Mv `maybe_raise_from_masking_exc()` to `.trionics`

Factor the `@acm`-closure it out of the
`test_trioisms::test_acm_embedded_nursery_propagates_enter_err` suite
for real use internally.

tests/test_trioisms.py

@@ -112,55 +112,11 @@ def test_acm_embedded_nursery_propagates_enter_err(
     '''
     import tractor
 
-    @acm
-    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(
+            tractor.trionics.maybe_raise_from_masking_exc(
                 tn=tn,
                 unmask_from=(
                     trio.Cancelled
@@ -202,3 +158,60 @@ def test_acm_embedded_nursery_propagates_enter_err(
     assert_eg, rest_eg = eg.split(AssertionError)
 
     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..
+
+    '''
+    from tractor import (
+        trionics,
+    )
+
+    @acm
+    async def open_memchan() -> trio.abc.ReceiveChannel:
+
+        task: trio.Task = trio.lowlevel.current_task()
+        print(
+            f'Opening {task!r}\n'
+        )
+
+        # 1 to force eager sending
+        send, recv = trio.open_memory_channel(16)
+
+        try:
+            async with send:
+                yield recv
+        finally:
+            print(
+                f'Closed {task!r}\n'
+            )
+
+
+    async def main():
+        async with (
+            # XXX should ensure ONLY the KBI
+            # is relayed upward
+            trionics.collapse_eg(),
+            trio.open_nursery(
+                # strict_exception_groups=False,
+            ), # as tn,
+
+            trionics.gather_contexts([
+                open_memchan(),
+                open_memchan(),
+            ]) as recv_chans,
+        ):
+            assert len(recv_chans) == 2
+
+            await trio.sleep(1)
+            raise KeyboardInterrupt
+            # tn.cancel_scope.cancel()
+
+    with pytest.raises(KeyboardInterrupt):
+        trio.run(main)

tractor/trionics/__init__.py

@@ -34,3 +34,6 @@ from ._beg import (
     maybe_collapse_eg as maybe_collapse_eg,
     is_multi_cancelled as is_multi_cancelled,
 )
+from ._taskc import (
+    maybe_raise_from_masking_exc as maybe_raise_from_masking_exc,
+)

tractor/trionics/_taskc.py

@@ -0,0 +1,71 @@
+# tractor: structured concurrent "actors".
+# Copyright 2018-eternity Tyler Goodlet.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+'''
+`trio.Task` cancellation helpers, extensions and "holsters".
+
+'''
+from __future__ import annotations
+from contextlib import (
+    asynccontextmanager as acm,
+)
+
+import trio
+# from trio import TaskStatus
+
+
+@acm
+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