eth: dial nodes from discv5

Unlike discv4, the discv5 random nodes iterator will always provide full ENRs. This means
we can apply filtering to the results and will only try dialing nodes which explictly opt
into the eth protocol with a matching chain.

I have also removed the dial iterator from snap. We don't have an official DNS list for
snap anymore, and I doubt anyone else is running one. While we could potentially filter
for snap on discv5, there will be very few nodes announcing it, and the extra iterator
would just stall the dialer.

eth/backend.go

@@ -23,6 +23,7 @@ import (
 	"math/big"
 	"runtime"
 	"sync"
+	"time"
 
 	"github.com/ethereum/go-ethereum/accounts"
 	"github.com/ethereum/go-ethereum/common"
@@ -64,15 +65,13 @@ type Config = ethconfig.Config
 
 // Ethereum implements the Ethereum full node service.
 type Ethereum struct {
+	// core protocol objects
 	config *ethconfig.Config
-
-	// Handlers
 	txPool *txpool.TxPool
+	blockchain *core.BlockChain
 
-	blockchain         *core.BlockChain
-	handler            *handler
-	ethDialCandidates  enode.Iterator
-	snapDialCandidates enode.Iterator
+	handler    *handler
+	discmix    *enode.FairMix
 
 	// DB interfaces
 	chainDb ethdb.Database // Block chain database
@@ -162,6 +161,7 @@ func New(stack *node.Node, config *ethconfig.Config) (*Ethereum, error) {
 		bloomRequests:     make(chan chan *bloombits.Retrieval),
 		bloomIndexer:      core.NewBloomIndexer(chainDb, params.BloomBitsBlocks, params.BloomConfirms),
 		p2pServer:         stack.Server(),
+		discmix:     enode.NewFairMix(2 * time.Second),
 		shutdownTracker:   shutdowncheck.NewShutdownTracker(chainDb),
 	}
 	bcVersion := rawdb.ReadDatabaseVersion(chainDb)
@@ -266,17 +266,6 @@ func New(stack *node.Node, config *ethconfig.Config) (*Ethereum, error) {
 	}
 	eth.APIBackend.gpo = gasprice.NewOracle(eth.APIBackend, config.GPO, config.Miner.GasPrice)
 
-	// Setup DNS discovery iterators.
-	dnsclient := dnsdisc.NewClient(dnsdisc.Config{})
-	eth.ethDialCandidates, err = dnsclient.NewIterator(eth.config.EthDiscoveryURLs...)
-	if err != nil {
-		return nil, err
-	}
-	eth.snapDialCandidates, err = dnsclient.NewIterator(eth.config.SnapDiscoveryURLs...)
-	if err != nil {
-		return nil, err
-	}
-
 	// Start the RPC service
 	eth.netRPCService = ethapi.NewNetAPI(eth.p2pServer, networkID)
 
@@ -359,17 +348,17 @@ func (s *Ethereum) BloomIndexer() *core.ChainIndexer   { return s.bloomIndexer }
 // Protocols returns all the currently configured
 // network protocols to start.
 func (s *Ethereum) Protocols() []p2p.Protocol {
-	protos := eth.MakeProtocols((*ethHandler)(s.handler), s.networkID, s.ethDialCandidates)
+	protos := eth.MakeProtocols((*ethHandler)(s.handler), s.networkID, s.discmix)
 	if s.config.SnapshotCache > 0 {
-		protos = append(protos, snap.MakeProtocols((*snapHandler)(s.handler), s.snapDialCandidates)...)
+		protos = append(protos, snap.MakeProtocols((*snapHandler)(s.handler))...)
 	}
 	return protos
 }
 
 // Start implements node.Lifecycle, starting all internal goroutines needed by the
 // Ethereum protocol implementation.
 func (s *Ethereum) Start() error {
-	eth.StartENRUpdater(s.blockchain, s.p2pServer.LocalNode())
+	s.setupDiscovery()
 
 	// Start the bloom bits servicing goroutines
 	s.startBloomHandlers(params.BloomBitsBlocks)
@@ -382,12 +371,40 @@ func (s *Ethereum) Start() error {
 	return nil
 }
 
+func (s *Ethereum) setupDiscovery() error {
+	eth.StartENRUpdater(s.blockchain, s.p2pServer.LocalNode())
+
+	dnsclient := dnsdisc.NewClient(dnsdisc.Config{})
+	if len(s.config.EthDiscoveryURLs) > 0 {
+		iter, err := dnsclient.NewIterator(s.config.EthDiscoveryURLs...)
+		if err != nil {
+			return err
+		}
+		s.discmix.AddSource(iter)
+	}
+
+	if len(s.config.SnapDiscoveryURLs) > 0 {
+		iter, err := dnsclient.NewIterator(s.config.SnapDiscoveryURLs...)
+		if err != nil {
+			return err
+		}
+		s.discmix.AddSource(iter)
+	}
+
+	if s.p2pServer.DiscoveryV5() != nil {
+		filter := eth.NewNodeFilter(s.blockchain)
+		iter := enode.Filter(s.p2pServer.DiscoveryV5().RandomNodes(), filter)
+		s.discmix.AddSource(iter)
+	}
+
+	return nil
+}
+
 // Stop implements node.Lifecycle, terminating all internal goroutines used by the
 // Ethereum protocol.
 func (s *Ethereum) Stop() error {
 	// Stop all the peer-related stuff first.
-	s.ethDialCandidates.Close()
-	s.snapDialCandidates.Close()
+	s.discmix.Close()
 	s.handler.Stop()
 
 	// Then stop everything else.

eth/protocols/eth/discovery.go

@@ -64,3 +64,15 @@ func currentENREntry(chain *core.BlockChain) *enrEntry {
 		ForkID: forkid.NewID(chain.Config(), chain.Genesis(), head.Number.Uint64(), head.Time),
 	}
 }
+
+func NewNodeFilter(chain *core.BlockChain) func(*enode.Node) bool {
+	filter := forkid.NewFilter(chain)
+	return func(n *enode.Node) bool {
+		var entry enrEntry
+		if err := n.Load(entry); err != nil {
+			return false
+		}
+		err := filter(entry.ForkID)
+		return err == nil
+	}
+}

eth/protocols/eth/handler.go

@@ -113,8 +113,7 @@ func MakeProtocols(backend Backend, network uint64, dnsdisc enode.Iterator) []p2
 			PeerInfo: func(id enode.ID) interface{} {
 				return backend.PeerInfo(id)
 			},
-			Attributes:     []enr.Entry{currentENREntry(backend.Chain())},
-			DialCandidates: dnsdisc,
+			Attributes: []enr.Entry{currentENREntry(backend.Chain())},
 		})
 	}
 	return protocols

eth/protocols/snap/handler.go

@@ -82,13 +82,7 @@ type Backend interface {
 }
 
 // MakeProtocols constructs the P2P protocol definitions for `snap`.
-func MakeProtocols(backend Backend, dnsdisc enode.Iterator) []p2p.Protocol {
-	// Filter the discovery iterator for nodes advertising snap support.
-	dnsdisc = enode.Filter(dnsdisc, func(n *enode.Node) bool {
-		var snap enrEntry
-		return n.Load(&snap) == nil
-	})
-
+func MakeProtocols(backend Backend) []p2p.Protocol {
 	protocols := make([]p2p.Protocol, len(ProtocolVersions))
 	for i, version := range ProtocolVersions {
 		version := version // Closure
@@ -108,8 +102,7 @@ func MakeProtocols(backend Backend, dnsdisc enode.Iterator) []p2p.Protocol {
 			PeerInfo: func(id enode.ID) interface{} {
 				return backend.PeerInfo(id)
 			},
-			Attributes:     []enr.Entry{&enrEntry{}},
-			DialCandidates: dnsdisc,
+			Attributes: []enr.Entry{&enrEntry{}},
 		}
 	}
 	return protocols