Re: Interesting difference in memory management of MacOS Foundation and GNUstep

[Eric Wasylishen]

Hey David,

I ran in to the same problem Mathias mentioned. It's easy to hit if
you're managing some resource with GCD, and that resource needs to be
closed in -dealloc (in my case, it's the database connection in
CoreObject that I only access within a particular dispatch queue, and
I need to close the connection in -dealloc).

You can trigger the failure just by using a block in dealloc that
causes self to be retained:
- (void) dealloc
{
    void (^myBlock)() = ^() {
        id foo = self;
        NSLog(@"inside myBlock, foo = %p", foo);
    };
    myBlock();
}

I attached this reduced test case and a naiive patch to arc.m that
makes the test case work, but it's an ugly hack and I'm sure it's
broken in various ways.

In CoreObject, I think I can work around this for now by creating the
block ahead of time, in -init.

Eric

On Mon, Feb 17, 2014 at 6:39 AM, David Chisnall
<David.Chisnall@cl.cam.ac.uk> wrote:
> On 17 Feb 2014, at 13:33, Mathias Bauer <mathias_bauer@gmx.net> wrote:
>
>> in case somebody else is also interested in this: it seems that Apple's 
>> runtime "protects" the developer by ignoring changes to the retain count as 
>> soon as the object entered its deallocate method. Wrong decision, IMHO.
>
> It is likely that this is a side effect of weak reference support.  Classes 
> must notify the runtime when they start deallocation now, so that concurrent 
> loads of weak references abort the deallocation.  Apple's implementation 
> stores objects' refcounts in a map table, so once the object has entered 
> deallocation it's likely just a separate path.  I wouldn't be surprised if 
> this is not an active decision at all, however it does make adding cycle 
> detection to ARC easier...
>
> David
>
>
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