[Libguestfs] what size should NBD_OPT_INFO report?

I noticed during integration testing that nbd-server blindly reports a size of 0 for all NBD_OPT_INFO requests, unless I pass a size argument on the command line to nbd-server. At first, I thought it was a side effect of me trying to use nbd-server on a block device (an LVM partition), as it is a common bug to rely on stat().st_size which only works for regular files (a block device has to use lseek(SEEK_END)); but then I noticed it happening when using nbd-server to serve regular files as well. I then turned to the source code, where I see that client->exportsize is set in just these places: commit_client() - exportsize = OFFT_MAX, then try setupexport() setupexport() - default to client->server->expected_size (if one was provided), further validating that actual size is large enough when actual size can be computed - if neither treefile or F_WAIT is set, compute actual size by opening one or more files and using size_autodetect() (which does the right thing for block devices, so my earlier thought about over-reliance on stat() was wrong) but these functions are only reached for NBD_OPT_EXPORT_NAME and NBD_OPT_GO, not NBD_OPT_INFO. The upshot is that for NBD_OPT_GO, there are some scenarios (treefile, F_WAIT) where nbd-server advertises a size of 9223372036854775807 (0x7fffffff_ffffffff) meaning unknown, but a size of 0 there is only possible if the file was successfully opened and really is zero bytes in length. Conversely, NBD_OPT_INFO is always advertising a size of 0, which means most of the time, the size changes between NBD_OPT_INFO and NBD_OPT_GO. For comparison, both nbdkit and qemu as an NBD server always advertise the same size for both NBD_OPT_INFO and NBD_OPT_GO (but it should also be noted that neither of these has the complexity of multifile like nbd-server). Should we tweak the NBD standard to recommend that a server advertise a size of OFFT_MAX for NBD_OPT_INFO if it is prohibitive to determine an actual size, rather than 0? Furthermore, is it worth adding code to make NBD_OPT_INFO try harder to provide a sensible value when possible (if expected_size was set, or if we are not multifile or F_WAIT, then a stat() is enough to get the size for serving a regular file; and if stat() says we have a block device, we can still try the open/lseek/close)? Also, is it worth trying to specify that since no known NBD servers allow exports with sizes larger than OFFT_MAX, a server MUST NOT report a size equal or larger than 9223372036854775808 (0x80000000_00000000)? There are definitely a few places that I could simplify in libnbd if we have a protocol guarantee that a valid export size will never have the most significant bit set, and therefore we don't have to worry about whether size is represented as a signed or unsigned value (which also implies that libnbd's function nbd_get_size() returning ssize_t instead of size_t is acceptable). But if we do that, then maybe 0x80000000_00000000 or even 0xffffffff_ffffffff would serve as a better recommended sentinel than 0x7fffffff_ffffffff for representing an indeterminate size. -- Eric Blake, Principal Software Engineer Red Hat, Inc. Virtualization: qemu.org | libguestfs.org