Pickle _contextvars.Context objects, for threads in Python 3.14+

[hroncok]

Fixes #716

Summary

Checklist

Documentation and Tests

Release Management

• Added "Fixes #NNN" in the PR body, referencing the issue (#NNN) it closes.
• Added a comment to issue #NNN, linking back to this PR.
• Requested a review.

[hroncok]

OK, this makes the tests pass. However, I am not entirely sure this is correct. I created it by trial and error.

[mmckerns]

This is on my short list to get a fix in for, so thanks for submitting. I'll review, test, and follow up here.

[mmckerns]

See also #586

[mmckerns]

This fix only works when the Context is empty. If you add a ContextVar, so the Context is not empty, it fails on the pickling of a ContextVar. Simple test code:

import contextvars as cv
var = cv.ContextVar('var')
t0 = var.set(0)
foo = cv.ContextVar('foo', default=0)
t1 = foo.set(1)
zap = cv.ContextVar('zap', default=3)
t4 = zap.set(4)
zap.reset(t4)
qux = cv.ContextVar('qux', default=0)
t2 = qux.set(2)
t3 = qux.set(3)
qux.reset(t3)
ctx = cv.copy_context()
print(list(ctx.items()))

True, the tests pass with an empty Context, and I believe you function is correct, it's not a full solution. Potentially we can merge this, and open a new ticket for a ContextVar and Token. However, I think ContextVar and Token can probably be handled with a petty simple registration function utilizing get and set. The tricky part may be whether a ContextVar and Token get referenced appropriately in the enclosing Context.

[eli-schwartz]

Looks like this was silently included in #726?

[mmckerns]

Yeah... I think I started the new branch with the edited version of _dill.py and it slid in. The issue with the ContextVar still remains. We will still get that in with this PR.

[mmckerns]

Started testing against 3.15t where this triggers a test failure.