You may try the following, which is probably a hack:
In the dealloc method of the Database, do something like this:
- (void) dealloc {
NSCache* cache = self.cache;
dispatch_async(private_queue, ^{
cache = nil;
});
}
Now, if `cache_remove_with_block` executes on a different thread than where the
block `cache = nil;` executes, respectively, if it starts executing when
`cache_remove_with_block` already has been finished, then the dead lock might
get resolved.
Andreas
On 21.10.2013, at 21:11, Jens Alfke <j...@mooseyard.com> wrote:
> I’ve just gotten a nasty bug report involving an iOS app hanging when it goes
> into the background. The problem is a deadlock involving NSCache. I can see
> what the problem is, but I don’t know what to do about it.
>
> In a nutshell: An NSCache is evicting objects, and as a result of that its
> last reference goes away so the cache gets dealloced. Only the dealloc method
> needs the same (non-recursive) lock the eviction call is using, so it
> deadlocks.
>
> Specifically, there is a “database” object that has a strong reference to an
> NSCache which maps to various ‘document’ objects (indexed by key.) The
> document objects have strong references back to the database.
>
> In the situation that hangs, there is a database with one document, neither
> of which have any external strong references to them. (They probably used to
> at one point, but the app stopped using that database.) That’s fine, it’s not
> a reference cycle because the NSCache will get cleaned up. Only the cleanup
> doesn’t work because:
>
> 1. OS tells NSCache to flush value objects (`cache_remove_with_block`)
> 2. NSCache releases the document, causing it to be dealloced
> 3. Document’s dealloc implicitly releases its reference to the database
> 4. Database is released and dealloced, implicitly releasing the NSCache
> 5. NSCache is dealloced … but the `cache_destroy` call needs the mutex that’s
> already being held by `cache_remove_with_block`.
>
> (You can see the full backtrace in the bug report on github.)
>
> Has anyone else run into this? Is there a workaround? This has come up once
> before for me, and I was able to work around it by making the cache-owner
> object call -autorelease instead of -release on the NSCache, to defer the
> call to the cache’s dealloc. But I’m now using ARC so that isn’t an option.
>
> —Jens
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