consensus/clique: Proof of Authority (#3753)

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

eth/backend.go

@@ -27,6 +27,7 @@ import (
 	"github.com/ethereum/go-ethereum/accounts"
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/consensus"
+	"github.com/ethereum/go-ethereum/consensus/clique"
 	"github.com/ethereum/go-ethereum/consensus/ethash"
 	"github.com/ethereum/go-ethereum/core"
 	"github.com/ethereum/go-ethereum/core/types"
@@ -230,6 +231,11 @@ func CreateDB(ctx *node.ServiceContext, config *Config, name string) (ethdb.Data
 
 // CreateConsensusEngine creates the required type of consensus engine instance for an Ethereum service
 func CreateConsensusEngine(ctx *node.ServiceContext, config *Config, chainConfig *params.ChainConfig, db ethdb.Database) consensus.Engine {
+	// If proof-of-authority is requested, set it up
+	if chainConfig.Clique != nil {
+		return clique.New(chainConfig.Clique, db)
+	}
+	// Otherwise assume proof-of-work
 	switch {
 	case config.PowFake:
 		log.Warn("Ethash used in fake mode")
@@ -333,6 +339,14 @@ func (s *Ethereum) StartMining(local bool) error {
 		log.Error("Cannot start mining without etherbase", "err", err)
 		return fmt.Errorf("etherbase missing: %v", err)
 	}
+	if clique, ok := s.engine.(*clique.Clique); ok {
+		wallet, err := s.accountManager.Find(accounts.Account{Address: eb})
+		if wallet == nil || err != nil {
+			log.Error("Etherbase account unavailable locally", "err", err)
+			return fmt.Errorf("singer missing: %v", err)
+		}
+		clique.Authorize(eb, wallet.SignHash)
+	}
 	if local {
 		// If local (CPU) mining is started, we can disable the transaction rejection
 		// mechanism introduced to speed sync times. CPU mining on mainnet is ludicrous