[libdispatch-dev] Replacement for dispatch_get_current_queue

[Daniel A. Steffen]

Allan,

the dispatch_retain() here is not safe and can crash with a resurrection assertion if the queue in question has already been released by its owner.

as the documentation says, you must not modify the queue returned by dispatch_get_current_queue() in any way, including retaining/releasing it or submitting blocks to it.

If you need something like this, you must track & memory manage the queues of interest yourself, e.g. by passing them as parameters, or setting them as queue-specific context etc.

Daniel

On Apr 19, 2013, at 19:33, Allan Odgaard <lists+libdispatch at simplit.com> wrote:

> On Apr 20, 2013, at 4:13, Thomas Clement <tclementdev at gmail.com> wrote:
> 
>> On 19 avr. 2013, at 22:48, Adam Ernst <adamjernst at fb.com> wrote:
>> 
>>> The design of dispatch_get_current_queue is clearly problematic, as its docs indicate. I've heard rumors the function will be deprecated and removed soon.
>>> […]
>> Take a look at dispatch_queue_get_specific and dispatch_queue_set_specific.
> 
> I have another use-case where objects can only be accessed from a single thread.
> 
> For example:
> 
>  class MyClass
>  {
>    void set_foo (Foo* foo);
>    …
>  };
> 
>  std::shared_ptr<MyClass> create_object ()
>  {
>    dispatch_queue_t original_queue = dispatch_get_current_queue();
>    dispatch_retain(original_queue);
> 
>    std::shared_ptr<MyClass> res(new MyClass);
> 
>    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
>      Foo* foo = new Foo; // could be expensive
>      dispatch_async(original_queue, ^{
>        res->set_foo(foo);
>        dispatch_release(original_queue);
>      });
>    });
> 
>    return res;
>  }
> 
> One solution is to pass the current queue to create_object(), which normally would be the main queue, however, when running tests, it’s the testing framework that decides which queue a test executes in, so tests would have to create their own “inner” queue and run the actual code in that, so that they have a valid queue to pass to create_object().
> 
> Another solution would be to allow set_foo() to be executed on arbitrary threads. This however will make the implementation of MyClass harder, and it would basically only be for the benefit of our tests, so I don’t think this cost/extra complexity is justified.
> 
> Probably the first solution is the best, but I’d be interested in hearing others’ take on this.
> 
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