[Libguestfs] Re: [RFC PATCH] server: Advertise nicer size for NBD_OPT_INFO

NBD_OPT_GO either advertises the actual size (possibly with an override from the command line or config file), the value OFFT_MAX (if multitree or F_WAIT means computing a real size would take too long), or the value UINT64_MAX (if size_autodetect fails, such as when fd is non-seekable); the only time it ever advertises a size of 0 is when it is serving a regular file that really is empty. On the other hand, NBD_OPT_INFO had been blindly advertising 0 no matter what. While we can't always compute a real size, we can do a lot better by advertising the same sentinels as NBD_OPT_GO. Signed-off-by: Eric Blake <eblake(a)redhat.com&gt; --- Rich has already chimed in on my question about what size SHOULD nbd-server report when NBD_OPT_GO wants to list an indeterminate size; if we start changing the spec, then this may be incomplete or need tweaking. Here, I only tackled the simple case of a single-file (or block device) export; there are multifile cases where NBD_OPT_GO still reports a more accurate number than this. There's also the question of whether we want to address the information leak (right now, the code can succeed on NBD_OPT_INFO even when it will later fail for NBD_OPT_GO because the client is not authorized to used the export).