p2p: limit number of lingering inbound pre-handshake connections

This is supposed to apply some back pressure so Server is not accepting
more connections than it can actually handle. The current limit is 50.
This doesn't really need to be configurable, but we'll see how it
behaves in our test nodes and adjust accordingly.

p2p/server.go

@@ -21,6 +21,11 @@ const (
 	defaultDialTimeout   = 10 * time.Second
 	refreshPeersInterval = 30 * time.Second
 
+	// This is the maximum number of inbound connection
+	// that are allowed to linger between 'accepted' and
+	// 'added as peer'.
+	maxAcceptConns = 50
+
 	// total timeout for encryption handshake and protocol
 	// handshake in both directions.
 	handshakeTimeout = 5 * time.Second
@@ -269,15 +274,28 @@ func (srv *Server) Self() *discover.Node {
 // main loop for adding connections via listening
 func (srv *Server) listenLoop() {
 	defer srv.loopWG.Done()
+
+	// This channel acts as a semaphore limiting
+	// active inbound connections that are lingering pre-handshake.
+	// If all slots are taken, no further connections are accepted.
+	slots := make(chan struct{}, maxAcceptConns)
+	for i := 0; i < maxAcceptConns; i++ {
+		slots <- struct{}{}
+	}
+
 	glog.V(logger.Info).Infoln("Listening on", srv.listener.Addr())
 	for {
+		<-slots
 		conn, err := srv.listener.Accept()
 		if err != nil {
 			return
 		}
 		glog.V(logger.Debug).Infof("Accepted conn %v\n", conn.RemoteAddr())
 		srv.peerWG.Add(1)
-		go srv.startPeer(conn, nil)
+		go func() {
+			srv.startPeer(conn, nil)
+			slots <- struct{}{}
+		}()
 	}
 }