graceful shutdown of uTP sockets

include/libtorrent/utp_socket_manager.hpp

@@ -82,6 +82,7 @@ namespace libtorrent
 
 		void mtu_for_dest(address const& addr, int& link_mtu, int& utp_mtu);
 		void set_sock_buf(int size);
+		int num_sockets() const { return m_utp_sockets.size(); }
 
 	private:
 		udp_socket& m_sock;

src/session_impl.cpp

@@ -2902,7 +2902,16 @@ namespace aux {
 #if defined TORRENT_VERBOSE_LOGGING
 //		(*m_logger) << time_now_string() << " session_impl::on_tick\n";
 #endif
-		if (m_abort) return;
+
+		// we have to keep ticking the utp socket manager
+		// until they're all closed
+		if (m_abort && m_utp_socket_manager.num_sockets() == 0)
+		{
+#if defined TORRENT_ASIO_DEBUGGING
+			fprintf(stderr, "uTP sockets left: %d\n", m_utp_socket_manager.num_sockets());
+#endif
+			return;
+		}
 
 		if (e == asio::error::operation_aborted) return;