On Thu, Oct 30, 2008 at 10:16 PM, Colin Barrett <email@hidden> wrote:
> On Thu, Oct 30, 2008 at 5:30 PM, Michael Ash <email@hidden> wrote:
>> Based on the state of the program when it crashes, it appears that the
>> problem is caused by a race condition which occasionally causes two of
>> the worker threads that NSOperationQueue spawns to dequeue and execute
>> the same NSOperation. Since an NSOperation is only supposed to run
>> once, things fall down go boom. This is just a theory, mind, and I'm
>> not sure of it yet.
>
> I'm not sure it would help, but it might. You could try using the
> dependency mechanism in addition to setting maxConcurrentOperations to
> 1. Keep track of the last NSOperation in your for loop and assign in
> as you go.
>
> If that fixes it, it could be a race in maxConcurrentOperations.
>
> This is all highly speculative.
Thanks for the suggestion. I tried it out and it still crashes with
that same exception. I even tried it without setting the
maxConcurrentOperations and it still crashed. When I tried it in the
case where the operations get enqueued in a separate thread instead of
from the operation method, then it still crashed but took a long time
(5-10 minutes, didn't count exactly) to do so. So it would seem that
this helps somehow but not quite enough.
This last case is particularly scary to me. I'm not doing anything
that NSOperationQueue isn't explicitly set up for. I'm just posting
NSOperations to queues that have been created and are sitting in
memory. I'm not fiddling with the maxConcurrentOperations, I'm not
doing anything weird like enqueueing a new operation from the middle
of an existing one, I'm just spawning some threads which post
operations to queues and let them run. And it *still* crashes.
I hate to make this sort of sweeping pronouncement because it seems
like a great way to end up looking like a fool, but at this point I
can only conclude that NSOperationQueue is fundamentally broken and
should not be used until this problem is fixed. Of course I would love
to be wrong.
Mike
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