linters: allow "categories" for subfiltering

[tigarmo]

This commit adds the notion of "categories" for Linter subclasses. For linters that perform multiple different "types" of linting, this lets the author ignore specific paths for specific types of linting issues, with a finer granularity than ignoring the whole linter wholesale.

Currently the only Linter type that supports this is the LibraryLinter, which provides the "unused-library" and "missing-library" categories to disable checking for unused or missing libraries.

CRAFT-1642

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• Have you successfully run make lint?
• Have you successfully run pytest tests/unit?

---

First commit has the general linter changes, second commit has the update of the ros extension.

[codecov-commenter]

Codecov Report

Merging #4077 (729462a) into main (dcd59d8) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 729462a differs from pull request most recent head 4ac70be. Consider uploading reports for the commit 4ac70be to get more accurate results

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@@            Coverage Diff             @@
##             main    #4077      +/-   ##
==========================================
- Coverage   94.60%   94.60%   -0.01%     
==========================================
  Files         641      641              
  Lines       51842    51822      -20     
==========================================
- Hits        49045    49026      -19     
+ Misses       2797     2796       -1     

Impacted Files	Coverage Δ
snapcraft/extensions/ros2_humble.py	96.96% <ø> (ø)
tests/unit/parts/extensions/test_ros2_humble.py	95.65% <ø> (ø)
snapcraft/linters/base.py	98.43% <100.00%> (+0.28%)	⬆️
snapcraft/linters/library_linter.py	87.95% <100.00%> (+1.87%)	⬆️
snapcraft/linters/linters.py	97.67% <100.00%> (+0.08%)	⬆️
tests/unit/linters/test_library_linter.py	100.00% <100.00%> (ø)
tests/unit/linters/test_linters.py	100.00% <100.00%> (ø)

... and 9 files with indirect coverage changes

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[sergiusens]

Looks good, nice and clean.
Can we extend the ros related spread test that uses the extension to verify this is working as expected too?
Additionally, can we have a spread test that declares linter entries?

[tigarmo]

Can we extend the ros related spread test that uses the extension to verify this is working as expected too?

Do you mean by e.g. checking the snapcraft output and verifying that no linter issues are emitted? Sure

Additionally, can we have a spread test that declares linter entries?

Yes. I'll do both in a little bit.

[tigarmo]

@sergiusens thank you for the suggestion, it pointed out an unexpected interaction. First, I added a new spread test that uses the ros2-humble extension and verifies that no linter warnings are generated. This worked as expected.

Then I tried to add a new spread with a "user-provided" lint ignore like:

lint:
  ignore:
    - unused-library:
      - usr/lib/*/libpng16.so*

... the resulting behavior is that this new unused-library entry "overrides" the extension-provided one, so the snap generates all the ros-related unused library warnings. I think it would be quite straightforward to update this behavior to have the user-generated lint ignores "add" to the extension-provided ones, but I also think we should do that in a separate task/PR.

What do you think?

[sergiusens]

@tigarmo let's ask that question during the demo