Crash when illegal file encoding `utf` in the first line

[raininboat]

Bug description

When parsing the following file:

# -*- encoding: utf -*-

pylint crashed with a AstroidError and with the following stacktrace:

Traceback (most recent call last):
  File "/home/ubuntu/.local/lib/python3.8/site-packages/pylint/lint/pylinter.py", line 782, in _lint_file
    check_astroid_module(module)
  File "/home/ubuntu/.local/lib/python3.8/site-packages/pylint/lint/pylinter.py", line 1052, in check_astroid_module
    retval = self._check_astroid_module(
  File "/home/ubuntu/.local/lib/python3.8/site-packages/pylint/lint/pylinter.py", line 1098, in _check_astroid_module
    raw_checker.process_module(node)
  File "/home/ubuntu/.local/lib/python3.8/site-packages/pylint/checkers/unicode.py", line 531, in process_module
    line = _remove_bom(line, codec)
  File "/home/ubuntu/.local/lib/python3.8/site-packages/pylint/checkers/unicode.py", line 223, in _remove_bom
    bom = UNICODE_BOMS[encoding]
KeyError: 'utf'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/ubuntu/.local/lib/python3.8/site-packages/pylint/lint/pylinter.py", line 747, in _lint_files
    self._lint_file(fileitem, module, check_astroid_module)
  File "/home/ubuntu/.local/lib/python3.8/site-packages/pylint/lint/pylinter.py", line 784, in _lint_file
    raise astroid.AstroidError from e
astroid.exceptions.AstroidError

Configuration

No response

Command used

pylint main.py

Pylint output

************* Module main
main.py:1:0: C2503: PEP8 recommends UTF-8 as encoding for Python files (bad-file-encoding)
main.py:1:0: F0002: main.py: Fatal error while checking 'main.py'. Please open an issue in our bug tracker so we address this. There is a pre-filled template that you can use in '/home/ubuntu/.cache/pylint/pylint-crash-2022-10-22-01-14-12.txt'. (astroid-error)

Expected behavior

pylint should show the error about the illegal file encoding utf and do not crash

Pylint version

pylint 2.15.4
astroid 2.12.12
Python 3.8.10 (default, Mar 15 2022, 12:22:08) 
[GCC 9.4.0]

OS / Environment

Linux 5.10.102.1-microsoft-standard-WSL2 #1 SMP Wed Mar 2 00:30:59 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
Ubuntu 20.04 LTS

Additional dependencies

No response

[Pierre-Sassoulas]

Thank you for opening the issue. It's very similar to #3860, seems like the fix was fragile.

[clavedeluna]

I can work on this over the next couple days. I can also see #7097 didn't add a test, so I can add that. @Pierre-Sassoulas is testing on modules best done via functional tests, or follow the examples in test_self.py which seem to dynamically create a module or load an existing module?

[Pierre-Sassoulas]

I think I tried to add a test at first in 865b1b8 but it seems I did not manage to reproduce later on (as something was fixed in astroid). So I would say this kind of crash is best tested with mocks in pylint. Due to the nature of the crash we're going to also want to fix astroid and add tests there. Edit: It seems in this issue the error is different and come from pylint (bom = UNICODE_BOMS[encoding] in pylint/checkers/unicode.py) so functional would probably work but there are a lot of unit tests for the unicode checker so the choice is yours,