support for callable unique_on taking args, kwargs

[rooterkyberian]

This is useful if you have a task which takes a complex object, but in fact uniqueness is only done based on single field of it, such as tasks taking User, but uniqueness check only needs user.id.

Of course you could argue, you shouldn't have such celery tasks with such complex object as params, but I think it should be left to the user to decide if is right for them or not.

At the same time it seems very easy to implement - for now I'm using custom subclass:

class UniqueFunctionSingleton(celery_singleton.Singleton):
    def generate_lock(self, task_name, task_args=None, task_kwargs=None):
        unique_on = self.unique_on
        task_args = task_args or []
        task_kwargs = task_kwargs or {}
        if callable(unique_on):
            unique_args = unique_on(*task_args, **task_kwargs)
            unique_kwargs = None
        else:
            if unique_on:
                if isinstance(unique_on, str):
                    unique_on = [unique_on]
                sig = inspect.signature(self.run)
                bound = sig.bind(*task_args, **task_kwargs).arguments

                unique_args = []
                unique_kwargs = {key: bound[key] for key in unique_on}
            else:
                unique_args = task_args
                unique_kwargs = task_kwargs
        return util.generate_lock(
            task_name,
            unique_args,
            unique_kwargs,
            key_prefix=self.singleton_config.key_prefix,
        )