> 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.

Neat idea, thanks, I have not thought about that.  Just pushed a commit
which uses a va_list in the callback prototype instead of a pointer to a
structure.  I chose this way because I really do not like passing a lot
of arguments around.  I think this solution also better conveys the
intent.  Could you please take a look?

> (Actually it's not a 'callback' but a function that does things directly, right?)

Correct, but I failed to find a better term and "callback" should give
the reader the right idea.  Plain "function" sounds too generic to me.
If you have any suggestions, I am all ears (or you can even push a
relevant tweak to the branch if you feel like it, as far as I am
concerned).