On Wed Nov 29 14:41:21 2017, michal wrote:
> The two for loops you mentioned do different things.  However, I agree
> the purpose of this code is hard to grasp in its current form.
> 
> I am intentionally not telling you what the code does; please look at
> the few refactoring commits I just pushed to rt46718 and let me know if
> it is clear now.  If it is not, even more comments are likely due.

Ah, it's much better now.

Just one nit - why struct and not va_list (or just pass all four arguments) for the callback?  The callback_data makes sense when you need to store the callback_data in an async structure.  (Actually it's not a 'callback' but a function that does things directly, right?)