Hi,
I have a very big and highly threaded program that generates a BlockedIndefinitelyOnMVar exception when run. I have spent a reasonable amount of time pouring over the source code, as has Max Bolingbroke. Neither of us have the slightest idea why it raises the exception.
Some questions:
* Does anyone know the exact sequence of actions that causes this exception to be thrown? I couldn't find it written down. * How confident are people that this exception does really mean that it is in a blocked state? Is there any chance the error could be raised incorrectly? * Any debugging tips for this problem?
My understanding was that this error occurred when one thread was blocked, waiting on an MVar, and no other thread in the program has a reference to that MVar (this can be detected during GC). Ergo, the blocked thread will end up waiting forever because no-one can ever wake it up again. Whenever I have had this error (or its STM equivalent) I think it was always telling the truth. I seem to remember it was often a symptom of another thread terminating unexpectedly. So if thread A is blocked on an MVar that it is expecting thread B to write to, then thread B terminating can cause this error to arise in thread A, even though the real problem is in thread B. Do you actually have use of MVars in your program directly, or are they being used via a library? And do you at least know which thread is throwing this exception? It should be catchable so you can probably wrap the arguments to your forkIO calls with a catcher than indicates which thread blew up. Thanks, Neil.