make the listen argument configurable

docs/manual.rst

@@ -3879,6 +3879,8 @@ session_settings
 		int unchoke_slots_limit;
 		int half_open_limit;
 		int connections_limit;
+
+		int listen_queue_size;
 	};
 
 ``version`` is automatically set to the libtorrent version you're using
@@ -4596,6 +4598,13 @@ opened. The number of connections is set to a hard minimum of at least two per
 torrent, so if you set a too low connections limit, and open too many torrents,
 the limit will not be met.
 
+``listen_queue_size`` is the value passed in to listen() for the listen socket.
+It is the number of outstanding incoming connections to queue up while we're not
+actively waiting for a connection to be accepted. The default is 5 which should
+be sufficient for any normal client. If this is a high performance server which
+expects to receive a lot of connections, or used in a simulator or test, it
+might make sense to raise this number. It will not take affect until listen_on()
+is called again (or for the first time).
 
 pe_settings
 ===========

include/libtorrent/session_settings.hpp

@@ -233,6 +233,7 @@ namespace libtorrent
 			, unchoke_slots_limit(8)
 			, half_open_limit(0)
 			, connections_limit(200)
+			, listen_queue_size(5)
 		{}
 
 		// libtorrent version. Used for forward binary compatibility
@@ -892,6 +893,11 @@ namespace libtorrent
 
 		// the max number of connections in the session
 		int connections_limit;
+
+		// this is the number passed in to listen(). i.e.
+		// the number of connections to accept while we're
+		// not waiting in an accept() call.
+		int listen_queue_size;
 	};
 
 #ifndef TORRENT_DISABLE_DHT

src/session_impl.cpp

@@ -1455,7 +1455,7 @@ namespace aux {
 			return listen_socket_t();
 		}
 		s.external_port = s.sock->local_endpoint(ec).port();
-		s.sock->listen(5, ec);
+		s.sock->listen(m_settings.listen_queue_size, ec);
 		if (ec)
 		{
 			if (m_alerts.should_post<listen_failed_alert>())