Xdebug does not have functionality for this. Using pcntl_signal and then using xdebug_print_function_stack is an option, but what you can also do is to create a "function trace". This is something that makes Xdebug write every function call to disk as soon as they happen. This should be able to tell you where your script hangs. From what I can see though is that is waiting for IO.
There is more information about function traces at http://www.xdebug.org/docs/execution_trace