Re: [Libguestfs] [PATCH libnbd] lib: Remove cookie parameter from completion callbacks.

On 7/30/19 11:28 AM, Richard W.M. Jones wrote: Eww, you're right. We return -1 if nbd_internal_run queued the command but encountered an error moving the state machine along; but that is much trickier for the user to recover from (if they aren't using the _callback form, then they NEED to know the cookie id to eventually retire the command during nbd_aio_command_completed). Maybe we should guarantee that we return the cookie in that case, even though a failure was detected? I don't think it is a good idea to change the API from 'int64 nbd_aio_pread()' to 'int nbd_aio_pread(int64 *cookie)' where *cookie gets assigned on successful queue whether or not the overall command returns 0 or -1. In the long term, if nbd_internal_run() fails, then the next nbd_* call that tries to manipulate the state machine will see that the state machine is in the DEAD state; learning that the state machine failed during nbd_aio_pread probably doesn't buy us much benefit. So I'm leaning towards just ignoring nbd_internal_run failure, and guaranteeing that we return > 0 if we queue the command, no matter whether we then encounter a state machine error after that point. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org