path: root/core/state_transition.go

// Copyright 2014 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

package core

import (
    "errors"
    "fmt"
    "math/big"

    "github.com/ethereum/go-ethereum/common"
    "github.com/ethereum/go-ethereum/common/math"
    "github.com/ethereum/go-ethereum/core/vm"
    "github.com/ethereum/go-ethereum/log"
    "github.com/ethereum/go-ethereum/params"
)

var (
    Big0                         = big.NewInt(0)
    errInsufficientBalanceForGas = errors.New("insufficient balance to pay for gas")
)

/*
The State Transitioning Model

A state transition is a change made when a transaction is applied to the current world state
The state transitioning model does all all the necessary work to work out a valid new state root.

1) Nonce handling
2) Pre pay gas
3) Create a new state object if the recipient is \0*32
4) Value transfer
== If contract creation ==
  4a) Attempt to run transaction data
  4b) If valid, use result as code for the new state object
== end ==
5) Run Script section
6) Derive new state root
*/
type StateTransition struct {
    gp         *GasPool
    msg        Message
    gas        uint64
    gasPrice   *big.Int
    initialGas *big.Int
    value      *big.Int
    data       []byte
    state      vm.StateDB

    evm *vm.EVM
}

// Message represents a message sent to a contract.
type Message interface {
    From() common.Address
    //FromFrontier() (common.Address, error)
    To() *common.Address

    GasPrice() *big.Int
    Gas() *big.Int
    Value() *big.Int

    Nonce() uint64
    CheckNonce() bool
    Data() []byte
}

// IntrinsicGas computes the 'intrinsic gas' for a message
// with the given data.
//
// TODO convert to uint64
func IntrinsicGas(data []byte, contractCreation, homestead bool) *big.Int {
    igas := new(big.Int)
    if contractCreation && homestead {
        igas.SetUint64(params.TxGasContractCreation)
    } else {
        igas.SetUint64(params.TxGas)
    }
    if len(data) > 0 {
        var nz int64
        for _, byt := range data {
            if byt != 0 {
                nz++
            }
        }
        m := big.NewInt(nz)
        m.Mul(m, new(big.Int).SetUint64(params.TxDataNonZeroGas))
        igas.Add(igas, m)
        m.SetInt64(int64(len(data)) - nz)
        m.Mul(m, new(big.Int).SetUint64(params.TxDataZeroGas))
        igas.Add(igas, m)
    }
    return igas
}

// NewStateTransition initialises and returns a new state transition object.
func NewStateTransition(evm *vm.EVM, msg Message, gp *GasPool) *StateTransition {
    return &StateTransition{
        gp:         gp,
        evm:        evm,
        msg:        msg,
        gasPrice:   msg.GasPrice(),
        initialGas: new(big.Int),
        value:      msg.Value(),
        data:       msg.Data(),
        state:      evm.StateDB,
    }
}

// ApplyMessage computes the new state by applying the given message
// against the old state within the environment.
//
// ApplyMessage returns the bytes returned by any EVM execution (if it took place),
// the gas used (which includes gas refunds) and an error if it failed. An error always
// indicates a core error meaning that the message would always fail for that particular
// state and would never be accepted within a block.
func ApplyMessage(evm *vm.EVM, msg Message, gp *GasPool) ([]byte, *big.Int, error) {
    st := NewStateTransition(evm, msg, gp)

    ret, _, gasUsed, err := st.TransitionDb()
    return ret, gasUsed, err
}

func (st *StateTransition) from() vm.AccountRef {
    f := st.msg.From()
    if !st.state.Exist(f) {
        st.state.CreateAccount(f)
    }
    return vm.AccountRef(f)
}

func (st *StateTransition) to() vm.AccountRef {
    if st.msg == nil {
        return vm.AccountRef{}
    }
    to := st.msg.To()
    if to == nil {
        return vm.AccountRef{} // contract creation
    }

    reference := vm.AccountRef(*to)
    if !st.state.Exist(*to) {
        st.state.CreateAccount(*to)
    }
    return reference
}

func (st *StateTransition) useGas(amount uint64) error {
    if st.gas < amount {
        return vm.ErrOutOfGas
    }
    st.gas -= amount

    return nil
}

func (st *StateTransition) buyGas() error {
    mgas := st.msg.Gas()
    if mgas.BitLen() > 64 {
        return vm.ErrOutOfGas
    }

    mgval := new(big.Int).Mul(mgas, st.gasPrice)

    var (
        state  = st.state
        sender = st.from()
    )
    if state.GetBalance(sender.Address()).Cmp(mgval) < 0 {
        return errInsufficientBalanceForGas
    }
    if err := st.gp.SubGas(mgas); err != nil {
        return err
    }
    st.gas += mgas.Uint64()

    st.initialGas.Set(mgas)
    state.SubBalance(sender.Address(), mgval)
    return nil
}

func (st *StateTransition) preCheck() error {
    msg := st.msg
    sender := st.from()

    // Make sure this transaction's nonce is correct
    if msg.CheckNonce() {
        if n := st.state.GetNonce(sender.Address()); n != msg.Nonce() {
            return fmt.Errorf("invalid nonce: have %d, expected %d", msg.Nonce(), n)
        }
    }
    return st.buyGas()
}

// TransitionDb will transition the state by applying the current message and returning the result
// including the required gas for the operation as well as the used gas. It returns an error if it
// failed. An error indicates a consensus issue.
func (st *StateTransition) TransitionDb() (ret []byte, requiredGas, usedGas *big.Int, 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
    // TODO convert to uint64
    intrinsicGas := IntrinsicGas(st.data, contractCreation, homestead)
    if intrinsicGas.BitLen() > 64 {
        return nil, nil, nil, vm.ErrOutOfGas
    }
    if err = st.useGas(intrinsicGas.Uint64()); err != nil {
        return nil, nil, nil, 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, nil, nil, vmerr
        }
    }
    requiredGas = new(big.Int).Set(st.gasUsed())

    st.refundGas()
    st.state.AddBalance(st.evm.Coinbase, new(big.Int).Mul(st.gasUsed(), st.gasPrice))

    return ret, requiredGas, st.gasUsed(), err
}

func (st *StateTransition) refundGas() {
    // Return eth for remaining gas to the sender account,
    // exchanged at the original rate.
    sender := st.from() // err already checked
    remaining := new(big.Int).Mul(new(big.Int).SetUint64(st.gas), st.gasPrice)
    st.state.AddBalance(sender.Address(), remaining)

    // Apply refund counter, capped to half of the used gas.
    uhalf := remaining.Div(st.gasUsed(), common.Big2)
    refund := math.BigMin(uhalf, st.state.GetRefund())
    st.gas += refund.Uint64()

    st.state.AddBalance(sender.Address(), refund.Mul(refund, st.gasPrice))

    // Also return remaining gas to the block gas counter so it is
    // available for the next transaction.
    st.gp.AddGas(new(big.Int).SetUint64(st.gas))
}

func (st *StateTransition) gasUsed() *big.Int {
    return new(big.Int).Sub(st.initialGas, new(big.Int).SetUint64(st.gas))
}