Re: [Libguestfs] no way to force-close NBD handle in nbdsh

I'm out of time this weekend, but while trying to write a test for an nbdkit bug fix (a nasty assertion failure when the client disconnects uncleanly without NBD_CMD_DISC, which is what nbdcopy does if it gets an EIO read response from the server), I realized it is extremely hard to trigger this using nbdsh, but easy to do in C or the Go bindings. In the Go bindings, we intentionally coded things so that the Go structure knows if its underlying C pointer is live or not; it exposes a way to force early closure, then the bindings return a closed_handle_error() if early closure has happened. We probably need to support something similar in the Python bindings. As a short-term hack, I tried directly calling h.__del__() - this is not a good idea, as our current __del__ implementation is not designed to be called twice, and when the later garbage collection calls it again, we segfault trying to free invalid memory.

But less drastic measures, such as: import gc del h h = None gc.collect() were still insufficient to trigger a normal __del__ of h, because I can't figure out how to force the garbage collector to see that I want to close the handle but keep python open. In the end, I did get a working test written (by just quit()ing python instead of trying to keep it alive). See my next email for the nbdkit patch that spawned my request here.