以太坊源码深入分析（9）-- 以太坊通过EVM执行交易过程分析

上一节分析了同步一个新的区块准备插入本地BlockChain之前需要重放并执行新区块的所有交易，并产生交易收据和日志。以太坊是如何执行这些交易呢？这就要请出大名鼎鼎的以太坊虚拟机。
以太坊虚拟机在执行交易分为两个部分，第一部分是创建EVM，计算交易金额，设置交易对象，计算交易gas花销；第二部分是EVM 的虚拟机解析器通过合约指令，执行智能合约代码，具体来看看源码。

一，创建EVM，通过EVM执行交易流程

上一节分析BlockChain调用processor.Process（）遍历block的所有交易，然后调用：

receipt, _, err := ApplyTransaction(p.config, p.bc, nil, gp, statedb, header, tx, usedGas, cfg)。

执行交易并返回收据数据

func ApplyTransaction(config *params.ChainConfig, bc *BlockChain, author *common.Address, gp *GasPool, statedb *state.StateDB, header *types.Header, tx *types.Transaction, usedGas *uint64, cfg vm.Config) (*types.Receipt, uint64, error) {
	msg, err := tx.AsMessage(types.MakeSigner(config, header.Number))
	if err != nil {
		return nil, 0, err
	}
	// Create a new context to be used in the EVM environment
	context := NewEVMContext(msg, header, bc, author)
	// Create a new environment which holds all relevant information
	// about the transaction and calling mechanisms.
	vmenv := vm.NewEVM(context, statedb, config, cfg)
	// Apply the transaction to the current state (included in the env)
	_, gas, failed, err := ApplyMessage(vmenv, msg, gp)
	if err != nil {
		return nil, 0, err
	}
	// Update the state with pending changes
	var root []byte
	if config.IsByzantium(header.Number) {
		statedb.Finalise(true)
	} else {
		root = statedb.IntermediateRoot(config.IsEIP158(header.Number)).Bytes()
	}
	*usedGas += gas


	// Create a new receipt for the transaction, storing the intermediate root and gas used by the tx
	// based on the eip phase, we're passing wether the root touch-delete accounts.
	receipt := types.NewReceipt(root, failed, *usedGas)
	receipt.TxHash = tx.Hash()
	receipt.GasUsed = gas
	// if the transaction created a contract, store the creation address in the receipt.
	if msg.To() == nil {
		receipt.ContractAddress = crypto.CreateAddress(vmenv.Context.Origin, tx.Nonce())
	}
	// Set the receipt logs and create a bloom for filtering
	receipt.Logs = statedb.GetLogs(tx.Hash())
	receipt.Bloom = types.CreateBloom(types.Receipts{receipt})


	return receipt, gas, err
}

1，首先调用tx.Message()方法产生交易Message。这个方法通过txdata数据来拼接Message对象，并通过签名方法signer.Sender(tx)，对txdata 的V、R 、S三个数进行解密得到这个交易的签名公钥（也是就是发送方的地址）。发送方的地址在交易数据中是没有的，这主要是为了防止交易数据被篡改，任何交易数据的变化后通过signer.Sender方法都不能得到正确的地址。
2，调用 NewEVMContext(msg, header, bc, author)创建EVM的上下文环境，调用vm.NewEVM(context, statedb, config, cfg)创建EVM对象，并在内部创建一个evm.interpreter（虚拟机解析器）。
3，调用ApplyMessage(vmenv, msg, gp)方法通过EVM对象来执行Message。

重点看看ApplyMessage()方法的实现：

func ApplyMessage(evm *vm.EVM, msg Message, gp *GasPool) ([]byte, uint64, bool, error) {
	return NewStateTransition(evm, msg, gp).TransitionDb()
}

创建stateTransition对象，执行TransitionDb（）方法：

func (st *StateTransition) TransitionDb() (ret []byte, usedGas uint64, failed bool, err error) {
	if err = st.preCheck(); err != nil {
		return
	}
	msg := st.msg
	sender := st.from() // err checked in preCheck


	homestead := st.evm.ChainConfig().IsHomestead(st.evm.BlockNumber)
	contractCreation := msg.To() == nil


	// Pay intrinsic gas
	gas, err := IntrinsicGas(st.data, contractCreation, homestead)
	if err != nil {
		return nil, 0, false, err
	}
	if err = st.useGas(gas); err != nil {
		return nil, 0, false, err
	}


	var (
		evm = st.evm
		// vm errors do not effect consensus and are therefor
		// not assigned to err, except for insufficient balance
		// error.
		vmerr error
	)
	if contractCreation {
		ret, _, st.gas, vmerr = evm.Create(sender, st.data, st.gas, st.value)
	} else {
		// Increment the nonce for the next transaction
		st.state.SetNonce(sender.Address(), st.state.GetNonce(sender.Address())+1)
		ret, st.gas, vmerr = evm.Call(sender, st.to().Address(), st.data, st.gas, st.value)
	}
	if vmerr != nil {
		log.Debug("VM returned with error", "err", vmerr)
		// The only possible consensus-error would be if there wasn't
		// sufficient balance to make the transfer happen. The first
		// balance transfer may never fail.
		if vmerr == vm.ErrInsufficientBalance {
			return nil, 0, false, vmerr
		}
	}
	st.refundGas()
	st.state.AddBalance(st.evm.Coinbase, new(big.Int).Mul(new(big.Int).SetUint64(st.gasUsed()), st.gasPrice))


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

3.1，调用IntrinsicGas()方法，通过计算消息的大小以及是否是合约创建交易，来计算此次交易需消耗的gas。

3.2，如果是合约创建交易，调用evm.Create(sender, st.data, st.gas, st.value)来执行message

func (evm *EVM) Create(caller ContractRef, code []byte, gas uint64, value *big.Int) (ret []byte, contractAddr common.Address, leftOverGas uint64, err error) {


	// Depth check execution. Fail if we're trying to execute above the
	// limit.
	if evm.depth > int(params.CallCreateDepth) {
		return nil, common.Address{}, gas, ErrDepth
	}
	if !evm.CanTransfer(evm.StateDB, caller.Address(), value) {
		return nil, common.Address{}, gas, ErrInsufficientBalance
	}
	// Ensure there's no existing contract already at the designated address
	nonce := evm.StateDB.GetNonce(caller.Address())
	evm.StateDB.SetNonce(caller.Address(), nonce+1)


	contractAddr = crypto.CreateAddress(caller.Address(), nonce)
	contractHash := evm.StateDB.GetCodeHash(contractAddr)
	if evm.StateDB.GetNonce(contractAddr) != 0 || (contractHash != (common.Hash{}) && contractHash != emptyCodeHash) {
		return nil, common.Address{}, 0, ErrContractAddressCollision
	}
	// Create a new account on the state
	snapshot := evm.StateDB.Snapshot()
	evm.StateDB.CreateAccount(contractAddr)
	if evm.ChainConfig().IsEIP158(evm.BlockNumber) {
		evm.StateDB.SetNonce(contractAddr, 1)
	}
	evm.Transfer(evm.StateDB, caller.Address(), contractAddr, value)


	// initialise a new contract and set the code that is to be used by the
	// E The contract is a scoped evmironment for this execution context
	// only.
	contract := NewContract(caller, AccountRef(contractAddr), value, gas)
	contract.SetCallCode(&contractAddr, crypto.Keccak256Hash(code), code)


	if evm.vmConfig.NoRecursion && evm.depth > 0 {
		return nil, contractAddr, gas, nil
	}


	if evm.vmConfig.Debug && evm.depth == 0 {
		evm.vmConfig.Tracer.CaptureStart(caller.Address(), contractAddr, true, code, gas, value)
	}
	start := time.Now()


	ret, err = run(evm, contract, nil)


	// check whether the max code size has been exceeded
	maxCodeSizeExceeded := evm.ChainConfig().IsEIP158(evm.BlockNumber) && len(ret) > params.MaxCodeSize
	// if the contract creation ran successfully and no errors were returned
	// calculate the gas required to store the code. If the code could not
	// be stored due to not enough gas set an error and let it be handled
	// by the error checking condition below.
	if err == nil && !maxCodeSizeExceeded {
		createDataGas := uint64(len(ret)) * params.CreateDataGas
		if contract.UseGas(createDataGas) {
			evm.StateDB.SetCode(contractAddr, ret)
		} else {
			err = ErrCodeStoreOutOfGas
		}
	}


	// When an error was returned by the EVM or when setting the creation code
	// above we revert to the snapshot and consume any gas remaining. Additionally
	// when we're in homestead this also counts for code storage gas errors.
	if maxCodeSizeExceeded || (err != nil && (evm.ChainConfig().IsHomestead(evm.BlockNumber) || err != ErrCodeStoreOutOfGas)) {
		evm.StateDB.RevertToSnapshot(snapshot)
		if err != errExecutionReverted {
			contract.UseGas(contract.Gas)
		}
	}
	// Assign err if contract code size exceeds the max while the err is still empty.
	if maxCodeSizeExceeded && err == nil {
		err = errMaxCodeSizeExceeded
	}
	if evm.vmConfig.Debug && evm.depth == 0 {
		evm.vmConfig.Tracer.CaptureEnd(ret, gas-contract.Gas, time.Since(start), err)
	}
	return ret, contractAddr, contract.Gas, err
}

3.2.1，evm执行栈深度不能超过1024，发送方持有的以太坊数量大于此次合约交易金额。
3.2.2，对该发送方地址的nonce值+1，通过地址和nonce值生成合约地址，通过合约地址得到合约hash值。
3.2.3，记录一个状态快照，用来后见失败回滚。
3.2.4，为这个合约地址创建一个合约账户，并为这个合约账户设置nonce值为1
3.2.5，产生以太坊资产转移，发送方地址账户金额减value值，合约账户的金额加value值。
3.2.6，根据发送方地址和合约地址，以及金额value 值和gas，合约代码和代码hash值，创建一个合约对象
3.2.7，run方法来执行合约，内部调用evm的解析器来执行合约指令，如果是预编译好的合约，则预编译执行合约就行。
3.2.8，如果执行ok，setcode更新这个合约地址状态，设置usegas为创建合约的gas。如果执行出错，则回滚到之前快照状态，设置usegas为传入的合约gas。

3.3，如果不是新创建的合约，则调用evm.Call(sender, st.to().Address(), st.data, st.gas, st.value)方法，同时更新发送方地址nonce值+1.

func (evm *EVM) Call(caller ContractRef, addr common.Address, input []byte, gas uint64, value *big.Int) (ret []byte, leftOverGas uint64, err error) {
	if evm.vmConfig.NoRecursion && evm.depth > 0 {
		return nil, gas, nil
	}


	// Fail if we're trying to execute above the call depth limit
	if evm.depth > int(params.CallCreateDepth) {
		return nil, gas, ErrDepth
	}
	// Fail if we're trying to transfer more than the available balance
	if !evm.Context.CanTransfer(evm.StateDB, caller.Address(), value) {
		return nil, gas, ErrInsufficientBalance
	}


	var (
		to       = AccountRef(addr)
		snapshot = evm.StateDB.Snapshot()
	)
	if !evm.StateDB.Exist(addr) {
		precompiles := PrecompiledContractsHomestead
		if evm.ChainConfig().IsByzantium(evm.BlockNumber) {
			precompiles = PrecompiledContractsByzantium
		}
		if precompiles[addr] == nil && evm.ChainConfig().IsEIP158(evm.BlockNumber) && value.Sign() == 0 {
			return nil, gas, nil
		}
		evm.StateDB.CreateAccount(addr)
	}
	evm.Transfer(evm.StateDB, caller.Address(), to.Address(), value)


	// Initialise a new contract and set the code that is to be used by the EVM.
	// The contract is a scoped environment for this execution context only.
	contract := NewContract(caller, to, value, gas)
	contract.SetCallCode(&addr, evm.StateDB.GetCodeHash(addr), evm.StateDB.GetCode(addr))


	start := time.Now()


	// Capture the tracer start/end events in debug mode
	if evm.vmConfig.Debug && evm.depth == 0 {
		evm.vmConfig.Tracer.CaptureStart(caller.Address(), addr, false, input, gas, value)


		defer func() { // Lazy evaluation of the parameters
			evm.vmConfig.Tracer.CaptureEnd(ret, gas-contract.Gas, time.Since(start), err)
		}()
	}
	ret, err = run(evm, contract, input)


	// When an error was returned by the EVM or when setting the creation code
	// above we revert to the snapshot and consume any gas remaining. Additionally
	// when we're in homestead this also counts for code storage gas errors.
	if err != nil {
		evm.StateDB.RevertToSnapshot(snapshot)
		if err != errExecutionReverted {
			contract.UseGas(contract.Gas)
		}
	}
	return ret, contract.Gas, err
}

evm.call方法和evm.create方法大致相同，我们来说说不一样的地方。
3.3.1，call方法调用的是一个存在的合约地址的合约，所以不用创建合约账户。如果call方法发现本地没有合约接收方的账户，则需要创建一个接收方的账户，并更新本地状态数据库。
3.3.2，create方法的资金transfer转移是在创建合约用户账户和这个合约账户之间发生，而call方法的资金转移是在合约的发送方和合约的接收方之间产生。

3.4，TransitionDb（）方法执行完合约，调用st.refundGas()方法计算合约退税，调用evm SSTORE指令 或者evm SUICIDE指令销毁合约十都会产生退税。
3.5，计算合约产生的gas总数，加入到矿工账户，作为矿工收入。

4，回到最开始的ApplyTransaction()方法，根据EVM的执行结果，拼接交易receipt数据，其中receipt.Logs日志数据是EVM执行指令代码的时候产生的，receipt.Bloom根据日志数据建立bloom过滤器。

二，EVM 的虚拟机解析器通过运行合约指令，执行智能合约代码

我们从 3.2.7 执行合约的run()方法入手，它调用了evm.interpreter.Run(contract, input)方法

func (in *Interpreter) Run(contract *Contract, input []byte) (ret []byte, err error) {
	// Increment the call depth which is restricted to 1024
	in.evm.depth++
	defer func() { in.evm.depth-- }()


	// Reset the previous call's return data. It's unimportant to preserve the old buffer
	// as every returning call will return new data anyway.
	in.returnData = nil


	// Don't bother with the execution if there's no code.
	if len(contract.Code) == 0 {
		return nil, nil
	}


	var (
		op    OpCode        // current opcode
		mem   = NewMemory() // bound memory
		stack = newstack()  // local stack
		// For optimisation reason we're using uint64 as the program counter.
		// It's theoretically possible to go above 2^64. The YP defines the PC
		// to be uint256. Practically much less so feasible.
		pc   = uint64(0) // program counter
		cost uint64
		// copies used by tracer
		pcCopy  uint64 // needed for the deferred Tracer
		gasCopy uint64 // for Tracer to log gas remaining before execution
		logged  bool   // deferred Tracer should ignore already logged steps
	)
	contract.Input = input


	if in.cfg.Debug {
		defer func() {
			if err != nil {
				if !logged {
					in.cfg.Tracer.CaptureState(in.evm, pcCopy, op, gasCopy, cost, mem, stack, contract, in.evm.depth, err)
				} else {
					in.cfg.Tracer.CaptureFault(in.evm, pcCopy, op, gasCopy, cost, mem, stack, contract, in.evm.depth, err)
				}
			}
		}()
	}
	// The Interpreter main run loop (contextual). This loop runs until either an
	// explicit STOP, RETURN or SELFDESTRUCT is executed, an error occurred during
	// the execution of one of the operations or until the done flag is set by the
	// parent context.
	for atomic.LoadInt32(&in.evm.abort) == 0 {
		if in.cfg.Debug {
			// Capture pre-execution values for tracing.
			logged, pcCopy, gasCopy = false, pc, contract.Gas
		}


		// Get the operation from the jump table and validate the stack to ensure there are
		// enough stack items available to perform the operation.
		op = contract.GetOp(pc)
		operation := in.cfg.JumpTable[op]
		if !operation.valid {
			return nil, fmt.Errorf("invalid opcode 0x%x", int(op))
		}
		if err := operation.validateStack(stack); err != nil {
			return nil, err
		}
		// If the operation is valid, enforce and write restrictions
		if err := in.enforceRestrictions(op, operation, stack); err != nil {
			return nil, err
		}


		var memorySize uint64
		// calculate the new memory size and expand the memory to fit
		// the operation
		if operation.memorySize != nil {
			memSize, overflow := bigUint64(operation.memorySize(stack))
			if overflow {
				return nil, errGasUintOverflow
			}
			// memory is expanded in words of 32 bytes. Gas
			// is also calculated in words.
			if memorySize, overflow = math.SafeMul(toWordSize(memSize), 32); overflow {
				return nil, errGasUintOverflow
			}
		}


		if !in.cfg.DisableGasMetering {
			// consume the gas and return an error if not enough gas is available.
			// cost is explicitly set so that the capture state defer method cas get the proper cost
			cost, err = operation.gasCost(in.gasTable, in.evm, contract, stack, mem, memorySize)
			if err != nil || !contract.UseGas(cost) {
				return nil, ErrOutOfGas
			}
		}
		if memorySize > 0 {
			mem.Resize(memorySize)
		}


		if in.cfg.Debug {
			in.cfg.Tracer.CaptureState(in.evm, pc, op, gasCopy, cost, mem, stack, contract, in.evm.depth, err)
			logged = true
		}


		// execute the operation
		res, err := operation.execute(&pc, in.evm, contract, mem, stack)
		// verifyPool is a build flag. Pool verification makes sure the integrity
		// of the integer pool by comparing values to a default value.
		if verifyPool {
			verifyIntegerPool(in.intPool)
		}
		// if the operation clears the return data (e.g. it has returning data)
		// set the last return to the result of the operation.
		if operation.returns {
			in.returnData = res
		}


		switch {
		case err != nil:
			return nil, err
		case operation.reverts:
			return res, errExecutionReverted
		case operation.halts:
			return res, nil
		case !operation.jumps:
			pc++
		}
	}
	return nil, nil
}

我们直接看解析器处理的主循环，之前的代码都是在初始化一些临时变量。
1，首先调用contract.GetOp(pc)从和约二进制数据里取得第pc个opcode，opcode是以太坊虚拟机指令，一共不超过256个，正好一个byte大小能装下。

2，从解析器的JumpTable表中查到op对应的operation。比如opcode是SHA3（0x20），取到的operation就是

    SHA3: {
			execute:       opSha3,
			gasCost:       gasSha3,
			validateStack: makeStackFunc(2, 1),
			memorySize:    memorySha3,
			valid:         true,
		}

execute表示指令对应的执行方法
gasCost表示执行这个指令需要消耗的gas
validateStack计算是不是解析器栈溢出
memorySize用于计算operation的占用内存大小

3，如果operation可用，解析器栈不超过1024，且读写不冲突
4，计算operation的memorysize，不能大于64位。
5，根据不同的指令，指令的memorysize等，调用operation.gasCost()方法计算执行operation指令需要消耗的gas。
6，调用operation.execute(&pc, in.evm, contract, mem, stack)执行指令对应的方法。
7，operation.reverts值是true或者operation.halts值是true的指令，会跳出主循环，否则继续遍历下个op。
8，operation指令集里面有4个特殊的指令LOG0，LOG1，LOG2，LOG3，它们的指令执行方法makeLog()会产生日志数据，这些日志数据会写入到tx的Receipt的logs里面，并存入本地ldb数据库。

总结

EVM是以太坊的核心功能，得益于EVM，以太坊把区块链带入了2.0时代，这是一个非常伟大的进步。客观的说以太坊的EVM还不是很完善，存在一些诟病，比如支持address类型数据导致栈容易溢出，缺少标准库，智能合约一旦创建很难修改，不支持浮点数等等问题，但这些问题都不足以击溃以太坊EVM的前进步伐。相信这些问题都会在以太坊不断的迭代更新中加以完善改进。