path: root/consensus/clique/clique.go

// Copyright 2017 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 clique implements the proof-of-authority consensus engine.
package clique

import (
    "bytes"
    "errors"
    "math/big"
    "math/rand"
    "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/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 (
    checkpointInterval = 1024 // Number of blocks after which to save the vote snapshot to the database
    inmemorySnapshots  = 128  // Number of recent vote snapshots to keep in memory
    inmemorySignatures = 4096 // Number of recent block signatures to keep in memory

    wiggleTime = 500 * time.Millisecond // Random delay (per signer) to allow concurrent signers
)

// Clique proof-of-authority protocol constants.
var (
    epochLength = uint64(30000) // 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")

    // 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")

    // 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")
)

// 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 proof-of-authority
// 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
}

// 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
}

// Clique is the proof-of-authority consensus engine proposed to support the
// Ethereum testnet following the Ropsten attacks.
type Clique struct {
    config *params.CliqueConfig // 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

    proposals map[common.Address]bool // Current list of proposals we are pushing

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

// New creates a Clique proof-of-authority consensus engine with the initial
// signers set to the ones provided by the user.
func New(config *params.CliqueConfig, db ethdb.Database) *Clique {
    // 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
    recents, _ := lru.NewARC(inmemorySnapshots)
    signatures, _ := lru.NewARC(inmemorySignatures)

    return &Clique{
        config:     &conf,
        db:         db,
        recents:    recents,
        signatures: signatures,
        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 *Clique) Author(header *types.Header) (common.Address, error) {
    return ecrecover(header, c.signatures)
}

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

// 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 *Clique) VerifyHeaders(chain consensus.ChainReader, headers []*types.Header, seals []bool) (chan<- struct{}, <-chan error) {
    abort := make(chan struct{})
    results := make(chan error, len(headers))

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

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

// 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 *Clique) verifyHeader(chain consensus.ChainReader, header *types.Header, parents []*types.Header) error {
    if header.Number == nil {
        return errUnknownBlock
    }
    number := header.Number.Uint64()

    // 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 PoA
    if header.UncleHash != uncleHash {
        return errInvalidUncleHash
    }
    // Ensure that the block's difficulty is meaningful (may not be correct at this point)
    if number > 0 {
        if header.Difficulty == nil || (header.Difficulty.Cmp(diffInTurn) != 0 && header.Difficulty.Cmp(diffNoTurn) != 0) {
            return errInvalidDifficulty
        }
    }
    // 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)
}

// 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 *Clique) verifyCascadingFields(chain consensus.ChainReader, header *types.Header, parents []*types.Header) 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
    }
    // 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
    }
    // If the block is a checkpoint block, verify the signer list
    if number%c.config.Epoch == 0 {
        signers := make([]byte, len(snap.Signers)*common.AddressLength)
        for i, signer := range snap.signers() {
            copy(signers[i*common.AddressLength:], signer[:])
        }
        extraSuffix := len(header.Extra) - extraSeal
        if !bytes.Equal(header.Extra[extraVanity:extraSuffix], signers) {
            return errInvalidCheckpointSigners
        }
    }
    // All basic checks passed, verify the seal and return
    return c.verifySeal(chain, header, parents)
}

// snapshot retrieves the authorization snapshot at a given point in time.
func (c *Clique) 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
        if number%checkpointInterval == 0 {
            if s, err := loadSnapshot(c.config, c.signatures, c.db, hash); err == nil {
                log.Trace("Loaded voting snapshot from disk", "number", number, "hash", hash)
                snap = s
                break
            }
        }
        // If we're at an checkpoint block, make a snapshot if it's known
        if number%c.config.Epoch == 0 {
            checkpoint := chain.GetHeaderByNumber(number)
            if checkpoint != nil {
                hash := checkpoint.Hash()

                signers := make([]common.Address, (len(checkpoint.Extra)-extraVanity-extraSeal)/common.AddressLength)
                for i := 0; i < len(signers); i++ {
                    copy(signers[i][:], checkpoint.Extra[extraVanity+i*common.AddressLength:])
                }
                snap = newSnapshot(c.config, c.signatures, number, hash, signers)
                if err := snap.store(c.db); err != nil {
                    return nil, err
                }
                log.Info("Stored checkpoint snapshot to disk", "number", number, "hash", hash)
                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%checkpointInterval == 0 && len(headers) > 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 *Clique) 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 *Clique) VerifySeal(chain consensus.ChainReader, header *types.Header) error {
    return c.verifySeal(chain, header, nil)
}

// 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.
func (c *Clique) verifySeal(chain consensus.ChainReader, header *types.Header, parents []*types.Header) 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
    signer, err := ecrecover(header, c.signatures)
    if err != nil {
        return err
    }
    if _, ok := snap.Signers[signer]; !ok {
        return errUnauthorized
    }
    for seen, recent := range snap.Recents {
        if recent == signer {
            // Signer is among recents, only fail if the current block doesn't shift it out
            if limit := uint64(len(snap.Signers)/2 + 1); seen > number-limit {
                return errUnauthorized
            }
        }
    }
    // Ensure that the difficulty corresponds to the turn-ness of the signer
    inturn := snap.inturn(header.Number.Uint64(), signer)
    if inturn && header.Difficulty.Cmp(diffInTurn) != 0 {
        return errInvalidDifficulty
    }
    if !inturn && header.Difficulty.Cmp(diffNoTurn) != 0 {
        return errInvalidDifficulty
    }
    return nil
}

// Prepare implements consensus.Engine, preparing all the consensus fields of the
// header for running the transactions on top.
func (c *Clique) 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()
    }
    // Set the correct difficulty
    header.Difficulty = CalcDifficulty(snap, c.signer)

    // 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]

    if number%c.config.Epoch == 0 {
        for _, signer := range snap.signers() {
            header.Extra = append(header.Extra, signer[:]...)
        }
    }
    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
    parent := chain.GetHeader(header.ParentHash, number-1)
    if parent == nil {
        return consensus.ErrUnknownAncestor
    }
    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
}

// Finalize implements consensus.Engine, ensuring no uncles are set, nor block
// rewards given, and returns the final block.
func (c *Clique) Finalize(chain consensus.ChainReader, header *types.Header, state *state.StateDB, txs []*types.Transaction, uncles []*types.Header, receipts []*types.Receipt) (*types.Block, error) {
    // No block rewards in PoA, so 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 *Clique) Authorize(signer common.Address, signFn 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 *Clique) Seal(chain consensus.ChainReader, block *types.Block, results chan<- *types.Block, stop <-chan struct{}) error {
    header := block.Header()

    // Sealing the genesis block is not supported
    number := header.Number.Uint64()
    if number == 0 {
        return errUnknownBlock
    }
    // For 0-period chains, refuse to seal empty blocks (no reward but would spin sealing)
    if c.config.Period == 0 && len(block.Transactions()) == 0 {
        return 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 err
    }
    if _, authorized := snap.Signers[signer]; !authorized {
        return errUnauthorized
    }
    // If we're amongst the recent signers, wait for the next block
    for seen, recent := range snap.Recents {
        if recent == signer {
            // Signer is among recents, only wait if the current block doesn't shift it out
            if limit := uint64(len(snap.Signers)/2 + 1); number < limit || seen > number-limit {
                log.Info("Signed recently, must wait for others")
                return nil
            }
        }
    }
    // Sweet, the protocol permits us to sign the block, wait for our time
    delay := time.Unix(header.Time.Int64(), 0).Sub(time.Now()) // nolint: gosimple
    if header.Difficulty.Cmp(diffNoTurn) == 0 {
        // It's not our turn explicitly to sign, delay it a bit
        wiggle := time.Duration(len(snap.Signers)/2+1) * wiggleTime
        delay += time.Duration(rand.Int63n(int64(wiggle)))

        log.Trace("Out-of-turn signing requested", "wiggle", common.PrettyDuration(wiggle))
    }
    // Sign all the things!
    sighash, err := signFn(accounts.Account{Address: signer}, sigHash(header).Bytes())
    if err != nil {
        return err
    }
    copy(header.Extra[len(header.Extra)-extraSeal:], sighash)
    // Wait until sealing is terminated or delay timeout.
    log.Trace("Waiting for slot to sign and propagate", "delay", common.PrettyDuration(delay))
    go func() {
        select {
        case <-stop:
            return
        case <-time.After(delay):
        }

        select {
        case results <- block.WithSeal(header):
        default:
            log.Warn("Sealing result is not read by miner", "sealhash", c.SealHash(header))
        }
    }()

    return 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 *Clique) CalcDifficulty(chain consensus.ChainReader, time uint64, parent *types.Header) *big.Int {
    snap, err := c.snapshot(chain, parent.Number.Uint64(), parent.Hash(), nil)
    if err != nil {
        return nil
    }
    return CalcDifficulty(snap, c.signer)
}

// 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 CalcDifficulty(snap *Snapshot, signer common.Address) *big.Int {
    if snap.inturn(snap.Number+1, signer) {
        return new(big.Int).Set(diffInTurn)
    }
    return new(big.Int).Set(diffNoTurn)
}

// SealHash returns the hash of a block prior to it being sealed.
func (c *Clique) SealHash(header *types.Header) common.Hash {
    return sigHash(header)
}

// Close implements consensus.Engine. It's a noop for clique as there is are no background threads.
func (c *Clique) Close() error {
    return nil
}

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