Demonstrating the guilty `trio.Lock.acquire()` impl which puts
a checkpoint inside its `trio.WouldBlock` handler and which will always
appear to mask the "sync path" case on (graceful) cancellation.
This first script draft demos the issue from within a `tractor.context`
ep bc that's where it was orig discovered, however i'm going to factor
out the `tractor` code and instead just use
a `.trionics.maybe_raise_from_masking_exc()` to demo its low-level
ignore-case feature.
Further, this script exposed a previously unhandled remote graceful
cancellation case which hangs:
- parent actor spawns child and opens a >1 ctxs with it,
- the parent then OoB (out-of-band) cancels the child actor (with
`Portal.cancel_actor()`),
- since the open ctxs raise a ctxc with a `.canceller == parent.uid` the
`Context._is_self_cancelled()` will eval `True`,
- the `Context._scope` will NOT be cancelled in
`._maybe_cancel_and_set_remote_error()` resulting in any bg-task which
is waiting on a `Portal.open_context()` to not be cancelled/unblocked.
So my plan is to factor this ^^ scenario into a standalone unit test
as well as another test which consumes from al low-level `trio`-only
version of **this** script-scenario to sanity check the interaction
of the unmasker-with-ignore-cases usage implicitly around a ctx ep.
9c6b90ef04