anyio is unexpectedly cancelling tasks

[Booplicate]

Things to check first

• I have searched the existing issues and didn't find my bug already reported there

• I have checked that my bug is still present in the latest release

AnyIO version

4.3.0

Python version

3.11

What happened?

It appears that anyio is cancelling outer task. Originally I thought it was httpx, but the maintainer team helped to nail down the problem to anyio. Original issue with more examples and discussion.

How can we reproduce the bug?

Given example code:

import asyncio

import anyio


async def loop():
    while True:
        stream = await anyio.connect_tcp('www.example.com', 80)
        # await stream.aclose()
        await asyncio.sleep(1)

async def spawn_task():
    task = asyncio.create_task(loop())
    while True:
        await asyncio.sleep(1)
        print("cancelling: ", task.cancelling())

asyncio.run(spawn_task())

Output:

cancelling:  1
cancelling:  2
cancelling:  3
cancelling:  4
cancelling:  5

Expected output: anyio shouldn't increment the cancelling counter of the outer task.

[Booplicate]

Same (?) code but using asyncio:

import asyncio


async def loop():
    while True:
        stream = await asyncio.open_connection("www.example.com", 80)
        await asyncio.sleep(1)

async def spawn_task():
    task = asyncio.create_task(loop())
    while True:
        await asyncio.sleep(1)
        print("cancelling: ", task.cancelling())

asyncio.run(spawn_task())

Output

cancelling:  0
cancelling:  0
cancelling:  0
cancelling:  0
cancelling:  0

[agronholm]

I think this is an artifact of AnyIO's happy eyeballs implementation which makes multiple concurrent connection attempts and cancels the rest after one succeeds. I'm not sure yet why it would lead the incrementing of the host task's cancel counter though.

[agronholm]

Here's another repro that doesn't involve connect():

import asyncio

import anyio
from anyio import Event, create_task_group


async def set_or_cancel(tg, event):
    if not event.is_set():
        event.set()
    else:
        tg.cancel_scope.cancel()


async def loop():
    event = Event()
    async with create_task_group() as tg:
        for _ in range(2):
            tg.start_soon(set_or_cancel, tg, event)
            with anyio.CancelScope():
                await event.wait()


async def spawn_task():
    task = asyncio.create_task(loop())
    await task
    print("cancelling: ", task.cancelling())

asyncio.run(spawn_task())

[agronholm]

That with anyio.CancelScope() seemingly does nothing here, but if it's commented out, the snippet no longer reproduces the problem.

[agronholm]

I've been digging into this for a while, and it's looking like _deliver_cancellation() increments the cancel counter of the wrong cancel scope when cancellation cascades to derivative cancel scopes. At least a particular one-line change seems to have fixed the issue. I'll just make sure it doesn't break anything else.

[agronholm]

An even simpler repro:

import asyncio

import anyio


async def main():
    with anyio.CancelScope() as root_scope:
        with anyio.CancelScope():
            root_scope.cancel()
            await anyio.sleep(0)

    task = asyncio.current_task()
    print("cancelling: ", task.cancelling())

asyncio.run(main())