Demo of trio task waking without exception from `await trio.something()` despite being in a recently-cancelled cancel scope

trio_cancel_race.py

# Question: if we cancel a scope, and another task is active in that scope, is it *guaranteed* that
# it will raise CancelledError when it next wakes? (Even if it is already queued to wake for some
# other reason.)
#
# The answer, at least in practice, is no: in this example, send_and_cancel() sends some data to
# a memory channel then cancels a scope, and then receive() (which is running in that scope) wakes
# with the first value that was sent.
#
# On my computer, this code produces the following output:
#
#     Python 3.12.1 | packaged by Anaconda, Inc. | (main, Jan 19 2024, 15:44:08) [MSC v.1916 64 bit (AMD64)]
#     Trio 0.24.0
#     main(): running
#     receive(): starting
#     receive(): waiting
#     send_and_cancel(): starting
#     send_and_cancel(): values sent
#     send_and_cancel(): cancelled
#     receive(): got 1; cancelled: True
#     receive(): waiting
#     receive(): exception Cancelled()
#     main(): done
#

import sys
import trio

async def send_and_cancel(snd, cnc):
    print("send_and_cancel(): starting")
    await trio.sleep(0.5)
    snd.send_nowait(1)
    snd.send_nowait(2)
    snd.send_nowait(3)
    print("send_and_cancel(): values sent")
    cnc.cancel()
    print("send_and_cancel(): cancelled")


async def receive(rcv, cnc):
    print(f"receive(): starting")
    try:
        for _ in range(2):
            print("receive(): waiting")
            val = await rcv.receive()
            print(f"receive(): got {val}; cancelled: {cnc.cancel_called}")
        print(f"receive(): done")
    except BaseException as e:
        print(f"receive(): exception {e!r}")
        raise
    else:
        print(f"receive(): finished no exception")

async def main():
    print("Python", sys.version)
    print("Trio", trio.__version__)
    snd, rcv = trio.open_memory_channel(max_buffer_size=5)
    async with trio.open_nursery() as n:
        n.start_soon(receive, rcv, n.cancel_scope)
        n.start_soon(send_and_cancel, snd, n.cancel_scope)
        print("main(): running")
    print("main(): done")

trio.run(main)