGas multigas.MultiGas,
 		}
 
 		singleGasCost := arbmath.BigMulByUint(effectiveBaseFee, gasUsed)
-		shouldRefundMultiGas := arbmath.BigGreaterThan(singleGasCost, multiDimensionalCost)
+		shouldRefundMultiGas := multiDimensionalCost != nil && arbmath.BigGreaterThan(singleGasCost, multiDimensionalCost)
 
 		maxRefund := new(big.Int).Set(inner.MaxRefund)
 		refund := func(refundFrom common.Address, amount *big.Int, reason tracing.BalanceChangeReason) {
@@ -703,19 +708,21 @@ func (p *TxProcessor) EndTxHook(gasLeft uint64, usedMultiGas multigas.MultiGas,
 	}
 
 	// Multi-dimensional refund (normal tx path)
-	amount := new(big.Int).Sub(totalCost, multiDimensionalCost)
-	if amount.Sign() > 0 {
-		err := util.TransferBalance(
-			&networkFeeAccount,
-			&p.msg.From,
-			amount,
-			p.evm,
-			scenario,
-			tracing.BalanceChangeMultiGasRefund,
-		)
-		p.state.Restrict(err)
-	} else if amount.Sign() < 0 {
-		log.Warn("multi dimensional gas price exceeded simple gas price", "amount", amount)
+	if multiDimensionalCost != nil {
+		amount := new(big.Int).Sub(totalCost, multiDimensionalCost)
+		if amount.Sign() > 0 {
+			err := util.TransferBalance(
+				&networkFeeAccount,
+				&p.msg.From,
+				amount,
+				p.evm,
+				scenario,
+				tracing.BalanceChangeMultiGasRefund,
+			)
+			p.state.Restrict(err)
+		} else if amount.Sign() < 0 {
+			log.Warn("multi dimensional gas price exceeded simple gas price", "amount", amount)
+		}
 	}
 
 	if p.msg.GasPrice.Sign() > 0 { // in tests, gas price could be 0