File without read rights will abort diffoscope

When running with a file that had only root access permissions, running diffoscope as a regular user fails.

This was seen in a live image for the file /etc/.pwd.lock

Minimal case to reproduce the issue:

As root:

• touch a b
• chmod go= a b

As a regular user

• diffoscope a b

The result

Traceback (most recent call last):
  File "/usr/bin/diffoscope", line 36, in <module>
    main()
  File "/usr/lib/python3/dist-packages/diffoscope/main.py", line 767, in main
    sys.exit(run_diffoscope(parsed_args))
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/diffoscope/main.py", line 718, in run_diffoscope
    difference = compare_root_paths(path1, path2)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/diffoscope/comparators/utils/compare.py", line 66, in compare_root_paths
    file1 = specialize(FilesystemFile(path1, container=container1))
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/diffoscope/comparators/utils/specialize.py", line 75, in specialize
    if try_recognize(file, cls, cls.recognizes):
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/diffoscope/comparators/utils/specialize.py", line 35, in try_recognize
    if not recognizes(file):
           ^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/diffoscope/comparators/debian.py", line 253, in recognizes
    if not super().recognizes(file):
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/diffoscope/comparators/utils/file.py", line 176, in recognizes
    file.magic_file_type,
    ^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/diffoscope/comparators/utils/file.py", line 325, in magic_file_type
    self._magic_file_type = File.guess_file_type(self.path)
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/diffoscope/comparators/utils/file.py", line 108, in guess_file_type
    return maybe_decode(cls._mimedb.from_file(path))
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/magic/__init__.py", line 112, in from_file
    with _real_open(filename):
         ^^^^^^^^^^^^^^^^^^^^
PermissionError: [Errno 13] Permission denied: 'a'

Here comes the design question:

I think it's straightforward to fail in case of EACCESS on files directly specified on the shell, but should it always fail (=ret code 2) whenever diffoscope hits a EACCESS wherever in the unpacking tree, or be more clever, in your opinion?

If the file is unaccessible and has zero bytes, diffoscope only needs to compare the stat information to look for a difference. If the file size is non-zero, the output could be similar to the dev-handles when running as non-root:

Differences may be incomplete: create_inode: could not create character device ./dev/console, because you're not superuser!
create_inode: could not create character device ./dev/full, because you're not superuser!

PS: Note that you can treat this issue as low-prio, since in a follow-up build of the live images I have deleted this specific 0-byte file (which will always be recreated on demand)

BTW, note that there is no need to use root to trigger this, doing this as a regular user is more than enough:

touch a b
chmod 000 a b
diffoscope a b

See also https://bugs.debian.org/1065498

Ah, I thought that I had created a bug report during MiniDebCamp in Hamburg, but I somehow couldn't find it again and reported it here again. Sorry for the duplicate noise.

mentioned in commit rclobus-guest/live-build@e1c79ffe

mentioned in commit rclobus-guest/live-build@f290d319

mentioned in commit rclobus-guest/live-build@37606ed0

I recently ran into this issue again, while running diffoscope as non-root on an extracted live image.

diffoscope /etc/credstore /etc/credstore will reproduce this issue as well (so it is not specific to zero byte files).

changed title from Zero byte file without read rights will abort diffoscope to File without read rights will abort diffoscope

closed with commit a5486ebd