Solve another OoB cancellation case, the bg task one

Such that we are able to (finally) detect when we should
`Context._scope.cancel()` specifically when the `.parent_task` is
**not** blocking on receiving from the underlying `._rx_chan`, since if
the task is blocking on `.receive()` it will call `.cancel()`
implicitly.

This is a lot to explain with very little code actually needed for the
implementation (are we like `trio` yet anyone?? XD) but the main jist is
that `Context._maybe_cancel_and_set_remote_error()` needed the
additional case of calling `._scope.cancel()` whenever we know that
a remote-error/ctxc won't be immediately handled, bc user code is doing
non `Context`-API things, and result in a similar outcome as if that
task was waiting on `Context.wait_for_result()` or `.__aexite__()`.

Impl details,
- add a new `._is_blocked_on_rx_chan()` method which predicates whether
  the (new) `.parent_task` is blocking on `._rx_chan.receive()`.
  * see various stipulations about the current impl and how we might
    need to adjust for the future given `trio`'s commitment to the
    `Task.custom_sleep_data` attr..
- add `.parent_task`, a pub wrapper for `._task`.
- check for `not ._is_blocked_on_rx_chan()` before manually cancelling
  the local `.parent_task`
- minimize the surrounding branch case expressions.

Other,
- tweak a couple logs.
- add a new `.cancel()` pre-started msg.
- mask the `.cancel_called` setter, it's only (been) used for tracing.
- todos around maybe moving the `._nursery` allocation "around" the
  `.start_remote_task()` call and various subsequent tweaks therein.

tractor/_context.py

@@ -442,25 +442,25 @@ class Context:
         '''
         Records whether cancellation has been requested for this context
         by a call to  `.cancel()` either due to,
-        - either an explicit call by some local task,
+        - an explicit call by some local task,
         - or an implicit call due to an error caught inside
-          the ``Portal.open_context()`` block.
+          the `Portal.open_context()` block.
 
         '''
         return self._cancel_called
 
-    @cancel_called.setter
+    # XXX, to debug who frickin sets it..
-    def cancel_called(self, val: bool) -> None:
+    # @cancel_called.setter
-        '''
+    # def cancel_called(self, val: bool) -> None:
-        Set the self-cancelled request `bool` value.
+    #     '''
+    #     Set the self-cancelled request `bool` value.
 
-        '''
+    #     '''
-        # to debug who frickin sets it..
     #     if val:
     #         from .devx import pause_from_sync
     #         pause_from_sync()
 
-        self._cancel_called = val
+    #     self._cancel_called = val
 
     @property
     def canceller(self) -> tuple[str, str]|None:
@@ -635,6 +635,71 @@ class Context:
         '''
         await self.chan.send(Stop(cid=self.cid))
 
+    @property
+    def parent_task(self) -> trio.Task:
+        '''
+        This IPC context's "owning task" which is a `trio.Task`
+        on one of the "sides" of the IPC.
+
+        Note that the "parent_" prefix here refers to the local
+        `trio` task tree using the same interface as
+        `trio.Nursery.parent_task` whereas for IPC contexts,
+        a different cross-actor task hierarchy exists:
+
+        - a "parent"-side which originally entered
+          `Portal.open_context()`,
+
+        - the "child"-side which was spawned and scheduled to invoke
+          a function decorated with `@tractor.context`.
+
+        This task is thus a handle to mem-domain-distinct/per-process
+        `Nursery.parent_task` depending on in which of the above
+        "sides" this context exists.
+
+        '''
+        return self._task
+
+    def _is_blocked_on_rx_chan(self) -> bool:
+        '''
+        Predicate to indicate whether the owner `._task: trio.Task` is
+        currently blocked (by `.receive()`-ing) on its underlying RPC
+        feeder `._rx_chan`.
+
+        This knowledge is highly useful when handling so called
+        "out-of-band" (OoB) cancellation conditions where a peer
+        actor's task transmitted some remote error/cancel-msg and we
+        must know whether to signal-via-cancel currently executing
+        "user-code" (user defined code embedded in `ctx._scope`) or
+        simply to forward the IPC-msg-as-error **without calling**
+        `._scope.cancel()`.
+
+        In the latter case it is presumed that if the owner task is
+        blocking for the next IPC msg, it will eventually receive,
+        process and raise the equivalent local error **without**
+        requiring `._scope.cancel()` to be explicitly called by the
+        *delivering OoB RPC-task* (via `_deliver_msg()`).
+
+        '''
+        # NOTE, see the mem-chan meth-impls for *why* this
+        # logic works,
+        # `trio._channel.MemoryReceiveChannel.receive[_nowait]()`
+        #
+        # XXX realize that this is NOT an
+        # official/will-be-loudly-deprecated API:
+        # - https://trio.readthedocs.io/en/stable/reference-lowlevel.html#trio.lowlevel.Task.custom_sleep_data
+        #  |_https://trio.readthedocs.io/en/stable/reference-lowlevel.html#trio.lowlevel.wait_task_rescheduled
+        #
+        # orig repo intro in the mem-chan change over patch:
+        # - https://github.com/python-trio/trio/pull/586#issuecomment-414039117
+        #  |_https://github.com/python-trio/trio/pull/616
+        #  |_https://github.com/njsmith/trio/commit/98c38cef6f62e731bf8c7190e8756976bface8f0
+        #
+        return (
+            self._task.custom_sleep_data
+            is
+            self._rx_chan
+        )
+
     def _maybe_cancel_and_set_remote_error(
         self,
         error: BaseException,
@@ -787,13 +852,27 @@ class Context:
         if self._canceller is None:
             log.error('Ctx has no canceller set!?')
 
+        cs: trio.CancelScope = self._scope
+
+        # ?TODO? see comment @ .start_remote_task()`
+        #
+        # if not cs:
+        #     from .devx import mk_pdb
+        #     mk_pdb().set_trace()
+        #     raise RuntimeError(
+        #         f'IPC ctx was not be opened prior to remote error delivery !?\n'
+        #         f'{self}\n'
+        #         f'\n'
+        #         f'`Portal.open_context()` must be entered (somewhere) beforehand!\n'
+        #     )
+
         # Cancel the local `._scope`, catch that
         # `._scope.cancelled_caught` and re-raise any remote error
         # once exiting (or manually calling `.wait_for_result()`) the
         # `.open_context()`  block.
-        cs: trio.CancelScope = self._scope
         if (
             cs
+            and not cs.cancel_called
 
             # XXX this is an expected cancel request response
             # message and we **don't need to raise it** in the
@@ -802,8 +881,7 @@ class Context:
             # if `._cancel_called` then `.cancel_acked and .cancel_called`
             # always should be set.
             and not self._is_self_cancelled()
-            and not cs.cancel_called
+            # and not cs.cancelled_caught
-            and not cs.cancelled_caught
         ):
             if (
                 msgerr
@@ -814,7 +892,7 @@ class Context:
                 not self._cancel_on_msgerr
             ):
                 message: str = (
-                    'NOT Cancelling `Context._scope` since,\n'
+                    f'NOT Cancelling `Context._scope` since,\n'
                     f'Context._cancel_on_msgerr = {self._cancel_on_msgerr}\n\n'
                     f'AND we got a msg-type-error!\n'
                     f'{error}\n'
@@ -824,13 +902,43 @@ class Context:
                 # `trio.Cancelled` subtype here ;)
                 # https://github.com/goodboy/tractor/issues/368
                 message: str = 'Cancelling `Context._scope` !\n\n'
-                # from .devx import pause_from_sync
+                cs.cancel()
-                # pause_from_sync()
+
-                self._scope.cancel()
+        # TODO, explicit condition for OoB (self-)cancellation?
-        else:
+        # - we called `Portal.cancel_actor()` from this actor
-            message: str = 'NOT cancelling `Context._scope` !\n\n'
+        #   and the peer ctx task delivered ctxc due to it.
+        # - currently `self._is_self_cancelled()` will be true
+        #   since the ctxc.canceller check will match us even though it
+        #   wasn't from this ctx specifically!
+        elif (
+            cs
+            and self._is_self_cancelled()
+            and not cs.cancel_called
+        ):
+            message: str = (
+                'Cancelling `ctx._scope` due to OoB self-cancel ?!\n'
+                '\n'
+            )
             # from .devx import mk_pdb
             # mk_pdb().set_trace()
+            # TODO XXX, required to fix timeout failure in
+            # `test_cancelled_lockacquire_in_ipctx_not_unmaskeed`
+            #
+
+            # XXX NOTE XXX, this is SUPER SUBTLE!
+            # we only want to cancel our embedded `._scope`
+            # if the ctx's current/using task is NOT blocked
+            # on `._rx_chan.receive()` and on some other
+            # `trio`-checkpoint since in the former case
+            # any `._remote_error` will be relayed through
+            # the rx-chan and appropriately raised by the owning
+            # `._task` directly. IF the owner task is however
+            # blocking elsewhere we need to interrupt it **now**.
+            if not self._is_blocked_on_rx_chan():
+                cs.cancel()
+        else:
+            # rx_stats = self._rx_chan.statistics()
+            message: str = 'NOT cancelling `Context._scope` !\n\n'
 
         fmt_str: str = 'No `self._scope: CancelScope` was set/used ?\n'
         if (
@@ -854,6 +962,7 @@ class Context:
                 +
                 cs_fmt
             )
+
         log.cancel(
             message
             +
@@ -946,8 +1055,9 @@ class Context:
 
         '''
         side: str = self.side
-        # XXX for debug via the `@.setter`
+        self._cancel_called = True
-        self.cancel_called = True
+        # ^ XXX for debug via the `@.setter`
+        # self.cancel_called = True
 
         header: str = (
             f'Cancelling ctx from {side!r}-side\n'
@@ -2011,6 +2121,9 @@ async def open_context_from_portal(
             f'|_{portal.actor}\n'
         )
 
+    # ?TODO? could we move this to inside the `tn` block?
+    # -> would allow doing `ctx.parent_task = tn.parent_task` ?
+    # -> would allow a `if not ._scope: => raise RTE` ?
     ctx: Context = await portal.actor.start_remote_task(
         portal.channel,
         nsf=nsf,
@@ -2037,6 +2150,7 @@ async def open_context_from_portal(
     scope_err: BaseException|None = None
     ctxc_from_child: ContextCancelled|None = None
     try:
+        # from .devx import pause
         async with (
             collapse_eg(),
             trio.open_nursery() as tn,
@@ -2059,6 +2173,10 @@ async def open_context_from_portal(
             # the dialog, the `Error` msg should be raised from the `msg`
             # handling block below.
             try:
+                log.runtime(
+                    f'IPC ctx parent waiting on Started msg..\n'
+                    f'ctx.cid: {ctx.cid!r}\n'
+                )
                 started_msg, first = await ctx._pld_rx.recv_msg(
                     ipc=ctx,
                     expect_msg=Started,
@@ -2067,16 +2185,16 @@ async def open_context_from_portal(
                 )
             except trio.Cancelled as taskc:
                 ctx_cs: trio.CancelScope = ctx._scope
+                log.cancel(
+                    f'IPC ctx was cancelled during "child" task sync due to\n\n'
+                    f'.cid: {ctx.cid!r}\n'
+                    f'.maybe_error: {ctx.maybe_error!r}\n'
+                )
+                # await pause(shield=True)
+
                 if not ctx_cs.cancel_called:
                     raise
 
-                # from .devx import pause
-                # await pause(shield=True)
-
-                log.cancel(
-                    'IPC ctx was cancelled during "child" task sync due to\n\n'
-                    f'{ctx.maybe_error}\n'
-                )
                 # OW if the ctx's scope was cancelled manually,
                 # likely the `Context` was cancelled via a call to
                 # `._maybe_cancel_and_set_remote_error()` so ensure
@@ -2272,13 +2390,16 @@ async def open_context_from_portal(
         match scope_err:
             case trio.Cancelled():
                 logmeth = log.cancel
+                cause: str = 'cancelled'
 
             # XXX explicitly report on any non-graceful-taskc cases
             case _:
+                cause: str = 'errored'
                 logmeth = log.exception
 
         logmeth(
-            f'ctx {ctx.side!r}-side exited with {ctx.repr_outcome()!r}\n'
+            f'ctx {ctx.side!r}-side {cause!r} with,\n'
+            f'{ctx.repr_outcome()!r}\n'
         )
 
         if debug_mode():
@@ -2303,6 +2424,7 @@ async def open_context_from_portal(
         # told us it's cancelled ;p
         if ctxc_from_child is None:
             try:
+                # await pause(shield=True)
                 await ctx.cancel()
             except (
                 trio.BrokenResourceError,
@@ -2459,8 +2581,10 @@ async def open_context_from_portal(
                 log.cancel(
                     f'Context cancelled by local {ctx.side!r}-side task\n'
                     f'c)>\n'
-                    f' |_{ctx._task}\n\n'
+                    f'  |_{ctx.parent_task}\n'
-                    f'{repr(scope_err)}\n'
+                    f'   .cid={ctx.cid!r}\n'
+                    f'\n'
+                    f'{scope_err!r}\n'
                 )
 
             # TODO: should we add a `._cancel_req_received`