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Author njs
Recipients asvetlov, lukasz.langa, ned.deily, njs, yselivanov
Date 2020-02-11.06:35:22
SpamBayes Score -1.0
Marked as misclassified Yes
Message-id <1581402923.1.0.805434272092.issue39606@roundup.psfhosted.org>
In-reply-to
Content
In bpo-39386, the 'aclose' method for async generators was fixed so that the following broken code would correctly raise an error:

# -- bad code --
async def agen_fn():
    yield

async def do_bad_thing():
    agen = agen_fn()
    aclose_coro = agen.aclose()
    await aclose_coro
    # Should raise RuntimeError:
    await aclose_coro

asyncio.run(do_bad_thing())

# -----------------

However, the merged fix also broke the following correct code, which should *not* raise an error:

# -- good code --
async def agen_fn():
    yield

async def do_good_thing():
    agen = agen_fn()
    await agen.aclose()
    # Should *not* raise an error, but currently does in Python dev branches:
    await agen.aclose()

asyncio.run(do_good_thing())

# ----------------

It's not supported to iterate the same coroutine object twice. However, making two *independent* calls to aclose should return two independent coroutine objects, and it should be legal to iterate over each object once.

This can also occur even if there's only a single call to 'aclose'. For example, this is the recommended idiom for making sure that an async generator correctly closes all its resources:

# -- good code --
async def agen_fn():
    yield

async def careful_loop():
    agen = agen_fn()
    try:
        async for _ in agen:
            pass
    finally:
        await agen.aclose()

asyncio.run(careful_loop())

# -------------------

On released Python, the code above runs correctly. On the latest Python dev branches, it raises a RuntimeError.

So basically the problem is that the fix for bpo-39386 is storing the "was aclose iterated?" state on the async generator object, but in fact it needs to be stored on the aclose coroutine object.
History
Date User Action Args
2020-02-11 06:35:23njssetrecipients: + njs, ned.deily, asvetlov, lukasz.langa, yselivanov
2020-02-11 06:35:23njssetmessageid: <1581402923.1.0.805434272092.issue39606@roundup.psfhosted.org>
2020-02-11 06:35:23njslinkissue39606 messages
2020-02-11 06:35:22njscreate