Re: [Libguestfs] [PATCH libnbd v2 2/5] lib: Implement closure lifetimes.

Previously closures had a crude flag which tells if they are persistent or transient. Transient closures (flag = false) last for the lifetime of the currently called libnbd function. Persistent closures had an indefinite lifetime which could last for as long as the handle. In language bindings handling persistent closures was wasteful as we needed to register a "close callback" to free the closure when the handle is closed. But if you had submitted thousands of asynchronous commands you would end up registering thousands of close callbacks. We actually have far more information about the lifetime of closures. We know precisely when they will no longer be needed by the library. Callers can use this information to free up associated user data earlier. In particular in language bindings we can remove roots / decrement reference counts at the right place to free the closure, without waiting for the handle to be closed. The solution to this is to introduce the concept of a closure lifetime. The callback is called with an extra valid_flag parameter which is a bitmap containing LIBNBD_CALLBACK_VALID and/or LIBNBD_CALLBACK_FREE. LIBNBD_CALLBACK_VALID corresponds to a normal call of the callback function by the library. After the library has finished with the callback (declaring that this callback will never be needed or called again), it is called once more with valid_flag == LIBNBD_CALLBACK_FREE. (Note it is also possible for the library to call the callback with valid_flag == LIBNBD_CALLBACK_VALID|LIBNBD_CALLBACK_FREE, meaning it's the last valid call.)