Wrap `send_chan_aclose_masks_beg.py` as test suite

Call it `test_trioisms::test_unmask_aclose_as_checkpoint_on_aexit` and
parametrize all script-mod`.main()` toggles including `.xfails()` for
the `raise_unmasked=False` cases.

tests/test_trioisms.py

@@ -6,11 +6,18 @@ want to see changed.
 from contextlib import (
     asynccontextmanager as acm,
 )
+from types import ModuleType
+
+from functools import partial
 
 import pytest
+from _pytest import pathlib
 from tractor.trionics import collapse_eg
 import trio
 from trio import TaskStatus
+from tractor._testing import (
+    examples_dir,
+)
 
 
 @pytest.mark.parametrize(
@@ -106,8 +113,9 @@ def test_acm_embedded_nursery_propagates_enter_err(
     debug_mode: bool,
 ):
     '''
-    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:`.
+    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:`.
 
     '''
     import tractor
@@ -158,13 +166,13 @@ def test_acm_embedded_nursery_propagates_enter_err(
     assert len(assert_eg.exceptions) == 1
 
 
-
 def test_gatherctxs_with_memchan_breaks_multicancelled(
     debug_mode: bool,
 ):
     '''
-    Demo how a using an `async with sndchan` inside a `.trionics.gather_contexts()` task
-    will break a strict-eg-tn's multi-cancelled absorption..
+    Demo how a using an `async with sndchan` inside
+    a `.trionics.gather_contexts()` task will break a strict-eg-tn's
+    multi-cancelled absorption..
 
     '''
     from tractor import (
@@ -190,7 +198,6 @@ def test_gatherctxs_with_memchan_breaks_multicancelled(
                 f'Closed {task!r}\n'
             )
 
-
     async def main():
         async with (
             # XXX should ensure ONLY the KBI
@@ -211,3 +218,50 @@ def test_gatherctxs_with_memchan_breaks_multicancelled(
 
     with pytest.raises(KeyboardInterrupt):
         trio.run(main)
+
+
+@pytest.mark.parametrize(
+    'raise_unmasked', [
+        True,
+        pytest.param(
+            False,
+            marks=pytest.mark.xfail(
+                reason="see examples/trio/send_chan_aclose_masks.py"
+            )
+        ),
+    ]
+)
+@pytest.mark.parametrize(
+    'child_errors_mid_stream',
+    [True, False],
+)
+def test_unmask_aclose_as_checkpoint_on_aexit(
+    raise_unmasked: bool,
+    child_errors_mid_stream: bool,
+    debug_mode: bool,
+):
+    '''
+    Verify that our unmasker util works over the common case where
+    a mem-chan's `.aclose()` is included in an `@acm` stack
+    and it being currently a checkpoint, can `trio.Cancelled`-mask an embedded
+    exception from user code resulting in a silent failure which
+    appears like graceful cancellation.
+
+    This test suite is mostly implemented as an example script so it
+    could more easily be shared with `trio`-core peeps as `tractor`-less
+    minimum reproducing example.
+
+    '''
+    mod: ModuleType = pathlib.import_path(
+        examples_dir()
+        / 'trio'
+        / 'send_chan_aclose_masks_beg.py',
+        root=examples_dir(),
+        consider_namespace_packages=False,
+    )
+    with pytest.raises(RuntimeError):
+        trio.run(partial(
+            mod.main,
+            raise_unmasked=raise_unmasked,
+            child_errors_mid_stream=child_errors_mid_stream,
+        ))