Validate debitGasFees execution in tx pool

Txs must not fail during debitGasFees execution during the state
transition, so we must remove problematic txs during the tx pool
validation.

contracts/erc20gas/erc20gas.go

@@ -8,6 +8,7 @@ import (
 	"github.com/celo-org/celo-blockchain/common"
 	"github.com/celo-org/celo-blockchain/common/hexutil"
 	"github.com/celo-org/celo-blockchain/contracts/internal/n"
+	"github.com/celo-org/celo-blockchain/core/types"
 	"github.com/celo-org/celo-blockchain/core/vm"
 	"github.com/celo-org/celo-blockchain/log"
 )
@@ -17,17 +18,35 @@ const (
 	maxGasForCreditGasFeesTransactions uint64 = 1 * n.Million
 )
 
+var (
+	// Recalculate with `cast sig 'debitGasFees(address from, uint256 value)'`
+	debitGasFeesSelector = hexutil.MustDecode("0x58cf9672")
+)
+
+// Returns nil if debit is possible, used in tx pool validation
+func TryDebitFees(tx *types.Transaction, from common.Address, currentVMRunner vm.EVMRunner) error {
+	cost := new(big.Int).SetUint64(tx.Gas())
+	cost.Mul(cost, tx.GasFeeCap())
+
+	// The following code is similar to DebitFees, but that function does not work on a vm.EVMRunner,
+	// so we have to adapt it instead of reusing.
+	transactionData := common.GetEncodedAbi(debitGasFeesSelector, [][]byte{common.AddressToAbi(from), common.AmountToAbi(cost)})
+
+	ret, err := currentVMRunner.Query(*tx.FeeCurrency(), transactionData, maxGasForDebitGasFeesTransactions)
+	if err != nil {
+		revertReason, err2 := abi.UnpackRevert(ret)
+		if err2 == nil {
+			return fmt.Errorf("TryDebitFees reverted: %s", revertReason)
+		}
+	}
+	return nil
+}
+
 func DebitFees(evm *vm.EVM, address common.Address, amount *big.Int, feeCurrency *common.Address) error {
 	if amount.Cmp(big.NewInt(0)) == 0 {
 		return nil
 	}
-	// Function is "debitGasFees(address from, uint256 value)"
-	// selector is first 4 bytes of keccak256 of "debitGasFees(address,uint256)"
-	// Source:
-	// pip3 install pyethereum
-	// python3 -c 'from ethereum.utils import sha3; print(sha3("debitGasFees(address,uint256)")[0:4].hex())'
-	functionSelector := hexutil.MustDecode("0x58cf9672")
-	transactionData := common.GetEncodedAbi(functionSelector, [][]byte{common.AddressToAbi(address), common.AmountToAbi(amount)})
+	transactionData := common.GetEncodedAbi(debitGasFeesSelector, [][]byte{common.AddressToAbi(address), common.AmountToAbi(amount)})
 
 	// Run only primary evm.Call() with tracer
 	if evm.GetDebug() {

core/tx_pool.go

@@ -31,6 +31,7 @@ import (
 	"github.com/celo-org/celo-blockchain/consensus"
 	"github.com/celo-org/celo-blockchain/contracts/blockchain_parameters"
 	"github.com/celo-org/celo-blockchain/contracts/currency"
+	"github.com/celo-org/celo-blockchain/contracts/erc20gas"
 	gpm "github.com/celo-org/celo-blockchain/contracts/gasprice_minimum"
 	"github.com/celo-org/celo-blockchain/core/state"
 	"github.com/celo-org/celo-blockchain/core/types"
@@ -1721,8 +1722,7 @@ func (pool *TxPool) demoteUnexecutables() {
 //   - Post-Espresso: it ensures balance >= GasFeeCap * gas + value + gatewayFee (2)
 //
 // For non-native tokens(cUSD, cEUR, ...) as feeCurrency:
-//   - Pre-Espresso: it ensures balance > GasPrice * gas + gatewayFee (3)
-//   - Post-Espresso: it ensures balance >= GasFeeCap * gas + gatewayFee (4)
+//   - It executes a static call on debitGasFees, implicitly ensuring balance >= GasFeeCap * gas and that `from` is not on the token's block list
 func ValidateTransactorBalanceCoversTx(tx *types.Transaction, from common.Address, currentState *state.StateDB, currentVMRunner vm.EVMRunner, espresso bool) error {
 	if tx.FeeCurrency() == nil {
 		balance := currentState.GetBalance(from)
@@ -1749,31 +1749,7 @@ func ValidateTransactorBalanceCoversTx(tx *types.Transaction, from common.Addres
 			return ErrInsufficientFunds
 		}
 	} else {
-		balance, err := currency.GetBalanceOf(currentVMRunner, from, *tx.FeeCurrency())
-		if err != nil {
-			log.Debug("ValidateTransactorBalanceCoversTx: error in getting fee currency balance", "feeCurrency", tx.FeeCurrency())
-			return err
-		}
-
-		if espresso {
-			// cost = GasFeeCap * gas + gatewayFee, as in (4)
-			cost := new(big.Int).SetUint64(tx.Gas())
-			cost.Mul(cost, tx.GasFeeCap())
-			if tx.GatewayFeeRecipient() != nil {
-				cost.Add(cost, tx.GatewayFee())
-			}
-			if balance.Cmp(cost) < 0 {
-				log.Debug("ValidateTransactorBalanceCoversTx: insufficient funds", "feeCurrency", tx.FeeCurrency(), "balance", balance)
-				return ErrInsufficientFunds
-			}
-		} else {
-			// cost = GasPrice * gas + gatewayFee, as in (3)
-			cost := tx.Fee()
-			if balance.Cmp(cost) <= 0 {
-				log.Debug("ValidateTransactorBalanceCoversTx: insufficient funds", "feeCurrency", tx.FeeCurrency(), "balance", balance)
-				return ErrInsufficientFunds
-			}
-		}
+		return erc20gas.TryDebitFees(tx, from, currentVMRunner)
 	}
 
 	return nil