Issue 38013: AsyncGenerator breaks when not iterated fully with RuntimeError("can't send non-None value to a just-started coroutine")

Version: Python 3.7.3

When I run this code:

import asyncio


class TestAsyncGenerator:
    def __init__(self):
        pass

    async def aiter(self):
        yield 1
        yield 2


async def main():
    gen = TestAsyncGenerator()
    async for number in gen.aiter():
        break


asyncio.run(main())

# unhandled exception during asyncio.run() shutdown
# task: <Task finished coro=<<async_generator_athrow without __name__>()> exception=RuntimeError("can't send non-None value to a just-started coroutine")>
# RuntimeError: can't send non-None value to a just-started coroutine


There is a warning message that I don't expect to see. I would expect breaking from an async iteration to behave as if breaking from a normal iteration - that is, no problems.

However, I see the warning message shown above. Am I missing something? Otherwise, I believe this is a bug. Thanks!

I have a PR for the fix.
Yuri preliminary agrees with the patch but he requested more time for careful review.

I've added 'release blocker' priority to don't miss this bugfix for the next release.

> I've added 'release blocker' priority to don't miss this bugfix for the next release.

Note that 3.7.5rc1 is scheduled to be tagged today so we need to make a decision on whether to hold the release for a fix.  From a quick test, it looks like the behavior was introduced in 3.7.1 as the test does not fail for me with 3.7.0. If so, that seems to meet the criteria for a maintenance release regression and thus potentially a release blocker.

Thanks, Ned.
I love to include this in 3.7.5.
I've pinged Yuri offline with ask for review

New changeset c275312a6284bd319ea33c9abd7e15c230eca43f by Miss Islington (bot) (Andrew Svetlov) in branch 'master':
bpo-38013: make async_generator_athrow object tolerant to throwing exceptions (GH-16070)
https://github.com/python/cpython/commit/c275312a6284bd319ea33c9abd7e15c230eca43f

New changeset fc022f04b41a79cacdff380435c30c8042c82b99 by Miss Islington (bot) in branch '3.7':
bpo-38013: make async_generator_athrow object tolerant to throwing exceptions (GH-16070)
https://github.com/python/cpython/commit/fc022f04b41a79cacdff380435c30c8042c82b99

New changeset 3c1786f18b1542e71454f37e3f3ca1ef3eec0e5f by Miss Islington (bot) in branch '3.8':
bpo-38013: make async_generator_athrow object tolerant to throwing exceptions (GH-16070)
https://github.com/python/cpython/commit/3c1786f18b1542e71454f37e3f3ca1ef3eec0e5f

messages: + msg352486