Since we can't use it to `Task.set_exception()` (since that task method never
seems to work.. XD) and setting the private/internal always seems to do
the desired raising in the task? I realize it's an internal `asyncio`
runtime field but i'd rather take the risk of it breaking then having to
rely on our own equivalent hack..
Also, it seems like the case where the task's associated (and internal)
future-waiter field is null, we won't run into the (same?) prior hanging
issues (maybe since there's nothing for `asyncio` internals to use to
wait XD ??) when `Task.cancel()` is used..??
Main deats,
- add and `Future.set_exception()` a new signal-exception
`class TrioTaskExited(AsyncioCancelled):` whenever the trio-task exits
gracefully and the asyncio-side task is still doing blocking work (of
some sort) which *seem to* be predicated by a check that
`._fut_waiter is not None`.
- always call `asyncio.Queue.shutdown()` for the same^ as well as
whenever we decide to call `Task.cancel()`; in that case the shutdown
relays correctly?
Some further refinements,
- only warn about `Task.cancel()` usage when actually used ;)
- more local scope vars setting in the exit phase of
`translate_aio_errors()`.
- also in ^ use explicit caught-exc var names for each error-type.
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