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"
    "math"
    "math/big"

    "github.com/dexon-foundation/dexon/common"
    "github.com/dexon-foundation/dexon/core/vm"
    "github.com/dexon-foundation/dexon/log"
    "github.com/dexon-foundation/dexon/params"
)

var (
    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 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 uint64
    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() uint64
    Value() *big.Int

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

// 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
}

// 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(),
        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, uint64, bool, error) {
    return NewStateTransition(evm, msg, gp).TransitionDb()
}

// to returns the recipient of the message.
func (st *StateTransition) to() common.Address {
    if st.msg == nil || st.msg.To() == nil /* contract creation */ {
        return common.Address{}
    }
    return *st.msg.To()
}

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

    return nil
}

func (st *StateTransition) buyGas() error {
    mgval := new(big.Int).Mul(new(big.Int).SetUint64(st.msg.Gas()), st.gasPrice)
    if st.state.GetBalance(st.msg.From()).Cmp(mgval) < 0 {
        return errInsufficientBalanceForGas
    }
    if err := st.gp.SubGas(st.msg.Gas()); err != nil {
        return err
    }
    st.gas += st.msg.Gas()

    st.initialGas = st.msg.Gas()
    st.state.SubBalance(st.msg.From(), mgval)
    return nil
}

func (st *StateTransition) preCheck() error {
    // Make sure this transaction's nonce is correct.
    if st.msg.CheckNonce() {
        nonce := st.state.GetNonce(st.msg.From())
        if nonce < st.msg.Nonce() {
            return ErrNonceTooHigh
        } else if nonce > st.msg.Nonce() {
            return ErrNonceTooLow
        }
    }
    return st.buyGas()
}

// TransitionDb will transition the state by applying the current message and
// returning the result including the used gas. It returns an error if failed.
// An error indicates a consensus issue.
func (st *StateTransition) TransitionDb() (ret []byte, usedGas uint64, failed bool, err error) {
    if err = st.preCheck(); err != nil {
        return
    }
    msg := st.msg
    sender := vm.AccountRef(msg.From())
    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(msg.From(), st.state.GetNonce(sender.Address())+1)
        ret, st.gas, vmerr = evm.Call(sender, st.to(), 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
}

func (st *StateTransition) refundGas() {
    // Apply refund counter, capped to half of the used gas.
    refund := st.gasUsed() / 2
    if refund > st.state.GetRefund() {
        refund = st.state.GetRefund()
    }
    st.gas += refund

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

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

// gasUsed returns the amount of gas used up by the state transition.
func (st *StateTransition) gasUsed() uint64 {
    return st.initialGas - st.gas
}