Handle empty cache files

[CodingFeng101]

Summary

Treat empty or truncated cache files the same way as other malformed cache files.

Details

Cache.read() already ignores malformed pickle data so a later cache write can repair the file, but an empty cache file raises EOFError, which was not caught. This can happen if cache creation is interrupted or an external process truncates the file.

This change catches EOFError while reading the cache and adds a regression test for an empty cache file.

Test

• python -m pytest tests/test_black.py::TestCaching::test_cache_empty_file tests/test_black.py::TestCaching::test_cache_broken_file -q

[cobaltt7]

Thanks! Please add a changelog entry.

[github-actions]

diff-shades results comparing this PR (30e2f72) to main (ce441f2):

--preview style: no changes

--stable style: no changes

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What is this? | Workflow run | diff-shades documentation

[CodingFeng101]

Added the requested changelog entry in fcf00f6.

[cobaltt7]

Could you use one of the pre-existing categories instead of adding a new one? (Caching changes have gone in Configuration in the past)

[CodingFeng101]

Moved the entry under the existing Configuration category in 66e62ac.

[DTiming24]

Catching EOFError seems like the right symmetry with malformed pickle handling. One subtle edge case: if an external process truncates the file while Black is running, silently treating it as a cache miss is probably preferable to surfacing cache corruption to the user. The regression test covers the repair path well.

[DTiming24]

Nice fix. Since empty cache files now fall back the same way as other malformed cache data, a regression test for a zero-byte file created by an interrupted write would help lock in the new behavior.

[CodingFeng101]

Thanks for taking a look. The current PR already has a zero-byte cache regression test: test_cache_empty_file creates the cache path with cache_file.touch() and asserts Cache.read(mode).file_data == {}. That is the interrupted/truncated-write case this change is meant to keep on the same recovery path as malformed pickle data.