静态Gas的计算
以太坊黄皮书上说的gasLimit的计算方法：
gasLimit = Gtransaction + Gtxdatanonzero × dataByteLength
需要注意的是这只是静态的gas消耗，实际gas消耗还需要加上合约执行的开销。
计算 IntrinsicGas的源码位置 core/state_transition.go
// IntrinsicGas computes the 'intrinsic gas' for a message with the given data.
func IntrinsicGas(data []byte, contractCreation, homestead bool) (uint64, error) {
// Set the starting gas for the raw transaction
var gas uint64
if contractCreation && homestead {
gas = params.TxGasContractCreation
} else {
gas = params.TxGas
}
// Bump the required gas by the amount of transactional data
if len(data) > 0 {
// Zero and non-zero bytes are priced differently
var nz uint64
for _, byt := range data {
if byt != 0 {
nz++
}
}
// Make sure we don't exceed uint64 for all data combinations
if (math.MaxUint64-gas)/params.TxDataNonZeroGas < nz {
return 0, vm.ErrOutOfGas
}
gas += nz * params.TxDataNonZeroGas
z := uint64(len(data)) - nz
if (math.MaxUint64-gas)/params.TxDataZeroGas < z {
return 0, vm.ErrOutOfGas
}
gas += z * params.TxDataZeroGas
}
return gas, nil
}
Gas预估
相关源码位置：internal/ethapi/api.go
func (s *PublicBlockChainAPI) EstimateGas(ctx context.Context, args CallArgs) (hexutil.Uint64, error) {
// Binary search the gas requirement, as it may be higher than the amount used
var (
lo uint64 = params.TxGas - 1
hi uint64
cap uint64
)
if uint64(args.Gas) >= params.TxGas {
hi = uint64(args.Gas)
} else {
// Retrieve the current pending block to act as the gas ceiling
block, err := s.b.BlockByNumber(ctx, rpc.PendingBlockNumber)
if err != nil {
return 0, err
}
hi = block.GasLimit()
}
cap = hi
// Create a helper to check if a gas allowance results in an executable transaction
executable := func(gas uint64) bool {
args.Gas = hexutil.Uint64(gas)
_, _, failed, err := s.doCall(ctx, args, rpc.PendingBlockNumber, vm.Config{}, 0)
if err != nil || failed {
return false
}
return true
}
// Execute the binary search and hone in on an executable gas limit
for lo+1 < hi {
mid := (hi + lo) / 2
if !executable(mid) {
lo = mid
} else {
hi = mid
}
}
// Reject the transaction as invalid if it still fails at the highest allowance
if hi == cap {
if !executable(hi) {
return 0, fmt.Errorf("gas required exceeds allowance or always failing transaction")
}
}
return hexutil.Uint64(hi), nil
}
EstimateGas采用二分查找法获取要评估交易的gas值。二分查找的下限是param.TxGas, 如果args参数指定Gas大于param.Gas，那么二分查找的上限就是args.Gas，否则以当前pending块的block gas limit(后面简称BGL)作为二分查找的上限。doCall函数模拟智能合约的执行，经过多次尝试找到智能合约能够成功运行的最佳gas值。
由于二分查找的上限和BGL有关，而BGL和不是固定不变的，因此每次gas评估的结果不一定都是相同的，可能每个区块周期就会变动一次。
在实际进行gas评估的时候，可能会出现类似下面的错误
{"jsonrpc":"2.0","id":1,"error":{"code":-32000,"message":"gas required exceeds allowance or always failing transaction"}}
该错误出现的最可能是合约执行中出错。
参考链接
How do you calculate gas limit for transaction with data in Ethereum?
