[RFC] Implement PEP 567 (contextvars)

[fanninpm]

Summary

The contextvars module, from PEP 567, is necessary to upgrade asyncio (and decimal) to CPython 3.8. I started #2950 to implement contextvars, but the PR has since stalled. (@DimitrisJim urged me to open this issue.)

Detailed Explanation and Rationale

PEP 567, implemented in CPython 3.7, introduced the contextvars module and context variables, which are conceptually similar to thread-local variables. However, unlike thread-local variables, context variables correctly support asynchronous code. Following that implementation, the asyncio and decimal modules were re-written to take advantage of this new capability. As part of our endeavors to get as much of the codebase covered by the CPython test suite as possible (see #1671), and out of a desire support CPython 3.8 or later (see the README), I think it's necessary to add the test_asyncio module to the CPython test suite. The addition of the CPython 3.8 version of test_asyncio would benefit from upgrading the asyncio module itself to that same version. (@coolreader18 and I have noticed that test failures were often alleviated by upgrading the relevant Python module.) However, asyncio has a dependency — contextvars — that we don't have yet. Accordingly, I have started a draft PR (#2950) to implement this module here, but I got stymied when I was drafting the Rust implementation because I am unfamiliar with how RustPython deals with thread safety. For context, here are the parts of the C implementation that I think are relevant:

• https://github.com/python/cpython/blob/3.8/Include/context.h
• https://github.com/python/cpython/blob/3.8/Python/context.c
• https://github.com/python/cpython/blob/3.8/Python/clinic/context.c.h
• https://github.com/python/cpython/blob/3.8/Modules/_contextvarsmodule.c
• https://github.com/python/cpython/blob/3.8/Modules/clinic/_contextvarsmodule.c.h
• https://github.com/python/cpython/blob/3.8/Include/internal/pycore_context.h

Unresolved Questions

• What is the meaning of PyContext (defined in pyobject.rs), and does it have any bearing on the scope of PEP 567?
• The scaffolding is present, but how should it be fleshed out?
• How would each Context object relate to a thread in RustPython?