cdimage@nusakan:~$ TMPDIR=/srv/system-image.ubuntu.com/tmp /srv/system-image.ubuntu.com/bin/import-images -v
xz: /srv/system-image.ubuntu.com/www/full/pool/custom-51fb7bd3e49898416dd4d9ce47c71563743aefac760b034834a793fa19d2619a.tar.xz: File format not recognized
Traceback (most recent call last):
  File "/srv/system-image.ubuntu.com/bin/import-images", line 251, in <module>
    abspath)
  File "/srv/system-image.ubuntu.com/bin/../lib/systemimage/generators.py", line 114, in generate_delta
    os.path.join(tempdir, "target.tar"))
  File "/srv/system-image.ubuntu.com/bin/../lib/systemimage/diff.py", line 93, in __init__
    self.target_file = tarfile.open(target, 'r:', encoding='utf-8')
  File "/usr/lib/python2.7/tarfile.py", line 1678, in open
    return func(name, filemode, fileobj, **kwargs)
  File "/usr/lib/python2.7/tarfile.py", line 1705, in taropen
    return cls(name, mode, fileobj, **kwargs)
  File "/usr/lib/python2.7/tarfile.py", line 1574, in __init__
    self.firstmember = self.next()
  File "/usr/lib/python2.7/tarfile.py", line 2334, in next
    raise ReadError("empty file")
tarfile.ReadError: empty file
cdimage@nusakan:~$