EXC_BAD_ACCESS in nested dispatch_async with an NSAutoreleasePool

Question

I have some code which is similar to the following code:

dispatch_queue_t queue          = dispatch_queue_create("", 0);
dispatch_queue_t inner_queue    = dispatch_queue_create("", 0);
dispatch_async(queue,
^{
    NSAutoreleasePool* autoreleasePool = [[NSAutoreleasePool alloc] init];
    NSArray* objects = [self.otherObject getObjectsFromSlowQuery];

    [objects enumerateObjectsWithUsingBlock:^(id anObject, NSUInteger idx, BOOL *stop) 
    {
        [anObject prepare];
        dispatch_async(inner_queue,
        ^{
            InnerObject* innerObject = anObject.innerObject;
            [self.helper doSomethingExpensiveWithObject:innerObject];
        });
        dispatch_sync(self.syncQueue,
        ^{
             [self insertIntoCollection:anObject];
        });
    }];
    dispatch_release(inner_queue);
    [autoreleasePool drain];
});
dispatch_release(queue);

Where [anObject.innerObject] is a nonatomic property.

I got a crash report from a user which shows an EXC_BAD_ACCESS in objc_msgSend() when trying to access a property of innerObject within the doSomethingExpensiveWithObject: call.

At first I was thinking that perhaps the autoreleasePool was drained so the innerObject instance was released before returning from doSomethingExpensiveWithObject: but as far as I know anObject should be retained by the inner dispatch_async call which should also keep the innerObject alive.

What am I missing?

Answer 1

Instruments will make quick work of this - run with zombies and review the reference counts when it stops.