[Libguestfs] Re: 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.