bpo-43419: fix contextvars behaviors in asyncio REPL

[lanfon72]

bug tracker

I just make the little enhance to propagate Context between MainThread and REPLThread, to let contextvars work properly in asyncio REPL environment.

Here is the new behavior after this PR merged:

asyncio REPL 3.8.6 (default, Oct 18 2020, 00:27:24) 
[Clang 11.0.0 (clang-1100.0.33.16)] on darwin
Use "await" directly instead of "asyncio.run()".
Type "help", "copyright", "credits" or "license" for more information.
>>> import asyncio
>>> from contextvars import ContextVar
>>> ctx = ContextVar('ctx')
>>> ctx.get(None)
>>> ctx.set(1)
<Token var=<ContextVar name='ctx' at 0x10167c8b0> at 0x10167bd00>
>>> ctx.get()
1
>>> async def coro_f():
...   return ctx.get()
... 
>>> await coro_f()
1
>>> def normal_f():
...   return ctx.get()
... 
>>> normal_f()
1
>>> exit()

https://bugs.python.org/issue43419

[the-knights-who-say-ni]

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[lanfon72]

I think there still got a bug that REPLThread use loop.create_task to schedule a coroutine, which means it will implicit do copy_context() inside.
But when we await a coroutine, which means we use the same context instead of a copied version.

here is the expected behavior using aioconsole:

Python 3.8.6 (default, Oct 18 2020, 00:27:24) 
[Clang 11.0.0 (clang-1100.0.33.16)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
---
This console is running in an asyncio event loop.
It allows you to wait for coroutines using the '{0}' syntax.
Try: await asyncio.sleep(1, result=3)
---
>>> from contextvars import ContextVar
>>> ctx = ContextVar('ctx')
>>> ctx.set(1)
<Token var=<ContextVar name='ctx' at 0x10b077db0> at 0x10b0c1cc0>
>>> ctx.get()
1
>>> async def awtf():
...   ctx.set(2)
...   return ctx.get()
... 
>>> await awtf()
2
>>> ctx.get()
2
>>> 

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[bswck]

See also gh-124594 and gh-124595.

[bswck]

@ZeroIntensity I think you can close this one too.