fix: make Specifier / SpecifierSet pickle-safe

[henryiii]

This is a followup to #1163. I've had an model apply the same changes/logic in that PR to Specifier/SpecifierSet. I think this is likely in better shape, due to not referncing a missing module, but should be better long term.

I also had it generate similar tests for several packaging versions.

🤖 Assisted by both OpenCode w/ Kimi-K2.5 & Copilot CLI w/ (new for CLI) auto model selection (claude-sonnet-4.6).

CC @eachimei.

[eachimei]

Looks good! 👍
One observation: the indexed-tuple approach optimizes for compactness and performance, at the cost of relying on shape-sniffing (isinstance, len, nested type checks) to distinguish formats in __setstate__. That's perfectly reasonable for this and Version use cases, just noting that there's no explicit format tag or version marker, so future format changes will need another heuristic branch rather than a clean dispatch. Not a current risk, and if it does become a problem in the future, a format tag can then be introduced at that point...

[henryiii]

Yes, I usually start the tuple with an "format version" integer. But I don't think this will be likely to change much, and simply adding a value is safe. Longer term, we could probably use pattern matching here if it's not too much slower.

[eachimei]

Indeed, sounds good

[Copilot]

Pull request overview

This PR extends the pickle-safety/backward-compatibility work from #1163 to Specifier and SpecifierSet, aiming to ensure pickles remain loadable across packaging versions and don’t persist cache/internal state.

Changes:

• Add __getstate__/__setstate__ to Specifier and SpecifierSet, using compact tuple state and discarding caches on restore while supporting older pickle formats.
• Add pickle roundtrip tests for Specifier/SpecifierSet.
• Add fixture-based tests to load historical pickle payloads (packaging 25.0, 26.0, 26.1, 26.2-style).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/packaging/specifiers.py	Implements compact pickle state + legacy restore logic for Specifier and SpecifierSet, clearing caches on restore.
tests/test_specifiers.py	Adds roundtrip and historical-pickle loading tests for specifiers/specifiersets across multiple packaging versions.

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[henryiii]

The copilot review boils down to one thing: protect against malformed pickles. I've added it (generated tests, but threw away the model-implemented solution and added my own), but I'm not sure it's really needed to always check this (auto-generated pickle code doesn't have such checks), but I think it's probably a good idea for the "old" pickles. I've added it for everything, but I'd also be fine to drop it for new-style pickles (outer tuple). It's not too expensive, though, and it does make typing happy.

Pattern matching would be so nice here. :)

[henryiii]

Also, I've followed #1163 in generating old pickles and storing them, but we could also just store the intermediate state, which wouldn't be a somewhat opaque binary string, and would unit test the same. I think the current one is a bit better, but if there's a desire to avoid binary strings, we could make something work.