[Libguestfs] [libnbd] More thoughts on callbacks and more

[Richard W.M. Jones]

On Sat, Jul 20, 2019 at 07:38:45AM +0100, Richard W.M. Jones wrote:
> More thoughts on callbacks, etc. following on from:
> https://www.redhat.com/archives/libguestfs/2019-July/thread.html#00184
> 
> Closure lifetimes
> -----------------
> 
> Closures could have a lifetime if we had a little bit of support from
> the C library.  We would generate (from C only):
> 
>   nbd_set_free_<fn>_<closure> (nbd, free_closure);
> 
> which calls free_closure (user_data) as soon as the closure will no
> longer be called by the library.  This function would be used to
> decrement the refcount from Python or remove the global root from
> OCaml.
> 
> Note this is a family of functions, eg:
> 
>   nbd_set_free_set_debug_callback_debug_fn
> 
> corresponding to the debug_fn arg of nbd_set_debug_callback.  Luckily
> they can all be generated along with the internal machinery to call
> them.

As written above this doesn't quite work.  However it could work to
pass an optional free function with the closure.  In other words it
would look like:

  struct nbd_closure { .cl = my_debug_fn,
                       .user_data = foo, .free = my_free } cl;
  nbd_set_debug_callback (nbd, &cl);

cl->free (cl->user_data) is called if cl->free != NULL when the
closure is no longer used by the library.

This is a bit of a change to the API however.

Rich.

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