Dropped exceptions in `as_safe_channel`

[A5rocks]

async def test8() -> None:
    @trio.as_safe_channel
    async def agen():
        await chan.aclose()
        raise ValueError()
        yield

    async with agen() as chan:
        async for _ in chan:
            pass

This sometimes raises this:

Traceback (most recent call last):
  File "C:\Users\A5rocks\Documents\trio\repro.py", line 124, in <module>
    trio.run(test8)
    ~~~~~~~~^^^^^^^
  File "C:\Users\A5rocks\Documents\trio\src\trio\_core\_run.py", line 2547, in run
    raise runner.main_task_outcome.error
  File "C:\Users\A5rocks\Documents\trio\repro.py", line 121, in test8
    async for _ in chan:
        pass
  File "C:\Users\A5rocks\Documents\trio\src\trio\_abc.py", line 689, in __anext__
    return await self.receive()
           ^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\A5rocks\Documents\trio\src\trio\_channel.py", line 477, in receive
    return await self._recv_chan.receive()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\A5rocks\Documents\trio\src\trio\_channel.py", line 385, in receive
    return await trio.lowlevel.wait_task_rescheduled(abort_fn)  # type: ignore[no-any-return]
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\A5rocks\Documents\trio\src\trio\_core\_traps.py", line 208, in wait_task_rescheduled
    return (await _async_yield(WaitTaskRescheduled(abort_func))).unwrap()
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "C:\Users\A5rocks\Documents\trio\.venv\Lib\site-packages\outcome\_impl.py", line 213, in unwrap
    raise captured_error
trio.ClosedResourceError

Notice no ValueError! Spun off of #3331

[A5rocks]

Hypothesis: this is due to cancellation being suppressed.

[A5rocks]

Ok it is. I guess this is user error!! But such a footgun...

This prints yes, this was cancelled... if and only if we drop the ValueError:

async def test8() -> None:
    @trio.as_safe_channel
    async def agen():
        try:
            await chan.aclose()
        except trio.Cancelled:
            print("yes, this was cancelled...")
            raise
        raise ValueError()
        yield

    async with agen() as chan:
        async for _ in chan:
            pass

[Zac-HD]

😱 it's #455 again, because of course it is 😱

[A5rocks]

Actually, I thought so too but... this is just normal cancellation semantics now that I think more about it. I guess cancellation is just hard in general...

[jakkdl]

I think I was doing the exact same thing and confusing myself when writing tests for as_safe_channel. Cancellation is definitely tricky!