Add the minimal OoB cancel edge case from #391

Discovered while writing a `@context` sanity test to verify unmasker
ignore-cases support. Masked code is due to the process of finding the
minimal example causing the original hang discovered in the original
examples script. Details are in the test-fn doc strings and surrounding
comments; more refinement and cleanup coming obviously.

Also moved over the self-cancel todos from the inter-peer tests module.

tests/test_inter_peer_cancellation.py

@@ -24,14 +24,10 @@ from tractor._testing import (
 )
 
 # XXX TODO cases:
-# - [ ] peer cancelled itself - so other peers should
-#   get errors reflecting that the peer was itself the .canceller?
-
 # - [x] WE cancelled the peer and thus should not see any raised
 #   `ContextCancelled` as it should be reaped silently?
 #   => pretty sure `test_context_stream_semantics::test_caller_cancels()`
 #      already covers this case?
-
 # - [x] INTER-PEER: some arbitrary remote peer cancels via
 #   Portal.cancel_actor().
 #   => all other connected peers should get that cancel requesting peer's
@@ -44,16 +40,6 @@ from tractor._testing import (
 #   that also spawned a remote task task in that same peer-parent.
 
 
-# def test_self_cancel():
-#     '''
-#     2 cases:
-#     - calls `Actor.cancel()` locally in some task
-#     - calls LocalPortal.cancel_actor()` ?
-
-#     '''
-#     ...
-
-
 @tractor.context
 async def open_stream_then_sleep_forever(
     ctx: Context,

tests/test_oob_cancellation.py

@@ -0,0 +1,212 @@
+'''
+Define the details of inter-actor "out-of-band" (OoB) cancel
+semantics, that is how cancellation works when a cancel request comes
+from the different concurrency (primitive's) "layer" then where the
+eventual `trio.Task` actually raises a signal.
+
+'''
+from functools import partial
+# from contextlib import asynccontextmanager as acm
+# import itertools
+
+# import pytest
+import trio
+import tractor
+from tractor import (  # typing
+    ActorNursery,
+    Portal,
+    Context,
+    # ContextCancelled,
+    # RemoteActorError,
+)
+# from tractor._testing import (
+#     tractor_test,
+#     expect_ctxc,
+# )
+
+# XXX TODO cases:
+# - [ ] peer cancelled itself - so other peers should
+#   get errors reflecting that the peer was itself the .canceller?
+
+# def test_self_cancel():
+#     '''
+#     2 cases:
+#     - calls `Actor.cancel()` locally in some task
+#     - calls LocalPortal.cancel_actor()` ?
+#
+# things to ensure!
+# -[ ] the ctxc raised in a child should ideally show the tb of the
+#     underlying `Cancelled` checkpoint, i.e.
+#     `raise scope_error from ctxc`?
+#
+# -[ ] a self-cancelled context, if not allowed to block on
+#     `ctx.result()` at some point will hang since the `ctx._scope`
+#     is never `.cancel_called`; cases for this include,
+#     - an `open_ctx()` which never starteds before being OoB actor
+#       cancelled.
+#       |_ parent task will be blocked in `.open_context()` for the
+#         `Started` msg, and when the OoB ctxc arrives `ctx._scope`
+#         will never have been signalled..
+
+#     '''
+#     ...
+
+# TODO, sanity test against the case in `/examples/trio/lockacquire_not_unmasked.py`
+# but with the `Lock.acquire()` from a `@context` to ensure the
+# implicit ignore-case-non-unmasking.
+#
+# @tractor.context
+# async def acquire_actor_global_lock(
+#     ctx: tractor.Context,
+#     ignore_special_cases: bool,
+# ):
+
+#     async with maybe_unmask_excs(
+#         ignore_special_cases=ignore_special_cases,
+#     ):
+#         await ctx.started('locked')
+
+#     # block til cancelled
+#     await trio.sleep_forever()
+
+
+@tractor.context
+async def sleep_forever(
+    ctx: tractor.Context,
+    # ignore_special_cases: bool,
+    do_started: bool,
+):
+
+    # async with maybe_unmask_excs(
+    #     ignore_special_cases=ignore_special_cases,
+    # ):
+    #     await ctx.started('locked')
+    if do_started:
+        await ctx.started()
+
+    # block til cancelled
+    print('sleepin on child-side..')
+    await trio.sleep_forever()
+
+
+def test_cancel_ctx_with_parent_side_entered_in_bg_task(
+    debug_mode: bool,
+    loglevel: str,
+    cancel_ctx: bool = False,
+):
+    '''
+    The most "basic" out-of-band-task self-cancellation case where
+    `Portal.open_context()` is entered in a bg task and the
+    parent-task (of the containing nursery) calls `Context.cancel()`
+    without the child knowing; the `Context._scope` should be
+    `.cancel_called` when the IPC ctx's child-side relays
+    a `ContextCancelled` with a `.canceller` set to the parent
+    actor('s task).
+
+    '''
+    async def main():
+        with trio.fail_after(
+            2 if not debug_mode else 999,
+        ):
+            an: ActorNursery
+            async with (
+                tractor.open_nursery(
+                    debug_mode=debug_mode,
+                    loglevel='devx',
+                    enable_stack_on_sig=True,
+                ) as an,
+                trio.open_nursery() as tn,
+            ):
+                ptl: Portal = await an.start_actor(
+                    'sub',
+                    enable_modules=[__name__],
+                )
+
+                async def _open_ctx_async(
+                    do_started: bool = True,
+                    task_status=trio.TASK_STATUS_IGNORED,
+                ):
+                    # do we expect to never enter the
+                    # `.open_context()` below.
+                    if not do_started:
+                        task_status.started()
+
+                    async with ptl.open_context(
+                        sleep_forever,
+                        do_started=do_started,
+                    ) as (ctx, first):
+                        task_status.started(ctx)
+                        await trio.sleep_forever()
+
+                # XXX, this is the key OoB part!
+                #
+                # - start the `.open_context()` in a bg task which
+                #   blocks inside the embedded scope-body,
+                #
+                # -  when we call `Context.cancel()` it **is
+                #   not** from the same task which eventually runs
+                #   `.__aexit__()`,
+                #
+                # - since the bg "opener" task will be in
+                #   a `trio.sleep_forever()`, it must be interrupted
+                #   by the `ContextCancelled` delivered from the
+                #   child-side; `Context._scope: CancelScope` MUST
+                #   be `.cancel_called`!
+                #
+                print('ASYNC opening IPC context in subtask..')
+                maybe_ctx: Context|None = await tn.start(partial(
+                    _open_ctx_async,
+                ))
+
+                if (
+                    maybe_ctx
+                    and
+                    cancel_ctx
+                ):
+                    print('cancelling first IPC ctx!')
+                    await maybe_ctx.cancel()
+
+                # XXX, note that despite `maybe_context.cancel()`
+                # being called above, it's the parent (bg) task
+                # which was originally never interrupted in
+                # the `ctx._scope` body due to missing case logic in
+                # `ctx._maybe_cancel_and_set_remote_error()`.
+                #
+                # It didn't matter that the subactor process was
+                # already terminated and reaped, nothing was
+                # cancelling the ctx-parent task's scope!
+                #
+                print('cancelling subactor!')
+                await ptl.cancel_actor()
+
+    trio.run(main)
+
+
+# def test_parent_actor_cancels_subactor_with_gt1_ctxs_open_to_it(
+#     debug_mode: bool,
+#     loglevel: str,
+# ):
+#     '''
+#     Demos OoB cancellation from the perspective of a ctx opened with
+#     a child subactor where the parent cancels the child at the "actor
+#     layer" using `Portal.cancel_actor()` and thus the
+#     `ContextCancelled.canceller` received by the ctx's parent-side
+#     task will appear to be a "self cancellation" even though that
+#     specific task itself was not cancelled and thus
+#     `Context.cancel_called ==False`.
+#     '''
+                # TODO, do we have an existing implied ctx
+                # cancel test like this?
+                # with trio.move_on_after(0.5):# as cs:
+                #     await _open_ctx_async(
+                #         do_started=False,
+                #     )
+
+
+                # in-line ctx scope should definitely raise
+                # a ctxc with `.canceller = 'root'`
+                # async with ptl.open_context(
+                #     sleep_forever,
+                #     do_started=True,
+                # ) as pair:
+