Check the process status before waiting, it appears to be edge trigge…

…red. (#125)

ext/io/event/selector/epoll.c

@@ -504,6 +504,13 @@ VALUE IO_Event_Selector_EPoll_process_wait(VALUE self, VALUE fiber, VALUE _pid,
 
 	rb_update_max_fd(descriptor);
 
+	// `pidfd_open` (above) may be edge triggered, so we need to check if the process is already exited, and if so, return immediately, otherwise we will block indefinitely.
+	VALUE status = IO_Event_Selector_process_status_wait(pid, flags);
+	if (status != Qnil) {
+		close(descriptor);
+		return status;
+	}
+
 	struct IO_Event_Selector_EPoll_Waiting waiting = {
 		.list = {.type = &IO_Event_Selector_EPoll_process_wait_list_type},
 		.fiber = fiber,