Function set_file_contents 

fn set_file_contents(
    dirfd: &OwnedFd,
    name: &OsStr,
    stat: &Stat,
    data: &[u8],
) -> Result<()>

Attempt to use O_TMPFILE + rename to atomically set file contents. Will fall back to a non-atomic write if the target doesn’t support O_TMPFILE.