go-ethereum/consensus/XDPoS/XDPoS.go

// Copyright (c) 2018 XDCchain
//
// This program 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.
//
// This program 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 this program. If not, see <http://www.gnu.org/licenses/>.

// Package XDPoS implements the delegated-proof-of-stake consensus engine.
package XDPoS

import (
	"bytes"
	"encoding/json"
	"errors"
	"fmt"
	"io/ioutil"
	"math/big"
	"math/rand"
	"path/filepath"
	"reflect"
	"sort"
	"strconv"
	"sync"
	"time"

	"github.com/ethereum/go-ethereum/accounts"
	"github.com/ethereum/go-ethereum/common"
	"github.com/ethereum/go-ethereum/common/hexutil"
	"github.com/ethereum/go-ethereum/consensus"
	"github.com/ethereum/go-ethereum/consensus/clique"
	"github.com/ethereum/go-ethereum/consensus/misc"
	"github.com/ethereum/go-ethereum/core/state"
	"github.com/ethereum/go-ethereum/core/types"
	"github.com/ethereum/go-ethereum/crypto"
	"github.com/ethereum/go-ethereum/crypto/sha3"
	"github.com/ethereum/go-ethereum/ethdb"
	"github.com/ethereum/go-ethereum/log"
	"github.com/ethereum/go-ethereum/params"
	"github.com/ethereum/go-ethereum/rlp"
	"github.com/ethereum/go-ethereum/rpc"
	lru "github.com/hashicorp/golang-lru"
)

const (
	inmemorySnapshots      = 128 // Number of recent vote snapshots to keep in memory
	blockSignersCacheLimit = 9000
	M2ByteLength           = 4
)

type Masternode struct {
	Address common.Address
	Stake   *big.Int
}

// XDPoS delegated-proof-of-stake protocol constants.
var (
	epochLength = uint64(900) // Default number of blocks after which to checkpoint and reset the pending votes

	extraVanity = 32 // Fixed number of extra-data prefix bytes reserved for signer vanity
	extraSeal   = 65 // Fixed number of extra-data suffix bytes reserved for signer seal

	nonceAuthVote = hexutil.MustDecode("0xffffffffffffffff") // Magic nonce number to vote on adding a new signer
	nonceDropVote = hexutil.MustDecode("0x0000000000000000") // Magic nonce number to vote on removing a signer.

	uncleHash = types.CalcUncleHash(nil) // Always Keccak256(RLP([])) as uncles are meaningless outside of PoW.

	diffInTurn = big.NewInt(2) // Block difficulty for in-turn signatures
	diffNoTurn = big.NewInt(1) // Block difficulty for out-of-turn signatures
)

// Various error messages to mark blocks invalid. These should be private to
// prevent engine specific errors from being referenced in the remainder of the
// codebase, inherently breaking if the engine is swapped out. Please put common
// error types into the consensus package.
var (
	// errUnknownBlock is returned when the list of signers is requested for a block
	// that is not part of the local blockchain.
	errUnknownBlock = errors.New("unknown block")

	// errInvalidCheckpointBeneficiary is returned if a checkpoint/epoch transition
	// block has a beneficiary set to non-zeroes.
	errInvalidCheckpointBeneficiary = errors.New("beneficiary in checkpoint block non-zero")

	// errInvalidVote is returned if a nonce value is something else that the two
	// allowed constants of 0x00..0 or 0xff..f.
	errInvalidVote = errors.New("vote nonce not 0x00..0 or 0xff..f")

	// errInvalidCheckpointVote is returned if a checkpoint/epoch transition block
	// has a vote nonce set to non-zeroes.
	errInvalidCheckpointVote = errors.New("vote nonce in checkpoint block non-zero")

	// errMissingVanity is returned if a block's extra-data section is shorter than
	// 32 bytes, which is required to store the signer vanity.
	errMissingVanity = errors.New("extra-data 32 byte vanity prefix missing")

	// errMissingSignature is returned if a block's extra-data section doesn't seem
	// to contain a 65 byte secp256k1 signature.
	errMissingSignature = errors.New("extra-data 65 byte suffix signature missing")

	// errExtraSigners is returned if non-checkpoint block contain signer data in
	// their extra-data fields.
	errExtraSigners = errors.New("non-checkpoint block contains extra signer list")

	// errInvalidCheckpointSigners is returned if a checkpoint block contains an
	// invalid list of signers (i.e. non divisible by 20 bytes, or not the correct
	// ones).
	errInvalidCheckpointSigners = errors.New("invalid signer list on checkpoint block")

	errInvalidCheckpointPenalties = errors.New("invalid penalty list on checkpoint block")

	// errInvalidMixDigest is returned if a block's mix digest is non-zero.
	errInvalidMixDigest = errors.New("non-zero mix digest")

	// errInvalidUncleHash is returned if a block contains an non-empty uncle list.
	errInvalidUncleHash = errors.New("non empty uncle hash")

	// errInvalidDifficulty is returned if the difficulty of a block is not either
	// of 1 or 2, or if the value does not match the turn of the signer.
	errInvalidDifficulty = errors.New("invalid difficulty")

	// ErrInvalidTimestamp is returned if the timestamp of a block is lower than
	// the previous block's timestamp + the minimum block period.
	ErrInvalidTimestamp = errors.New("invalid timestamp")

	// errInvalidVotingChain is returned if an authorization list is attempted to
	// be modified via out-of-range or non-contiguous headers.
	errInvalidVotingChain = errors.New("invalid voting chain")

	// errUnauthorized is returned if a header is signed by a non-authorized entity.
	errUnauthorized = errors.New("unauthorized")

	errFailedDoubleValidation = errors.New("wrong pair of creator-validator in double validation")

	// errWaitTransactions is returned if an empty block is attempted to be sealed
	// on an instant chain (0 second period). It's important to refuse these as the
	// block reward is zero, so an empty block just bloats the chain... fast.
	errWaitTransactions = errors.New("waiting for transactions")

	ErrInvalidCheckpointValidators = errors.New("invalid validators list on checkpoint block")
)

// SignerFn is a signer callback function to request a hash to be signed by a
// backing account.
//type SignerFn func(accounts.Account, []byte) ([]byte, error)

// sigHash returns the hash which is used as input for the delegated-proof-of-stake
// signing. It is the hash of the entire header apart from the 65 byte signature
// contained at the end of the extra data.
//
// Note, the method requires the extra data to be at least 65 bytes, otherwise it
// panics. This is done to avoid accidentally using both forms (signature present
// or not), which could be abused to produce different hashes for the same header.
func sigHash(header *types.Header) (hash common.Hash) {
	hasher := sha3.NewKeccak256()

	rlp.Encode(hasher, []interface{}{
		header.ParentHash,
		header.UncleHash,
		header.Coinbase,
		header.Root,
		header.TxHash,
		header.ReceiptHash,
		header.Bloom,
		header.Difficulty,
		header.Number,
		header.GasLimit,
		header.GasUsed,
		header.Time,
		header.Extra[:len(header.Extra)-65], // Yes, this will panic if extra is too short
		header.MixDigest,
		header.Nonce,
	})
	hasher.Sum(hash[:0])
	return hash
}

func SigHash(header *types.Header) (hash common.Hash) {
	return sigHash(header)
}

// ecrecover extracts the Ethereum account address from a signed header.
func ecrecover(header *types.Header, sigcache *lru.ARCCache) (common.Address, error) {
	// If the signature's already cached, return that
	hash := header.Hash()
	if address, known := sigcache.Get(hash); known {
		return address.(common.Address), nil
	}
	// Retrieve the signature from the header extra-data
	if len(header.Extra) < extraSeal {
		return common.Address{}, errMissingSignature
	}
	signature := header.Extra[len(header.Extra)-extraSeal:]

	// Recover the public key and the Ethereum address
	pubkey, err := crypto.Ecrecover(sigHash(header).Bytes(), signature)
	if err != nil {
		return common.Address{}, err
	}
	var signer common.Address
	copy(signer[:], crypto.Keccak256(pubkey[1:])[12:])

	sigcache.Add(hash, signer)
	return signer, nil
}

// XDPoS is the delegated-proof-of-stake consensus engine proposed to support the
// Ethereum testnet following the Ropsten attacks.
type XDPoS struct {
	config *params.XDPoSConfig // Consensus engine configuration parameters
	db     ethdb.Database      // Database to store and retrieve snapshot checkpoints

	recents             *lru.ARCCache // Snapshots for recent block to speed up reorgs
	signatures          *lru.ARCCache // Signatures of recent blocks to speed up mining
	validatorSignatures *lru.ARCCache // Signatures of recent blocks to speed up mining
	verifiedHeaders     *lru.ARCCache
	proposals           map[common.Address]bool // Current list of proposals we are pushing

	signer common.Address  // Ethereum address of the signing key
	signFn clique.SignerFn // Signer function to authorize hashes with
	lock   sync.RWMutex    // Protects the signer fields

	BlockSigners               *lru.Cache
	HookReward                 func(chain consensus.ChainReader, state *state.StateDB, header *types.Header) (error, map[string]interface{})
	HookPenalty                func(chain consensus.ChainReader, blockNumberEpoc uint64) ([]common.Address, error)
	HookGetSignersFromContract func(blockHash common.Hash) ([]common.Address, error)
	HookPenaltyTIPSigning      func(chain consensus.ChainReader, header *types.Header, candidate []common.Address) ([]common.Address, error)
	HookValidator              func(header *types.Header, signers []common.Address) ([]byte, error)
	HookVerifyMNs              func(header *types.Header, signers []common.Address) error
}

// New creates a XDPoS delegated-proof-of-stake consensus engine with the initial
// signers set to the ones provided by the user.
func New(config *params.XDPoSConfig, db ethdb.Database) *XDPoS {
	// Set any missing consensus parameters to their defaults
	conf := *config
	if conf.Epoch == 0 {
		conf.Epoch = epochLength
	}
	// Allocate the snapshot caches and create the engine
	BlockSigners, _ := lru.New(blockSignersCacheLimit)
	recents, _ := lru.NewARC(inmemorySnapshots)
	signatures, _ := lru.NewARC(inmemorySnapshots)
	validatorSignatures, _ := lru.NewARC(inmemorySnapshots)
	verifiedHeaders, _ := lru.NewARC(inmemorySnapshots)
	return &XDPoS{
		config:              &conf,
		db:                  db,
		BlockSigners:        BlockSigners,
		recents:             recents,
		signatures:          signatures,
		verifiedHeaders:     verifiedHeaders,
		validatorSignatures: validatorSignatures,
		proposals:           make(map[common.Address]bool),
	}
}

// Author implements consensus.Engine, returning the Ethereum address recovered
// from the signature in the header's extra-data section.
func (c *XDPoS) Author(header *types.Header) (common.Address, error) {
	return ecrecover(header, c.signatures)
}

// Get signer coinbase
func (c *XDPoS) Signer() common.Address { return c.signer }

// VerifyHeader checks whether a header conforms to the consensus rules.
func (c *XDPoS) VerifyHeader(chain consensus.ChainReader, header *types.Header, fullVerify bool) error {
	return c.verifyHeaderWithCache(chain, header, nil, fullVerify)
}

// VerifyHeaders is similar to VerifyHeader, but verifies a batch of headers. The
// method returns a quit channel to abort the operations and a results channel to
// retrieve the async verifications (the order is that of the input slice).
func (c *XDPoS) VerifyHeaders(chain consensus.ChainReader, headers []*types.Header, fullVerifies []bool) (chan<- struct{}, <-chan error) {
	abort := make(chan struct{})
	results := make(chan error, len(headers))

	go func() {
		for i, header := range headers {
			err := c.verifyHeaderWithCache(chain, header, headers[:i], fullVerifies[i])

			select {
			case <-abort:
				return
			case results <- err:
			}
		}
	}()
	return abort, results
}

func (c *XDPoS) verifyHeaderWithCache(chain consensus.ChainReader, header *types.Header, parents []*types.Header, fullVerify bool) error {
	_, check := c.verifiedHeaders.Get(header.Hash())
	if check {
		return nil
	}
	err := c.verifyHeader(chain, header, parents, fullVerify)
	if err == nil {
		c.verifiedHeaders.Add(header.Hash(), true)
	}
	return err
}

// verifyHeader checks whether a header conforms to the consensus rules.The
// caller may optionally pass in a batch of parents (ascending order) to avoid
// looking those up from the database. This is useful for concurrently verifying
// a batch of new headers.
func (c *XDPoS) verifyHeader(chain consensus.ChainReader, header *types.Header, parents []*types.Header, fullVerify bool) error {
	if common.IsTestnet {
		fullVerify = false
	}
	if header.Number == nil {
		return errUnknownBlock
	}
	number := header.Number.Uint64()
	if fullVerify {
		if header.Number.Uint64() > c.config.Epoch && len(header.Validator) == 0 {
			return consensus.ErrNoValidatorSignature
		}
		// Don't waste time checking blocks from the future
		if header.Time.Cmp(big.NewInt(time.Now().Unix())) > 0 {
			return consensus.ErrFutureBlock
		}
	}
	// Checkpoint blocks need to enforce zero beneficiary
	checkpoint := (number % c.config.Epoch) == 0
	if checkpoint && header.Coinbase != (common.Address{}) {
		return errInvalidCheckpointBeneficiary
	}

	// Nonces must be 0x00..0 or 0xff..f, zeroes enforced on checkpoints
	if !bytes.Equal(header.Nonce[:], nonceAuthVote) && !bytes.Equal(header.Nonce[:], nonceDropVote) {
		return errInvalidVote
	}
	if checkpoint && !bytes.Equal(header.Nonce[:], nonceDropVote) {
		return errInvalidCheckpointVote
	}
	// Check that the extra-data contains both the vanity and signature
	if len(header.Extra) < extraVanity {
		return errMissingVanity
	}
	if len(header.Extra) < extraVanity+extraSeal {
		return errMissingSignature
	}
	// Ensure that the extra-data contains a signer list on checkpoint, but none otherwise
	signersBytes := len(header.Extra) - extraVanity - extraSeal
	if !checkpoint && signersBytes != 0 {
		return errExtraSigners
	}
	if checkpoint && signersBytes%common.AddressLength != 0 {
		return errInvalidCheckpointSigners
	}
	// Ensure that the mix digest is zero as we don't have fork protection currently
	if header.MixDigest != (common.Hash{}) {
		return errInvalidMixDigest
	}
	// Ensure that the block doesn't contain any uncles which are meaningless in XDPoS
	if header.UncleHash != uncleHash {
		return errInvalidUncleHash
	}
	// If all checks passed, validate any special fields for hard forks
	if err := misc.VerifyForkHashes(chain.Config(), header, false); err != nil {
		return err
	}
	// All basic checks passed, verify cascading fields
	return c.verifyCascadingFields(chain, header, parents, fullVerify)
}

// verifyCascadingFields verifies all the header fields that are not standalone,
// rather depend on a batch of previous headers. The caller may optionally pass
// in a batch of parents (ascending order) to avoid looking those up from the
// database. This is useful for concurrently verifying a batch of new headers.
func (c *XDPoS) verifyCascadingFields(chain consensus.ChainReader, header *types.Header, parents []*types.Header, fullVerify bool) error {
	// The genesis block is the always valid dead-end
	number := header.Number.Uint64()
	if number == 0 {
		return nil
	}
	// Ensure that the block's timestamp isn't too close to it's parent
	var parent *types.Header
	if len(parents) > 0 {
		parent = parents[len(parents)-1]
	} else {
		parent = chain.GetHeader(header.ParentHash, number-1)
	}
	if parent == nil || parent.Number.Uint64() != number-1 || parent.Hash() != header.ParentHash {
		return consensus.ErrUnknownAncestor
	}
	if parent.Time.Uint64()+c.config.Period > header.Time.Uint64() {
		return ErrInvalidTimestamp
	}

	if number%c.config.Epoch != 0 {
		return c.verifySeal(chain, header, parents, fullVerify)
	}

	/*
		BUG: snapshot returns wrong signers sometimes
		when it happens we get the signers list by requesting smart contract
	*/
	// Retrieve the snapshot needed to verify this header and cache it
	snap, err := c.snapshot(chain, number-1, header.ParentHash, parents)
	if err != nil {
		return err
	}

	signers := snap.GetSigners()
	err = c.checkSignersOnCheckpoint(chain, header, signers)
	if err == nil {
		return c.verifySeal(chain, header, parents, fullVerify)
	}
	signers, err = c.GetSignersFromContract(chain, header)
	if err != nil {
		return err
	}

	if err == nil {
		return c.verifySeal(chain, header, parents, fullVerify)
	}

	return err
}

func (c *XDPoS) checkSignersOnCheckpoint(chain consensus.ChainReader, header *types.Header, signers []common.Address) error {
	number := header.Number.Uint64()
	// ignore signerCheck at checkpoint block 27307800 due to wrong snapshot at gap 27307799
	if number == common.IgnoreSignerCheckBlock {
		return nil
	}
	penPenalties := []common.Address{}
	if c.HookPenalty != nil || c.HookPenaltyTIPSigning != nil {
		var err error
		if chain.Config().IsTIPSigning(header.Number) {
			penPenalties, err = c.HookPenaltyTIPSigning(chain, header, signers)
		} else {
			penPenalties, err = c.HookPenalty(chain, number)
		}
		if err != nil {
			return err
		}
		for _, address := range penPenalties {
			log.Debug("Penalty Info", "address", address, "number", number)
		}
		bytePenalties := common.ExtractAddressToBytes(penPenalties)
		if !bytes.Equal(header.Penalties, bytePenalties) {
			return errInvalidCheckpointPenalties
		}
	}
	signers = common.RemoveItemFromArray(signers, penPenalties)
	for i := 1; i <= common.LimitPenaltyEpoch; i++ {
		if number > uint64(i)*c.config.Epoch {
			signers = RemovePenaltiesFromBlock(chain, signers, number-uint64(i)*c.config.Epoch)
		}
	}
	extraSuffix := len(header.Extra) - extraSeal
	masternodesFromCheckpointHeader := common.ExtractAddressFromBytes(header.Extra[extraVanity:extraSuffix])
	validSigners := compareSignersLists(masternodesFromCheckpointHeader, signers)
	if !validSigners {
		log.Error("Masternodes lists are different in checkpoint header and snapshot", "number", number, "masternodes_from_checkpoint_header", masternodesFromCheckpointHeader, "masternodes_in_snapshot", signers, "penList", penPenalties)
		return errInvalidCheckpointSigners
	}
	if c.HookVerifyMNs != nil {
		err := c.HookVerifyMNs(header, signers)
		if err != nil {
			return err
		}
	}

	return nil
}

// compare 2 signers lists
// return true if they are same elements, otherwise return false
func compareSignersLists(list1 []common.Address, list2 []common.Address) bool {
	if len(list1) == 0 && len(list2) == 0 {
		return true
	}
	sort.Slice(list1, func(i, j int) bool {
		return list1[i].String() <= list1[j].String()
	})
	sort.Slice(list2, func(i, j int) bool {
		return list2[i].String() <= list2[j].String()
	})
	return reflect.DeepEqual(list1, list2)
}

func (c *XDPoS) GetSnapshot(chain consensus.ChainReader, header *types.Header) (*Snapshot, error) {
	number := header.Number.Uint64()
	log.Trace("take snapshot", "number", number, "hash", header.Hash())
	snap, err := c.snapshot(chain, number, header.Hash(), nil)
	if err != nil {
		return nil, err
	}
	return snap, nil
}

func (c *XDPoS) StoreSnapshot(snap *Snapshot) error {
	return snap.store(c.db)
}

func position(list []common.Address, x common.Address) int {
	for i, item := range list {
		if item == x {
			return i
		}
	}
	return -1
}

func (c *XDPoS) GetMasternodes(chain consensus.ChainReader, header *types.Header) []common.Address {
	n := header.Number.Uint64()
	e := c.config.Epoch
	switch {
	case n%e == 0:
		return c.GetMasternodesFromCheckpointHeader(header, n, e)
	case n%e != 0:
		h := chain.GetHeaderByNumber(n - (n % e))
		return c.GetMasternodesFromCheckpointHeader(h, n, e)
	default:
		return []common.Address{}
	}
}

func (c *XDPoS) GetPeriod() uint64 { return c.config.Period }

func whoIsCreator(snap *Snapshot, header *types.Header) (common.Address, error) {
	if header.Number.Uint64() == 0 {
		return common.Address{}, errors.New("Don't take block 0")
	}
	m, err := ecrecover(header, snap.sigcache)
	if err != nil {
		return common.Address{}, err
	}
	return m, nil
}

func (c *XDPoS) YourTurn(chain consensus.ChainReader, parent *types.Header, signer common.Address) (int, int, int, bool, error) {
	masternodes := c.GetMasternodes(chain, parent)

	if common.IsTestnet {
		// Only three mns hard code for XDC testnet.
		masternodes = []common.Address{
			common.HexToAddress("0xfFC679Dcdf444D2eEb0491A998E7902B411CcF20"),
			common.HexToAddress("0xd76fd76F7101811726DCE9E43C2617706a4c45c8"),
			common.HexToAddress("0x8A97753311aeAFACfd76a68Cf2e2a9808d3e65E8"),
		}
	}

	snap, err := c.GetSnapshot(chain, parent)
	if err != nil {
		log.Warn("Failed when trying to commit new work", "err", err)
		return 0, -1, -1, false, err
	}
	if len(masternodes) == 0 {
		return 0, -1, -1, false, errors.New("Masternodes not found")
	}
	pre := common.Address{}
	// masternode[0] has chance to create block 1
	preIndex := -1
	if parent.Number.Uint64() != 0 {
		pre, err = whoIsCreator(snap, parent)
		if err != nil {
			return 0, 0, 0, false, err
		}
		preIndex = position(masternodes, pre)
	}
	curIndex := position(masternodes, signer)
	if signer == c.signer {
		log.Debug("Masternodes cycle info", "number of masternodes", len(masternodes), "previous", pre, "position", preIndex, "current", signer, "position", curIndex)
	}
	for i, s := range masternodes {
		log.Debug("Masternode:", "index", i, "address", s.String())
	}
	if (preIndex+1)%len(masternodes) == curIndex {
		return len(masternodes), preIndex, curIndex, true, nil
	}
	return len(masternodes), preIndex, curIndex, false, nil
}

// snapshot retrieves the authorization snapshot at a given point in time.
func (c *XDPoS) snapshot(chain consensus.ChainReader, number uint64, hash common.Hash, parents []*types.Header) (*Snapshot, error) {
	// Search for a snapshot in memory or on disk for checkpoints
	var (
		headers []*types.Header
		snap    *Snapshot
	)
	for snap == nil {
		// If an in-memory snapshot was found, use that
		if s, ok := c.recents.Get(hash); ok {
			snap = s.(*Snapshot)
			break
		}
		// If an on-disk checkpoint snapshot can be found, use that
		// checkpoint snapshot = checkpoint - gap
		if (number+c.config.Gap)%c.config.Epoch == 0 {
			if s, err := loadSnapshot(c.config, c.signatures, c.db, hash); err == nil {
				log.Trace("Loaded voting snapshot form disk", "number", number, "hash", hash)
				snap = s
				break
			}
		}
		// If we're at block zero, make a snapshot
		if number == 0 {
			genesis := chain.GetHeaderByNumber(0)
			if err := c.VerifyHeader(chain, genesis, true); err != nil {
				return nil, err
			}
			signers := make([]common.Address, (len(genesis.Extra)-extraVanity-extraSeal)/common.AddressLength)
			for i := 0; i < len(signers); i++ {
				copy(signers[i][:], genesis.Extra[extraVanity+i*common.AddressLength:])
			}
			snap = newSnapshot(c.config, c.signatures, 0, genesis.Hash(), signers)
			if err := snap.store(c.db); err != nil {
				return nil, err
			}
			log.Trace("Stored genesis voting snapshot to disk")
			break
		}
		// No snapshot for this header, gather the header and move backward
		var header *types.Header
		if len(parents) > 0 {
			// If we have explicit parents, pick from there (enforced)
			header = parents[len(parents)-1]
			if header.Hash() != hash || header.Number.Uint64() != number {
				return nil, consensus.ErrUnknownAncestor
			}
			parents = parents[:len(parents)-1]
		} else {
			// No explicit parents (or no more left), reach out to the database
			header = chain.GetHeader(hash, number)
			if header == nil {
				return nil, consensus.ErrUnknownAncestor
			}
		}
		headers = append(headers, header)
		number, hash = number-1, header.ParentHash
	}
	// Previous snapshot found, apply any pending headers on top of it
	for i := 0; i < len(headers)/2; i++ {
		headers[i], headers[len(headers)-1-i] = headers[len(headers)-1-i], headers[i]
	}
	snap, err := snap.apply(headers)
	if err != nil {
		return nil, err
	}
	c.recents.Add(snap.Hash, snap)

	// If we've generated a new checkpoint snapshot, save to disk
	if (snap.Number+c.config.Gap)%c.config.Epoch == 0 {
		if err = snap.store(c.db); err != nil {
			return nil, err
		}
		log.Trace("Stored voting snapshot to disk", "number", snap.Number, "hash", snap.Hash)
	}
	return snap, err
}

// VerifyUncles implements consensus.Engine, always returning an error for any
// uncles as this consensus mechanism doesn't permit uncles.
func (c *XDPoS) VerifyUncles(chain consensus.ChainReader, block *types.Block) error {
	if len(block.Uncles()) > 0 {
		return errors.New("uncles not allowed")
	}
	return nil
}

// VerifySeal implements consensus.Engine, checking whether the signature contained
// in the header satisfies the consensus protocol requirements.
func (c *XDPoS) VerifySeal(chain consensus.ChainReader, header *types.Header) error {
	return c.verifySeal(chain, header, nil, true)
}

// verifySeal checks whether the signature contained in the header satisfies the
// consensus protocol requirements. The method accepts an optional list of parent
// headers that aren't yet part of the local blockchain to generate the snapshots
// from.
// verifySeal also checks the pair of creator-validator set in the header satisfies
// the double validation.
func (c *XDPoS) verifySeal(chain consensus.ChainReader, header *types.Header, parents []*types.Header, fullVerify bool) error {
	// Verifying the genesis block is not supported
	number := header.Number.Uint64()
	if number == 0 {
		return errUnknownBlock
	}
	// Retrieve the snapshot needed to verify this header and cache it
	snap, err := c.snapshot(chain, number-1, header.ParentHash, parents)
	if err != nil {
		return err
	}

	// Resolve the authorization key and check against signers
	creator, err := ecrecover(header, c.signatures)
	if err != nil {
		return err
	}
	var parent *types.Header
	if len(parents) > 0 {
		parent = parents[len(parents)-1]
	} else {
		parent = chain.GetHeader(header.ParentHash, number-1)
	}
	difficulty := c.calcDifficulty(chain, parent, creator)
	log.Debug("verify seal block", "number", header.Number, "hash", header.Hash(), "block difficulty", header.Difficulty, "calc difficulty", difficulty, "creator", creator)
	// Ensure that the block's difficulty is meaningful (may not be correct at this point)
	if number > 0 {
		if header.Difficulty.Int64() != difficulty.Int64() {
			return errInvalidDifficulty
		}
	}
	masternodes := c.GetMasternodes(chain, header)
	mstring := []string{}
	for _, m := range masternodes {
		mstring = append(mstring, m.String())
	}
	nstring := []string{}
	for _, n := range snap.GetSigners() {
		nstring = append(nstring, n.String())
	}
	if _, ok := snap.Signers[creator]; !ok {
		valid := false
		for _, m := range masternodes {
			if m == creator {
				valid = true
				break
			}
		}
		if !valid {
			log.Debug("Unauthorized creator found", "block number", number, "creator", creator.String(), "masternodes", mstring, "snapshot from parent block", nstring)
			return errUnauthorized
		}
	}
	if len(masternodes) > 1 {
		for seen, recent := range snap.Recents {
			if recent == creator {
				// Signer is among recents, only fail if the current block doesn't shift it out
				// There is only case that we don't allow signer to create two continuous blocks.
				if limit := uint64(2); seen > number-limit {
					// Only take into account the non-epoch blocks
					if number%c.config.Epoch != 0 {
						return errUnauthorized
					}
				}
			}
		}
	}

	// header must contain validator info following double validation design
	// start checking from epoch 2nd.
	if header.Number.Uint64() > c.config.Epoch && fullVerify {
		validator, err := c.RecoverValidator(header)
		if err != nil {
			return err
		}

		// verify validator
		assignedValidator, err := c.GetValidator(creator, chain, header)
		if err != nil {
			return err
		}
		if validator != assignedValidator {
			log.Debug("Bad block detected. Header contains wrong pair of creator-validator", "creator", creator, "assigned validator", assignedValidator, "wrong validator", validator)
			return errFailedDoubleValidation
		}
	}
	return nil
}

func (c *XDPoS) GetValidator(creator common.Address, chain consensus.ChainReader, header *types.Header) (common.Address, error) {
	epoch := c.config.Epoch
	no := header.Number.Uint64()
	cpNo := no
	if no%epoch != 0 {
		cpNo = no - (no % epoch)
	}
	if cpNo == 0 {
		return common.Address{}, nil
	}
	cpHeader := chain.GetHeaderByNumber(cpNo)
	if cpHeader == nil {
		if no%epoch == 0 {
			cpHeader = header
		} else {
			return common.Address{}, fmt.Errorf("couldn't find checkpoint header")
		}
	}
	m, err := GetM1M2FromCheckpointHeader(cpHeader, header, chain.Config())
	if err != nil {
		return common.Address{}, err
	}
	return m[creator], nil
}

// Prepare implements consensus.Engine, preparing all the consensus fields of the
// header for running the transactions on top.
func (c *XDPoS) Prepare(chain consensus.ChainReader, header *types.Header) error {
	// If the block isn't a checkpoint, cast a random vote (good enough for now)
	header.Coinbase = common.Address{}
	header.Nonce = types.BlockNonce{}

	number := header.Number.Uint64()
	// Assemble the voting snapshot to check which votes make sense
	snap, err := c.snapshot(chain, number-1, header.ParentHash, nil)
	if err != nil {
		return err
	}
	if number%c.config.Epoch != 0 {
		c.lock.RLock()

		// Gather all the proposals that make sense voting on
		addresses := make([]common.Address, 0, len(c.proposals))
		for address, authorize := range c.proposals {
			if snap.validVote(address, authorize) {
				addresses = append(addresses, address)
			}
		}
		// If there's pending proposals, cast a vote on them
		if len(addresses) > 0 {
			header.Coinbase = addresses[rand.Intn(len(addresses))]
			if c.proposals[header.Coinbase] {
				copy(header.Nonce[:], nonceAuthVote)
			} else {
				copy(header.Nonce[:], nonceDropVote)
			}
		}
		c.lock.RUnlock()
	}
	parent := chain.GetHeader(header.ParentHash, number-1)
	if parent == nil {
		return consensus.ErrUnknownAncestor
	}
	// Set the correct difficulty
	header.Difficulty = c.calcDifficulty(chain, parent, c.signer)
	log.Debug("CalcDifficulty ", "number", header.Number, "difficulty", header.Difficulty)
	// Ensure the extra data has all it's components
	if len(header.Extra) < extraVanity {
		header.Extra = append(header.Extra, bytes.Repeat([]byte{0x00}, extraVanity-len(header.Extra))...)
	}
	header.Extra = header.Extra[:extraVanity]
	masternodes := snap.GetSigners()
	if number >= c.config.Epoch && number%c.config.Epoch == 0 {
		if c.HookPenalty != nil || c.HookPenaltyTIPSigning != nil {
			var penMasternodes []common.Address = nil
			var err error = nil
			if chain.Config().IsTIPSigning(header.Number) {
				penMasternodes, err = c.HookPenaltyTIPSigning(chain, header, masternodes)
			} else {
				penMasternodes, err = c.HookPenalty(chain, number)
			}
			if err != nil {
				return err
			}
			if len(penMasternodes) > 0 {
				// penalize bad masternode(s)
				masternodes = common.RemoveItemFromArray(masternodes, penMasternodes)
				for _, address := range penMasternodes {
					log.Debug("Penalty status", "address", address, "number", number)
				}
				header.Penalties = common.ExtractAddressToBytes(penMasternodes)
			}
		}
		// Prevent penalized masternode(s) within 4 recent epochs
		for i := 1; i <= common.LimitPenaltyEpoch; i++ {
			if number > uint64(i)*c.config.Epoch {
				masternodes = RemovePenaltiesFromBlock(chain, masternodes, number-uint64(i)*c.config.Epoch)
			}
		}
		for _, masternode := range masternodes {
			header.Extra = append(header.Extra, masternode[:]...)
		}
		if c.HookValidator != nil {
			validators, err := c.HookValidator(header, masternodes)
			if err != nil {
				return err
			}
			header.Validators = validators
		}
	}
	header.Extra = append(header.Extra, make([]byte, extraSeal)...)

	// Mix digest is reserved for now, set to empty
	header.MixDigest = common.Hash{}

	// Ensure the timestamp has the correct delay

	header.Time = new(big.Int).Add(parent.Time, new(big.Int).SetUint64(c.config.Period))
	if header.Time.Int64() < time.Now().Unix() {
		header.Time = big.NewInt(time.Now().Unix())
	}
	return nil
}

func (c *XDPoS) UpdateMasternodes(chain consensus.ChainReader, header *types.Header, ms []Masternode) error {
	number := header.Number.Uint64()
	log.Trace("take snapshot", "number", number, "hash", header.Hash())
	// get snapshot
	snap, err := c.snapshot(chain, number, header.Hash(), nil)
	if err != nil {
		return err
	}
	newMasternodes := make(map[common.Address]struct{})
	for _, m := range ms {
		newMasternodes[m.Address] = struct{}{}
	}
	snap.Signers = newMasternodes
	nm := []string{}
	for _, n := range ms {
		nm = append(nm, n.Address.String())
	}
	c.recents.Add(snap.Hash, snap)
	log.Info("New set of masternodes has been updated to snapshot", "number", snap.Number, "hash", snap.Hash, "new masternodes", nm)
	return nil
}

// Finalize implements consensus.Engine, ensuring no uncles are set, nor block
// rewards given, and returns the final block.
func (c *XDPoS) Finalize(chain consensus.ChainReader, header *types.Header, state *state.StateDB, txs []*types.Transaction, uncles []*types.Header, receipts []*types.Receipt) (*types.Block, error) {
	// set block reward
	number := header.Number.Uint64()
	rCheckpoint := chain.Config().XDPoS.RewardCheckpoint

	// _ = c.CacheData(header, txs, receipts)

	if c.HookReward != nil && number%rCheckpoint == 0 {
		err, rewards := c.HookReward(chain, state, header)
		if err != nil {
			return nil, err
		}
		if len(common.StoreRewardFolder) > 0 {
			data, err := json.Marshal(rewards)
			if err == nil {
				err = ioutil.WriteFile(filepath.Join(common.StoreRewardFolder, header.Number.String()+"."+header.Hash().Hex()), data, 0644)
			}
			if err != nil {
				log.Error("Error when save reward info ", "number", header.Number, "hash", header.Hash().Hex(), "err", err)
			}
		}
	}

	// the state remains as is and uncles are dropped
	header.Root = state.IntermediateRoot(chain.Config().IsEIP158(header.Number))
	header.UncleHash = types.CalcUncleHash(nil)

	// Assemble and return the final block for sealing
	return types.NewBlock(header, txs, nil, receipts), nil
}

// Authorize injects a private key into the consensus engine to mint new blocks
// with.
func (c *XDPoS) Authorize(signer common.Address, signFn clique.SignerFn) {
	c.lock.Lock()
	defer c.lock.Unlock()

	c.signer = signer
	c.signFn = signFn
}

// Seal implements consensus.Engine, attempting to create a sealed block using
// the local signing credentials.
func (c *XDPoS) Seal(chain consensus.ChainReader, block *types.Block, stop <-chan struct{}) (*types.Block, error) {
	header := block.Header()

	// Sealing the genesis block is not supported
	number := header.Number.Uint64()
	if number == 0 {
		return nil, errUnknownBlock
	}
	// For 0-period chains, refuse to seal empty blocks (no reward but would spin sealing)
	// checkpoint blocks have no tx
	if c.config.Period == 0 && len(block.Transactions()) == 0 && number%c.config.Epoch != 0 {
		return nil, errWaitTransactions
	}
	// Don't hold the signer fields for the entire sealing procedure
	c.lock.RLock()
	signer, signFn := c.signer, c.signFn
	c.lock.RUnlock()

	// Bail out if we're unauthorized to sign a block
	snap, err := c.snapshot(chain, number-1, header.ParentHash, nil)
	if err != nil {
		return nil, err
	}
	masternodes := c.GetMasternodes(chain, header)
	if _, authorized := snap.Signers[signer]; !authorized {
		valid := false
		for _, m := range masternodes {
			if m == signer {
				valid = true
				break
			}
		}
		if !valid {
			return nil, errUnauthorized
		}
	}
	// If we're amongst the recent signers, wait for the next block
	// only check recent signers if there are more than one signer.
	if len(masternodes) > 1 {
		for seen, recent := range snap.Recents {
			if recent == signer {
				// Signer is among recents, only wait if the current block doesn't shift it out
				// There is only case that we don't allow signer to create two continuous blocks.
				if limit := uint64(2); number < limit || seen > number-limit {
					// Only take into account the non-epoch blocks
					if number%c.config.Epoch != 0 {
						log.Info("Signed recently, must wait for others ", "len(masternodes)", len(masternodes), "number", number, "limit", limit, "seen", seen, "recent", recent.String(), "snap.Recents", snap.Recents)
						<-stop
						return nil, nil
					}
				}
			}
		}
	}
	select {
	case <-stop:
		return nil, nil
	default:
	}
	// Sign all the things!
	sighash, err := signFn(accounts.Account{Address: signer}, sigHash(header).Bytes())
	if err != nil {
		return nil, err
	}
	copy(header.Extra[len(header.Extra)-extraSeal:], sighash)
	m2, err := c.GetValidator(signer, chain, header)
	if err != nil {
		return nil, fmt.Errorf("can't get block validator: %v", err)
	}
	if m2 == signer {
		header.Validator = sighash
	}
	return block.WithSeal(header), nil
}

// CalcDifficulty is the difficulty adjustment algorithm. It returns the difficulty
// that a new block should have based on the previous blocks in the chain and the
// current signer.
func (c *XDPoS) CalcDifficulty(chain consensus.ChainReader, time uint64, parent *types.Header) *big.Int {
	return c.calcDifficulty(chain, parent, c.signer)
}

func (c *XDPoS) calcDifficulty(chain consensus.ChainReader, parent *types.Header, signer common.Address) *big.Int {
	len, preIndex, curIndex, _, err := c.YourTurn(chain, parent, signer)
	if err != nil {
		return big.NewInt(int64(len + curIndex - preIndex))
	}
	return big.NewInt(int64(len - Hop(len, preIndex, curIndex)))
}

// APIs implements consensus.Engine, returning the user facing RPC API to allow
// controlling the signer voting.
func (c *XDPoS) APIs(chain consensus.ChainReader) []rpc.API {
	return []rpc.API{{
		Namespace: "XDPoS",
		Version:   "1.0",
		Service:   &API{chain: chain, XDPoS: c},
		Public:    false,
	}}
}

func (c *XDPoS) RecoverSigner(header *types.Header) (common.Address, error) {
	return ecrecover(header, c.signatures)
}

func (c *XDPoS) RecoverValidator(header *types.Header) (common.Address, error) {
	// If the signature's already cached, return that
	hash := header.Hash()
	if address, known := c.validatorSignatures.Get(hash); known {
		return address.(common.Address), nil
	}
	// Retrieve the signature from the header.Validator
	// len equals 65 bytes
	if len(header.Validator) != extraSeal {
		return common.Address{}, consensus.ErrFailValidatorSignature
	}
	// Recover the public key and the Ethereum address
	pubkey, err := crypto.Ecrecover(sigHash(header).Bytes(), header.Validator)
	if err != nil {
		return common.Address{}, err
	}
	var signer common.Address
	copy(signer[:], crypto.Keccak256(pubkey[1:])[12:])

	c.validatorSignatures.Add(hash, signer)
	return signer, nil
}

// Get master nodes over extra data of previous checkpoint block.
func (c *XDPoS) GetMasternodesFromCheckpointHeader(preCheckpointHeader *types.Header, n, e uint64) []common.Address {
	if preCheckpointHeader == nil {
		log.Info("Previous checkpoint's header is empty", "block number", n, "epoch", e)
		return []common.Address{}
	}
	masternodes := make([]common.Address, (len(preCheckpointHeader.Extra)-extraVanity-extraSeal)/common.AddressLength)
	for i := 0; i < len(masternodes); i++ {
		copy(masternodes[i][:], preCheckpointHeader.Extra[extraVanity+i*common.AddressLength:])
	}

	return masternodes
}

func (c *XDPoS) CacheData(header *types.Header, txs []*types.Transaction, receipts []*types.Receipt) []*types.Transaction {
	signTxs := []*types.Transaction{}
	for _, tx := range txs {
		if tx.IsSigningTransaction() {
			var b uint
			for _, r := range receipts {
				if r.TxHash == tx.Hash() {
					if len(r.PostState) > 0 {
						b = types.ReceiptStatusSuccessful
					} else {
						b = r.Status
					}
					break
				}
			}

			if b == types.ReceiptStatusFailed {
				continue
			}

			signTxs = append(signTxs, tx)
		}
	}

	log.Debug("Save tx signers to cache", "hash", header.Hash().String(), "number", header.Number, "len(txs)", len(signTxs))
	c.BlockSigners.Add(header.Hash(), signTxs)

	return signTxs
}

func (c *XDPoS) CacheSigner(hash common.Hash, txs []*types.Transaction) []*types.Transaction {
	signTxs := []*types.Transaction{}
	for _, tx := range txs {
		if tx.IsSigningTransaction() {
			signTxs = append(signTxs, tx)
		}
	}
	log.Debug("Save tx signers to cache", "hash", hash.String(), "len(txs)", len(signTxs))
	c.BlockSigners.Add(hash, signTxs)
	return signTxs
}

func (c *XDPoS) GetDb() ethdb.Database {
	return c.db
}

// Extract validators from byte array.
func RemovePenaltiesFromBlock(chain consensus.ChainReader, masternodes []common.Address, epochNumber uint64) []common.Address {
	if epochNumber <= 0 {
		return masternodes
	}
	header := chain.GetHeaderByNumber(epochNumber)
	block := chain.GetBlock(header.Hash(), epochNumber)
	penalties := block.Penalties()
	if penalties != nil {
		prevPenalties := common.ExtractAddressFromBytes(penalties)
		masternodes = common.RemoveItemFromArray(masternodes, prevPenalties)
	}
	return masternodes
}

// Get masternodes address from checkpoint Header.
func GetMasternodesFromCheckpointHeader(checkpointHeader *types.Header) []common.Address {
	masternodes := make([]common.Address, (len(checkpointHeader.Extra)-extraVanity-extraSeal)/common.AddressLength)
	for i := 0; i < len(masternodes); i++ {
		copy(masternodes[i][:], checkpointHeader.Extra[extraVanity+i*common.AddressLength:])
	}
	return masternodes
}

// Get m2 list from checkpoint block.
func GetM1M2FromCheckpointHeader(checkpointHeader *types.Header, currentHeader *types.Header, config *params.ChainConfig) (map[common.Address]common.Address, error) {
	if checkpointHeader.Number.Uint64()%common.EpocBlockRandomize != 0 {
		return nil, errors.New("This block is not checkpoint block epoc.")
	}
	// Get signers from this block.
	masternodes := GetMasternodesFromCheckpointHeader(checkpointHeader)
	validators := ExtractValidatorsFromBytes(checkpointHeader.Validators)
	m1m2, _, err := getM1M2(masternodes, validators, currentHeader, config)
	if err != nil {
		return map[common.Address]common.Address{}, err
	}
	return m1m2, nil
}

func getM1M2(masternodes []common.Address, validators []int64, currentHeader *types.Header, config *params.ChainConfig) (map[common.Address]common.Address, uint64, error) {
	m1m2 := map[common.Address]common.Address{}
	maxMNs := len(masternodes)
	moveM2 := uint64(0)
	if len(validators) < maxMNs {
		return nil, moveM2, errors.New("len(m2) is less than len(m1)")
	}
	if maxMNs > 0 {
		isForked := config.IsTIPRandomize(currentHeader.Number)
		if isForked {
			moveM2 = ((currentHeader.Number.Uint64() % config.XDPoS.Epoch) / uint64(maxMNs)) % uint64(maxMNs)
		}
		for i, m1 := range masternodes {
			m2Index := uint64(validators[i] % int64(maxMNs))
			m2Index = (m2Index + moveM2) % uint64(maxMNs)
			m1m2[m1] = masternodes[m2Index]
		}
	}
	return m1m2, moveM2, nil
}

// Extract validators from byte array.
func ExtractValidatorsFromBytes(byteValidators []byte) []int64 {
	lenValidator := len(byteValidators) / M2ByteLength
	var validators []int64
	for i := 0; i < lenValidator; i++ {
		trimByte := bytes.Trim(byteValidators[i*M2ByteLength:(i+1)*M2ByteLength], "\x00")
		intNumber, err := strconv.Atoi(string(trimByte))
		if err != nil {
			log.Error("Can not convert string to integer", "error", err)
			return []int64{}
		}
		validators = append(validators, int64(intNumber))
	}

	return validators
}

func Hop(len, pre, cur int) int {
	switch {
	case pre < cur:
		return cur - (pre + 1)
	case pre > cur:
		return (len - pre) + (cur - 1)
	default:
		return len - 1
	}
}

func (c *XDPoS) CheckMNTurn(chain consensus.ChainReader, parent *types.Header, signer common.Address) bool {
	masternodes := c.GetMasternodes(chain, parent)

	if common.IsTestnet {
		// Only three mns hard code for XDC testnet.
		masternodes = []common.Address{
			common.HexToAddress("0xfFC679Dcdf444D2eEb0491A998E7902B411CcF20"),
			common.HexToAddress("0xd76fd76F7101811726DCE9E43C2617706a4c45c8"),
			common.HexToAddress("0x8A97753311aeAFACfd76a68Cf2e2a9808d3e65E8"),
		}
	}

	snap, err := c.GetSnapshot(chain, parent)
	if err != nil {
		log.Warn("Failed when trying to commit new work", "err", err)
		return false
	}
	if len(masternodes) == 0 {
		return false
	}
	pre := common.Address{}
	// masternode[0] has chance to create block 1
	preIndex := -1
	if parent.Number.Uint64() != 0 {
		pre, err = whoIsCreator(snap, parent)
		if err != nil {
			return false
		}
		preIndex = position(masternodes, pre)
	}
	curIndex := position(masternodes, signer)
	if (preIndex)%len(masternodes) == curIndex {
		return true
	}
	return false
}

func (c *XDPoS) GetSignersFromContract(chain consensus.ChainReader, checkpointHeader *types.Header) ([]common.Address, error) {
	startGapBlockHeader := checkpointHeader
	number := checkpointHeader.Number.Uint64()
	for step := uint64(1); step <= chain.Config().XDPoS.Gap; step++ {
		startGapBlockHeader = chain.GetHeader(startGapBlockHeader.ParentHash, number-step)
	}
	signers, err := c.HookGetSignersFromContract(startGapBlockHeader.Hash())
	if err != nil {
		return []common.Address{}, fmt.Errorf("Can't get signers from Smart Contract . Err: %v", err)
	}
	return signers, nil
}