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

[David Chisnall]

Hi Eric,

The diff looks okay.  I'd make the increment a number that is exactly half of 
the size of the available space, and add a comment saying why it's a good idea, 
but please feel free to commit it.

David

On 27 Feb 2014, at 05:58, Eric Wasylishen <ewasylishen@gmail.com> wrote:

> 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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