consensus/clique: Proof of Authority (#3753)

This PR is a prototype implementation of plugable consensus engines and the
Clique PoA protocol ethereum/EIPs#225

1 files changed, 638 insertions, 0 deletions

diff --git a/consensus/clique/clique.go b/consensus/clique/clique.go
new file mode 100644
index 000000000..e516d5057
--- /dev/null
+++ b/consensus/clique/clique.go
@@ -0,0 +1,638 @@
+// 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/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 = 1024 // Number of recent blocks 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
+	blockPeriod = uint64(15)    // Default minimum difference between two consecutive block's timestamps
+
+	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")
+
+	// drrInvalidCheckpointSigners is returned if a checkpoint block contains an
+	// invalid list of signers (i.e. non divisible by 20 bytes, or not the correct
+	// ones).
+	drrInvalidCheckpointSigners = 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")
+)
+
+// 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) (common.Address, error) {
+	// 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:])
+
+	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
+	}
+	if conf.Period == 0 {
+		conf.Period = blockPeriod
+	}
+	// 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),
+	}
+}
+
+// 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 drrInvalidCheckpointSigners
+	}
+	// 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
+		}
+	}
+	// 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 drrInvalidCheckpointSigners
+		}
+	}
+	// 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.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, false); 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, 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%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
+	}
+	c.recents.Add(snap.Hash, snap)
+
+	// Resolve the authorization key and check against signers
+	signer, err := ecrecover(header)
+	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 doens't shift it out
+			if limit := uint64(len(snap.Signers)/2 + 1); seen > number-limit {
+				return errUnauthorized
+			}
+		}
+	}
+	// Ensure that the difficulty corresponts 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 fror now)
+	header.Coinbase = common.Address{}
+	header.Nonce = types.BlockNonce{}
+
+	number := header.Number.Uint64()
+	if number%c.config.Epoch != 0 {
+		c.lock.RLock()
+		if len(c.proposals) > 0 {
+			addresses := make([]common.Address, 0, len(c.proposals))
+			for address := range c.proposals {
+				addresses = append(addresses, address)
+			}
+			header.Coinbase = addresses[rand.Intn(len(addresses))]
+			if c.proposals[header.Coinbase] {
+				copy(header.Nonce[:], nonceAuthVote)
+			} else {
+				copy(header.Nonce[:], nonceDropVote)
+			}
+		}
+		c.lock.RUnlock()
+	}
+	// Assemble the voting snapshot and set the correct difficulty
+	snap, err := c.snapshot(chain, number-1, header.ParentHash, nil)
+	if err != nil {
+		return err
+	}
+	header.Difficulty = diffNoTurn
+	if snap.inturn(header.Number.Uint64(), c.signer) {
+		header.Difficulty = diffInTurn
+	}
+	// 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, 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
+	}
+	// 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
+	}
+	if _, authorized := snap.Signers[signer]; !authorized {
+		return nil, errUnauthorized
+	}
+	// If we're amongs 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 doens't shift it out
+			if limit := uint64(len(snap.Signers)/2 + 1); seen > number-limit {
+				log.Info("Signed recently, must wait for others")
+				<-stop
+				return nil, nil
+			}
+		}
+	}
+	// Sweet, the protocol permits us to sign the block, wait for our time
+	delay := time.Unix(header.Time.Int64(), 0).Sub(time.Now())
+	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))
+	}
+	log.Trace("Waiting for slot to sign and propagate", "delay", common.PrettyDuration(delay))
+
+	select {
+	case <-stop:
+		return nil, nil
+	case <-time.After(delay):
+	}
+	// 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)
+
+	return block.WithSeal(header), 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,
+	}}
+}