All RPC communication of a rpctrace:d programs is catched by rpctrace.  When signal that terminates or suspends the traced task is sent, the special thread of the traced task that handles signals calls RPC proc_dostop_request to the proc server.  This call, by design, suspends all threads of the traced task except the one that handles the signal.  Unfortunately, actually all the threads are suspended, thus making the whole process to hang.  Moreover, when a key is pressed the whole terminal hangs.  This is probably because the traced task is no longer able to handle any input.

So, why proc_dostop_request can't recognize the signal thread of the traced task?  Because proc_dostop_request receives 2 arguments: task_t process and thread_t contthread.  The function gets all process threads and compare them to contthread.  But contthread is changed by rpctrace!  That's why all threads are suspended.

The easiest solution is to add special case to rpctrace processing.  Though it is questionable whether this is the right thing to do.