[Libguestfs] libnbd: When are callbacks freed

Hello, While writing some tests for the Rust bindings, I discovered a memory leak with Valgrind due to a completion callback not being freed. More specificly, the completion callback of `aio_opt_info()` doesn't seem to be if `aio_opt_info()` failes. In this case, `aio_opt_info()` was called in the connected state, so it should indeed fail, but it should perhaps also call the free function associated with the callback. According to libnbd(3) <https://libguestfs.org/libnbd.3.html>;: After studying the code in lib/opt.c, I think that the situation with synchronous callbacks like `nbd_opt_list()` is even worse. Here the list callback will not be freed if the internal call to `nbd_unlocked_aio_opt_list()` failes, but it will be freed if the completion callback got an error which is returned by `nbd_opt_list()`. Here the list callback has not been freed. But here I think that the callback has been freed. The problem is that if `nbd_opt_list()` returns an error, I as a user cannot know wether I should free the data associated with the list callback myself or if that has been done already. Maybe I am just misunderstanding the code or the API, but if not then I wonder how I should know when I need to free the user data associated with a callback myself and when that is done by libnbd? Best regards, Tage