[Libguestfs] [PATCH libnbd 0/2] Use free callback to dereference NBD.Buffer.

In this patch series we use the newly introduced free callback on the completion function to dererence the OCaml NBD.Buffer. I will make the same kind of change for Python later in a separate series. The completion function is always called at the C level, even if the OCaml program didn't use the optional argument. That's because the free callback doesn't run otherwise. There is a case for having the free callback run even if there is no registered callback: nbd_aio_pread (nbd, buf, sizeof buf, 0, (nbd_completion_callback) { .callback = NULL, .free = completion_free }, 0); but the semantics of that are a bit weird. Why would you need to "free" a callback which doesn't exist? Would it be correct for the library to free the callback immediately, rather than after the theoretical completion callback would have run? Rich.