[Libguestfs] nbdkit filter error handling

I've given some more thoughts to error handling in nbdkit filters, and am writing this while it is still fresh on my mind (even though I won't have time to code any of it until next week). Right now, plugins are allowed to call either nbdkit_set_error OR to leave a value in errno; the latter works only if errno_is_preserved was set. If a plugin returns -1 but does not set an error through either method, then nbdkit assumes EIO as the error value. But filters may want/need to perform error recovery (perhaps recognizing an ENOSPC on write, by resizing and then retrying the operation). Filters have access to errno after a client call fails, but cannot rely on errno having the right value - they have to be able to get at the thread-local value that was stored during nbdkit_set_error. Furthermore, if a filter does ANYTHING non-trivial after the client returns an error, it is likely to clobber the errno value that the plugin set (if the client relied on errno instead of nbdkit_set_error) - which may be okay if the filter itself wants to return a new value, but not so good if it was just logging things but wants to preserve the plugin's error. At the same time, if the plugin set the error via nbdkit_set_error, then changing errno won't make a difference; the way nbdkit handles errors is that the thread-local value takes priority and errno is only a fallback if the thread-local value was not set. So, we can either mandate that filters that want to do non-trivial work after calling into next_ops must check what the plugin set (hoist the connections.c:get_error into nbdkit_get_error), and then MUST call nbdkit_set_error to change the error (meaning that filters should not rely on errno for setting the value) - or we can change the semantics of the backend and filters to say that instead of returning -1, we return the actual error code. Plugins still return -1 for backward compatibility, but the conversion from plugin to backend semantics in plugins.c will convert the errno-or-nbdkit_set_error value into its return; and then the filter stack will see the return value directly, and can then return that same value without having to mess with either errno or nbdkit_set_error. At this point, I'm leaning towards this semantic change as being less boilerplate in filters. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org