[Libguestfs] Re: what size should NBD_OPT_INFO report?

On Wed, Feb 21, 2024 at 12:26:08PM -0600, Eric Blake wrote: > On Wed, Feb 21, 2024 at 12:19:49PM -0600, Eric Blake wrote: > > 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. > > Amending myself: reading size_autodetect(), it tries ioctl(fd, > BLKGETSIZE64) first (even when fd is not a block device!), then falls > back to fstat(fd) coupled with lseek(SEEK_END) if fstat() reported > st_size of 0 and not a regular file; but if all of those fail, it > reports UINT64_MAX, which is different than OFFT_MAX. Another wrinkle: On BSD systems, lseek(fd_of_block, 0, SEEK_END) behaves identically to lseek(fd_of_block, 0, SEEK_START), because BSD has the idea that the size of a seekable file descriptor is whatever gets reported in stat().st_size which is 0 on block devices (Linux is the odd system out where lseek() and stat() disagree on this point - all other systems surveyed either reliably report the block capacity in st_size and SEEK_END, or use offset 0 in both places). See this POSIX bug for more details: https://austingroupbugs.net/view.php?id=1817

If we are going to report correct capacity of block devices, it requires a LOT more code; compare what nbdkit is adding: https://gitlab.com/nbdkit/nbdkit/-/merge_requests/54/diffs?commit_id=532b...