[Libguestfs] Re: what size should NBD_OPT_INFO report?

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.