cmd/devp2p, p2p: dial using node iterator, discovery crawler (#20132)

* p2p/enode: add Iterator and associated utilities

* p2p/discover: add RandomNodes iterator

* p2p: dial using iterator

* cmd/devp2p: add discv4 crawler

* cmd/devp2p: WIP nodeset filter

* cmd/devp2p: fixup lesFilter

* core/forkid: add NewStaticFilter

* cmd/devp2p: make -eth-network filter actually work

* cmd/devp2p: improve crawl timestamp handling

* cmd/devp2p: fix typo

* p2p/enode: fix comment typos

* p2p/discover: fix comment typos

* p2p/discover: rename lookup.next to 'advance'

* p2p: lower discovery mixer timeout

* p2p/enode: implement dynamic FairMix timeouts

* cmd/devp2p: add ropsten support in -eth-network filter

* cmd/devp2p: tweak crawler log message

p2p/server.go

@@ -45,6 +45,11 @@ import (
 const (
 	defaultDialTimeout = 15 * time.Second
 
+	// This is the fairness knob for the discovery mixer. When looking for peers, we'll
+	// wait this long for a single source of candidates before moving on and trying other
+	// sources.
+	discmixTimeout = 5 * time.Second
+
 	// Connectivity defaults.
 	maxActiveDialTasks     = 16
 	defaultMaxPendingPeers = 50
@@ -167,16 +172,20 @@ type Server struct {
 	lock    sync.Mutex // protects running
 	running bool
 
-	nodedb       *enode.DB
-	localnode    *enode.LocalNode
-	ntab         discoverTable
 	listener     net.Listener
 	ourHandshake *protoHandshake
-	DiscV5       *discv5.Network
 	loopWG       sync.WaitGroup // loop, listenLoop
 	peerFeed     event.Feed
 	log          log.Logger
 
+	nodedb    *enode.DB
+	localnode *enode.LocalNode
+	ntab      *discover.UDPv4
+	DiscV5    *discv5.Network
+	discmix   *enode.FairMix
+
+	staticNodeResolver nodeResolver
+
 	// Channels into the run loop.
 	quit                    chan struct{}
 	addstatic               chan *enode.Node
@@ -470,7 +479,7 @@ func (srv *Server) Start() (err error) {
 	}
 
 	dynPeers := srv.maxDialedConns()
-	dialer := newDialState(srv.localnode.ID(), srv.ntab, dynPeers, &srv.Config)
+	dialer := newDialState(srv.localnode.ID(), dynPeers, &srv.Config)
 	srv.loopWG.Add(1)
 	go srv.run(dialer)
 	return nil
@@ -521,6 +530,18 @@ func (srv *Server) setupLocalNode() error {
 }
 
 func (srv *Server) setupDiscovery() error {
+	srv.discmix = enode.NewFairMix(discmixTimeout)
+
+	// Add protocol-specific discovery sources.
+	added := make(map[string]bool)
+	for _, proto := range srv.Protocols {
+		if proto.DialCandidates != nil && !added[proto.Name] {
+			srv.discmix.AddSource(proto.DialCandidates)
+			added[proto.Name] = true
+		}
+	}
+
+	// Don't listen on UDP endpoint if DHT is disabled.
 	if srv.NoDiscovery && !srv.DiscoveryV5 {
 		return nil
 	}
@@ -562,7 +583,10 @@ func (srv *Server) setupDiscovery() error {
 			return err
 		}
 		srv.ntab = ntab
+		srv.discmix.AddSource(ntab.RandomNodes())
+		srv.staticNodeResolver = ntab
 	}
+
 	// Discovery V5
 	if srv.DiscoveryV5 {
 		var ntab *discv5.Network
@@ -620,6 +644,7 @@ func (srv *Server) run(dialstate dialer) {
 	srv.log.Info("Started P2P networking", "self", srv.localnode.Node().URLv4())
 	defer srv.loopWG.Done()
 	defer srv.nodedb.Close()
+	defer srv.discmix.Close()
 
 	var (
 		peers        = make(map[enode.ID]*Peer)