The fix for [bpo-39386](https://bugs.python.org/issue39386) attempted to make it so you couldn't reuse a
agen.aclose() coroutine object. It accidentally also prevented you
from calling aclose() at all on an async generator that was already
closed or exhausted. This commit fixes it so we're only blocking the
actually illegal cases, while allowing the legal cases.
The new tests failed before this patch. Also confirmed that this fixes
the test failures we were seeing in Trio with Python dev builds:
https://github.com/python-trio/trio/pull/1396https://bugs.python.org/issue39606
Ignore `GeneratorExit` exceptions when throwing an exception into the `aclose` coroutine of an asynchronous generator.
https://bugs.python.org/issue35409
Even when the helper is not started yet.
This behavior follows conventional generator one.
There is no reason for `async_generator_athrow` to handle `gen.throw()` differently.
https://bugs.python.org/issue38013
Lidi Zheng
Nathaniel J. Smith
Andrew Svetlov
Vincent Michel
Yury Selivanov
Brett Cannon
Victor Stinner
Serhiy Storchaka
Martin Panter