p2p: implement naive version of discv4 forkid filtering

Simple version that does the filtering, but misses pipelining,
waiting for ENRs to be retrieved one-by-one.

Signed-off-by: Csaba Kiraly <csaba.kiraly@gmail.com>

eth/backend.go

@@ -487,6 +487,15 @@ func (s *Ethereum) setupDiscovery() error {
 		s.discmix.AddSource(iter)
 	}
 
+	// Add DHT nodes from discv4.
+	if s.p2pServer.DiscoveryV4() != nil {
+		asyncFilter := s.p2pServer.DiscoveryV4().RequestENR
+		filter := eth.NewNodeFilter(s.blockchain)
+		iter := enode.AsyncFilter(s.p2pServer.DiscoveryV4().RandomNodes(), asyncFilter)
+		iter = enode.Filter(iter, filter)
+		s.discmix.AddSource(iter)
+	}
+
 	// Add DHT nodes from discv5.
 	if s.p2pServer.DiscoveryV5() != nil {
 		filter := eth.NewNodeFilter(s.blockchain)

p2p/enode/iter.go

@@ -152,6 +152,37 @@ func (f *filterIter) Next() bool {
 	return false
 }
 
+// AsyncFilter wraps an iterator such that Next only returns nodes for which
+// the 'check' function returns a (possibly modified) node.
+func AsyncFilter(it Iterator, check func(*Node) (*Node, error)) Iterator {
+	return &AsyncFilterIter{it, nil, check}
+}
+
+type AsyncFilterIter struct {
+	it     Iterator
+	buffer *Node
+	check  func(*Node) (*Node, error)
+}
+
+func (f *AsyncFilterIter) Next() bool {
+	for f.it.Next() {
+		nn, err := f.check(f.it.Node())
+		if err == nil {
+			f.buffer = nn
+			return true
+		}
+	}
+	return false
+}
+
+func (f *AsyncFilterIter) Node() *Node {
+	return f.buffer
+}
+
+func (f *AsyncFilterIter) Close() {
+	f.it.Close()
+}
+
 // FairMix aggregates multiple node iterators. The mixer itself is an iterator which ends
 // only when Close is called. Source iterators added via AddSource are removed from the
 // mix when they end.

p2p/server.go

@@ -483,7 +483,6 @@ func (srv *Server) setupDiscovery() error {
 			return err
 		}
 		srv.discv4 = ntab
-		srv.discmix.AddSource(ntab.RandomNodes())
 	}
 	if srv.Config.DiscoveryV5 {
 		cfg := discover.Config{