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Fix bugs related to gas fees (ethereum#144)
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When paying gas fees in C$ or any other non-native currency

1. Charge the user upfront for three things
   - balanceOf - to determine their balance in that currency)
   - debitFrom - to debit gas from their account
   - creditTo - to credit miner at the end

2. At the end if the fee to be refunded is less than the transaction fee
to refund then pass that fee to the miner.

Prior to this diff, ethereum#1 was not happening properly and ethereum#2 was not
happening at all.

As a more concrete example, if user passes 100K C$ wei as gas amount
for a send tranaction then we will deduct 2195 to determine their
balance in C$, then we will deduct 10,000 for a debitFrom transaction
and another 10K for a creditTo transaction.
So, the available gas (`st.gas`) would be
100,000 - 22,195 = 77,805 for rest of the transaction.

At the end, we will pay the gas fees to the miner, which in this case is
22,195 + 21,000 (send fee) = 43,195, at the end, 56,805 gas is left, out
of which 10, 000 is further deducted and then 46,805 is refunded to the
sender and 43, 195 + 10, 000 is sent to the miner.

Sample logs with gas price = 999
```
DEBUG[03-21|18:18:51.003|core/state_transition.go:252]           debitGas                                 amount=99900000 gasCurrency=0x13f252CFCd0AED2FBAdAE507AA812aa1De01d8a4
TRACE[03-21|18:18:51.003|core/state_transition.go:262]           debitGas                                 rootCaller=0 customTokenContractAddress=0x13f252CFCd0AED2FBAdAE507AA812aa1De01d8a4 gas=10000 value=0 transactionData=0x362a5f80000000000000000000000000fee1a22f43beecb912b5a4912ba87527682ef0fc0000000000000000000000000000000000000000000000000000000005f45a60
DEBUG[03-21|18:18:51.003|core/state_transition.go:273]           debitGas successful                      ret=0x00000000000000000000000000000000000000000000006c4fd1ee2454c9dd59 leftoverGas=2351
TRACE[03-21|18:18:51.003|core/state_transition.go:192]           buyGas after debitGas                    upfrontGasCharges=22195 available gas=77805   initial gas=100000  gasCurrency=0x13f252CFCd0AED2FBAdAE507AA812aa1De01d8a4
TRACE[03-21|18:18:51.003|core/state_transition.go:463]           Refunding gas to sender                  sender=0xfeE1a22F43BeeCB912B5a4912ba87527682ef0fC refundAmount=46758195 gas Currency=0x13f252CFCd0AED2FBAdAE507AA812aa1De01d8a4
DEBUG[03-21|18:18:51.003|core/state_transition.go:252]           creditGas                                amount=46758195 gasCurrency=0x13f252CFCd0AED2FBAdAE507AA812aa1De01d8a4
TRACE[03-21|18:18:51.003|core/state_transition.go:262]           creditGas                                rootCaller=0 customTokenContractAddress=0x13f252CFCd0AED2FBAdAE507AA812aa1De01d8a4 gas=10000 value=0 transactionData=0x9951b90c000000000000000000000000fee1a22f43beecb912b5a4912ba87527682ef0fc0000000000000000000000000000000000000000000000000000000002c97933
DEBUG[03-21|18:18:51.004|core/state_transition.go:273]           creditGas successful                     ret=0x00000000000000000000000000000000000000000000006c4fd1ee245793568c leftoverGas=2057
TRACE[03-21|18:18:51.004|core/state_transition.go:425]           Paying gas fees to miner                 unrefundedGas=0 gas used=53195 gasFeesForMiner=53195 miner Fee=53141805
TRACE[03-21|18:18:51.004|core/state_transition.go:428]           Paying gas fees to miner                 miner=0xfeE1a22F43BeeCB912B5a4912ba87527682ef0fC minerFee=53141805 gas Currency=0x13f252CFCd0AED2FBAdAE507AA812aa1De01d8a4
```
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@ashishb
ashishb committed Mar 23, 2019
1 parent 5081721 commit e48dad5
Showing 1 changed file with 56 additions and 35 deletions.
91 changes: 56 additions & 35 deletions core/state_transition.go
View file
Expand Up @@ -162,23 +162,37 @@ func (st *StateTransition) useGas(amount uint64) error {

func (st *StateTransition) buyGas() error {
mgval := new(big.Int).Mul(new(big.Int).SetUint64(st.msg.Gas()), st.gasPrice)
hasSufficientGas, gasUsed := st.canBuyGas(st.msg.From(), mgval, st.msg.GasCurrency())
// gasConsumedToDetermineBalance = Charge to determine user's balance in native or non-native currency
hasSufficientGas, gasConsumedToDetermineBalance := st.canBuyGas(st.msg.From(), mgval, st.msg.GasCurrency())
if !hasSufficientGas {
return errInsufficientBalanceForGas
}
if err := st.gp.SubGas(st.msg.Gas()); err != nil {
return err
}
st.gas += st.msg.Gas()
st.initialGas = st.msg.Gas()
gasCurrency := st.msg.GasCurrency()
// Charge user for the gas used to determine gas balance.
// It is 0 for native gas currency, and we charge maxGasForDebitAndCreditTransactions for safety and give
// that as the gas allowance for the debit and the credit transactions.
chargeForGasWithdrawal := new(big.Int).SetUint64(maxGasForDebitAndCreditTransactions)
mgval = new(big.Int).Add(mgval, chargeForGasWithdrawal.Mul(chargeForGasWithdrawal, st.gasPrice))
amount := new(big.Int).Add(mgval, new(big.Int).SetUint64(gasUsed))
err := st.debitGas(amount, gasCurrency)

if gasCurrency != nil {
// This gas is used for charging user for one `debitFrom` transaction to deduct their balance in
// non-native currency and one `creditTo` transaction to pay miner fee in non-native currency
// at the end. We do not charge the gas refund fee upfront, that's deducted only if if there is
// sufficient gas to refunded. These transactions are not made for the native currency and hence,
// these charges are not applicable in those cases either.
gasToMakeOneDebitAndOneCreditTransaction := 2 * maxGasForDebitAndCreditTransactions
upfrontGasCharges := gasConsumedToDetermineBalance + gasToMakeOneDebitAndOneCreditTransaction
if st.gas < upfrontGasCharges {
log.Debug("Gas too low during buy gas",
"gas left", st.gas,
"gasConsumedToDetermineBalance", gasConsumedToDetermineBalance,
"maxGasForDebitAndCreditTransactions", gasToMakeOneDebitAndOneCreditTransaction)
return errInsufficientBalanceForGas
}
st.gas -= upfrontGasCharges
log.Trace("buyGas before debitGas", "upfrontGasCharges", upfrontGasCharges, "available gas", st.gas, "initial gas", st.initialGas, "gasCurrency", gasCurrency)
}
st.initialGas = st.msg.Gas()
err := st.debitGas(mgval, gasCurrency)
return err
}

Expand All @@ -198,23 +212,14 @@ func (st *StateTransition) canBuyGas(
return balanceOf.Cmp(gasNeeded) > 0, gasUsed
}

type ZeroAddress int64

func (ZeroAddress) Address() common.Address {
var address common.Address
// Not required since address is, by default, initialized to 0
// copy(address[:], "0000000000000000000000000000000000000000")
return address
}

// contractAddress must have a function with following signature
// "function balanceOf(address _owner) public view returns (uint256)"
func (st *StateTransition) getBalanceOf(accountOwner common.Address, contractAddress *common.Address) (
balance *big.Int, gasUsed uint64, err error) {
evm := st.evm
functionSelector := getBalanceOfFunctionSelector()
transactionData := getEncodedAbiWithOneArg(functionSelector, addressToAbi(accountOwner))
anyCaller := ZeroAddress(0) // any caller will work
anyCaller := vm.AccountRef(common.HexToAddress("0x0")) // any caller will work
log.Trace("getBalanceOf", "caller", anyCaller, "customTokenContractAddress",
*contractAddress, "gas", st.gas, "transactionData", hexutil.Encode(transactionData))
ret, leftoverGas, err := evm.StaticCall(anyCaller, *contractAddress, transactionData, st.gas+st.msg.Gas())
Expand Down Expand Up @@ -242,7 +247,7 @@ func (st *StateTransition) debitOrCreditErc20Balance(
evm := st.evm
transactionData := getEncodedAbiWithTwoArgs(functionSelector, addressToAbi(address), amountToAbi(amount))

rootCaller := ZeroAddress(0)
rootCaller := vm.AccountRef(common.HexToAddress("0x0"))
maxGasForCall := st.gas
// Limit the gas used by these calls to prevent a gas stealing attack.
if maxGasForCall > maxGasForDebitAndCreditTransactions {
Expand Down Expand Up @@ -282,7 +287,7 @@ func (st *StateTransition) debitGas(amount *big.Int, gasCurrency *common.Address
func (st *StateTransition) creditGas(to common.Address, amount *big.Int, gasCurrency *common.Address) (err error) {
// native currency
if gasCurrency == nil {
st.state.AddBalance(to, new(big.Int).Mul(new(big.Int).SetUint64(st.gasUsed()), st.gasPrice))
st.state.AddBalance(to, amount)
return nil
}

Expand Down Expand Up @@ -405,42 +410,58 @@ func (st *StateTransition) TransitionDb() (ret []byte, usedGas uint64, failed bo
return nil, 0, false, vmerr
}
}
// Refund unused gas to sender
st.refundGas()
// Refund unused gas to sender. If the refund gas is lower than the transaction cost to refund it
// then don't refund the gas to the user, send it to the miner instead.
unrefundedGas := st.refundGas()
gasFeesForMiner := st.gasUsed() + unrefundedGas
// Pay gas fee to Coinbase chosen by the miner
minerFee := new(big.Int).Mul(new(big.Int).SetUint64(gasFeesForMiner), st.gasPrice)
log.Trace("Paying gas fees to miner", "unrefundedGas", unrefundedGas,
"gas used", st.gasUsed(), "gasFeesForMiner", gasFeesForMiner,
"miner Fee", minerFee)
log.Trace("Paying gas fees to miner", "miner", st.evm.Coinbase,
"minerFee", minerFee, "gas Currency", st.msg.GasCurrency())
st.creditGas(
st.evm.Coinbase,
new(big.Int).Mul(new(big.Int).SetUint64(st.gasUsed()), st.gasPrice),
minerFee,
st.msg.GasCurrency())

return ret, st.gasUsed(), vmerr != nil, err
}

func (st *StateTransition) refundGas() {
func (st *StateTransition) refundGas() (unrefundedGas uint64) {
refund := st.state.GetRefund()
// We charge the user for cost associated with gas refund to their account as well as the
// cost associated with paying the mining fee to Coinbase account.
if refund >= 2*maxGasForDebitAndCreditTransactions {
refund -= 2 * maxGasForDebitAndCreditTransactions
} else {
log.Info("refundGas not possible since refund amount is too small",
"refund", refund, "maxGasForDebitAndCreditTransactions", maxGasForDebitAndCreditTransactions)
return
}

// Apply refund counter, capped to half of the used gas.
if refund > st.gasUsed()/2 {
refund = st.gasUsed() / 2
}

st.gas += refund

// We charge the user for cost associated with non-native currency gas refund to their account.
// We have already charged user for crediting the miner's account in buyGas()
if st.msg.GasCurrency() != nil {
if st.gas > maxGasForDebitAndCreditTransactions {
st.gas -= maxGasForDebitAndCreditTransactions
} else {
log.Info(
"Refunding gas to sender not possible since refund amount is less than gas required to process refund",
"remaining", st.gas,
"gasFees", maxGasForDebitAndCreditTransactions)
return st.gas
}
}

// Return ETH for remaining gas, exchanged at the original rate.
remaining := new(big.Int).Mul(new(big.Int).SetUint64(st.gas), st.gasPrice)

log.Trace("Refunding gas to sender", "sender", st.msg.From(),
"refundAmount", remaining, "gas Currency", st.msg.GasCurrency())
st.creditGas(st.msg.From(), remaining, st.msg.GasCurrency())
// Also return remaining gas to the block gas counter so it is
// available for the next transaction.
st.gp.AddGas(st.gas)
return 0
}

// gasUsed returns the amount of gas used up by the state transition.
Expand Down

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