how to catch GHC "no threads to run" exception?

My program runs 3 threads, and after a few minutes' execution all threads come to a state that they all wait on some MVars, then the program halts with an error: test: no threads to run: infinite loop or deadlock? (test is my program name) My problem is that this behavior is actually desired, but how do I catch this exception and do some bookkeeping (closing external IO, etc.) and then a proper exit? Thanks in advance! Regards, .paul.