Impl a proto "unmasker" `@acm` alongside our test

Such that the suite verifies the wip `maybe_raise_from_masking_exc()`
will raise from a `trio.Cancelled.__context__` since I can't think of
any reason a `Cancelled` should ever be raised in-place of
a non-`Cancelled` XD

Not sure what should be raised instead (or maybe just a `log.warning()`
emitted?) but this starts a draft for refinement at the least. Use the
new `@pytest.mark.parametrize` explicit tuple-of-params form with an
`pytest.param + `.mark.xfail()` for the default behaviour case.

tests/test_trioisms.py

@@ -86,58 +86,113 @@ def test_stashed_child_nursery(use_start_soon):
         trio.run(main)
 
 
-# @pytest.mark.parametrize(
-#     'open_tn_outside_acm',
-#     [True, False]
-#     # ids='aio_err_triggered={}'.format
-# )
 @pytest.mark.parametrize(
-    'canc_from_finally',
-    [True, False]
-    # ids='aio_err_triggered={}'.format
+    ('unmask_from_canc', 'canc_from_finally'),
+    [
+        (True, False),
+        (True, True),
+        pytest.param(False, True,
+                     marks=pytest.mark.xfail(reason="never raises!")
+        ),
+    ],
+    # TODO, ask ronny how to impl this .. XD
+    # ids='unmask_from_canc={0}, canc_from_finally={1}',#.format,
 )
 def test_acm_embedded_nursery_propagates_enter_err(
     canc_from_finally: bool,
-    # open_tn_outside_acm: bool,
+    unmask_from_canc: bool,
 ):
-    # from tractor.trionics import maybe_open_nursery
+    '''
+    Demo how a masking `trio.Cancelled` could be handled by unmasking from the
+    `.__context__` field when a user (by accident) re-raises from a `finally:`.
 
-    # async def canc_then_checkpoint(tn):
-    #     tn.cancel_scope.cancel()
-    #     await trio.lowlevel.checkpoint()
+    '''
+    import tractor
 
     @acm
-    async def wraps_tn_that_always_cancels(
-        # maybe_tn: trio.Nursery|None = None
+    async def maybe_raise_from_masking_exc(
+        tn: trio.Nursery,
+        unmask_from: BaseException|None = trio.Cancelled
+
+        # TODO, maybe offer a collection?
+        # unmask_from: set[BaseException] = {
+        #     trio.Cancelled,
+        # },
+    ):
+        if not unmask_from:
+            yield
+            return
+
+        try:
+            yield
+        except* unmask_from as be_eg:
+
+            # TODO, if we offer `unmask_from: set`
+            # for masker_exc_type in unmask_from:
+
+            matches, rest = be_eg.split(unmask_from)
+            if not matches:
+                raise
+
+            for exc_match in be_eg.exceptions:
+                if (
+                    (exc_ctx := exc_match.__context__)
+                    and
+                    type(exc_ctx) not in {
+                        # trio.Cancelled,  # always by default?
+                        unmask_from,
+                    }
+                ):
+                    exc_ctx.add_note(
+                        f'\n'
+                        f'WARNING: the above error was masked by a {unmask_from!r} !?!\n'
+                        f'Are you always cancelling? Say from a `finally:` ?\n\n'
+
+                        f'{tn!r}'
+                    )
+                    raise exc_ctx from exc_match
+
+
+    @acm
+    async def wraps_tn_that_always_cancels():
+        async with (
+            trio.open_nursery() as tn,
+            maybe_raise_from_masking_exc(
+                tn=tn,
+                unmask_from=(
+                    trio.Cancelled
+                    if unmask_from_canc
+                    else None
+                ),
+            )
         ):
-        # async with maybe_open_nursery(maybe_tn) as tn:
-        async with trio.open_nursery() as tn:
             try:
                 yield tn
             finally:
                 if canc_from_finally:
-                    # await canc_then_checkpoint(tn)
                     tn.cancel_scope.cancel()
                     await trio.lowlevel.checkpoint()
 
     async def _main():
-        # open_nursery = (
-        #     trio.open_nursery if open_tn_outside_acm
-        #     else nullcontext
-        # )
+        with tractor.devx.open_crash_handler() as bxerr:
+            assert not bxerr.value
 
             async with (
-            # open_nursery() as tn,
-            # wraps_tn_that_always_cancels(maybe_tn=tn) as tn
-            wraps_tn_that_always_cancels() as tn
+                wraps_tn_that_always_cancels() as tn,
             ):
                 assert not tn.cancel_scope.cancel_called
                 assert 0
 
+        assert (
+            (err := bxerr.value)
+            and
+            type(err) is AssertionError
+        )
+
     with pytest.raises(ExceptionGroup) as excinfo:
         trio.run(_main)
 
-    eg = excinfo.value
+    eg: ExceptionGroup = excinfo.value
     assert_eg, rest_eg = eg.split(AssertionError)
 
     assert len(assert_eg.exceptions) == 1