Maintain contextvars between fixtures and tests

[bcmills]

The approach I've taken here is to inspect the context after running the fixture setup task and copy any changed values out into the test's ambient context. When we run the finalizer to tear down the fixture, we then reset the modified variables.

Some limitations:

• This requires Python 3.11 or higher (for Accept explicit contextvars.Context in asyncio create_task() API python/cpython#91150).
• Because we have to iterate over the fixture context to copy out changes, this approach may be slow for fixtures that run in contexts with a large number of variables.

Fixes #127.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.42%. Comparing base (a1cd861) to head (7e2da55).
Report is 5 commits behind head on main.

Additional details and impacted files

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+ Coverage   90.83%   91.42%   +0.58%     
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  Files           2        2              
  Lines         513      548      +35     
  Branches       66       72       +6     
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+ Hits          466      501      +35     
  Misses         28       28              
  Partials       19       19              

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[bcmills]

• If a synchronous fixture runs after the corresponding event loop has already been created, and it modifies context vars in the thread's current context, those changes will not propagate into async fixtures and functions.

It occurs to me that we can get rid of this caveat by copying the modified values out to the ambient context after running the setup phase, and resetting them after the finalizer. I'll look into that approach.

[bcmills]

It occurs to me that we can get rid of this caveat by copying the modified values out to the ambient context after running the setup phase, and resetting them after the finalizer.

Done, and I'm even happier with this approach!

[seifertm]

You put a lot of effort into this, thanks! I find this a fairly elegant way to address the long-standing problem with context vars in pytest-asyncio. Your use of helper functions makes the code easy to follow and there are tests to support the change.

I'd appreciate if:

1. You addressed my comment regarding the structure of the tests.
2. You added a corresponding entry in docs/reference/changelog.rst that mentions the support for contextvars and the limitation to Python >=3.11.

[bcmills]

Thanks for the review! I think I've addressed all of your comments, but please let me know if I missed something.

[seifertm]

That's exactly what I had in mind, thanks!