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

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. Buffer lifetimes ---------------- Similar to the above, persistent buffers (BytesPersist*) can have lifetimes. Remove nbd_add_close_callback ----------------------------- The above changes (actually, just the closure change) lets us remove nbd_add_close_callback. Fixing callback / cookie problem -------------------------------- I think an easier way to fix this would be to simply detect the problematic situation and queue up the callback to be call the next time the library is entered. Detecting the situation is fairly easy - we know the current cookie number, so can check it before we call the completion function. And calling the deferred callback(s) is easy from the generated code. The only questions in my mind are: (1) If this could cause a deadlock because the caller is waiting for the completion signal before reentering the library. (2) If there's some implicit guarantee that we should call the callback as easy as possible after the command finishes. I think (1) is not possible in ordinary code since callers must always enter the main loop after calling one of the nbd_aio_* functions. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com Fedora Windows cross-compiler. Compile Windows programs, test, and build Windows installers. Over 100 libraries supported. http://fedoraproject.org/wiki/MinGW