Try out always delivering `ContextCancelled`

Previously, on a task cancel request there was no real response other
then the `None` returned from the `Actor._cancel_task()` method and
sometimes this might get lost if the cancel task was cancelled by
a runtime cancel request (i.e. an "actor cancel"). Instead let's try
always checking if the task's cancel scope is cancelled and if so relay
back to the caller a `ContextCancelled` which can then be explicitly
handled by actor nursery machinery as well as individual cancel APIs
(`Portal.cancel_actor()`, and maybe later if we decide to expose the
`tractor.Context` on every `Portal.run()` call).

Also,
- fix up a bunch of cancellation related logging
- add an `Actor.cancel_called` flag much like `trio`'s cancel scope

tractor/_actor.py

@@ -194,7 +194,17 @@ async def _invoke(
                 task_status.started(cs)
                 await chan.send({'return': await coro, 'cid': cid})
 
-    except (Exception, trio.MultiError) as err:
+            if cs.cancelled_caught:
+                raise ContextCancelled(
+                    'cancelled',
+                    suberror_type=trio.Cancelled,
+                )
+
+    except (
+        Exception,
+        trio.MultiError,
+        # trio.Cancelled,
+    ) as err:
 
         if not is_multi_cancelled(err):
 
@@ -248,7 +258,7 @@ async def _invoke(
                 # If we're cancelled before the task returns then the
                 # cancel scope will not have been inserted yet
                 log.warning(
-                    f"Task {func} likely errored or cancelled before it started")
+                    f"Task {func} likely errored or cancelled before start")
         finally:
             if not actor._rpc_tasks:
                 log.runtime("All RPC tasks have completed")
@@ -358,6 +368,14 @@ class Actor:
             Tuple[Any, Any, Any, Any, Any]] = None
         self._actoruid2nursery: Dict[str, 'ActorNursery'] = {}  # type: ignore  # noqa
 
+    @property
+    def cancel_called(self) -> bool:
+        '''
+        Same principle as ``trio.CancelScope.cancel_called``.
+
+        '''
+        return self._cancel_called
+
     async def wait_for_peer(
         self, uid: Tuple[str, str]
     ) -> Tuple[trio.Event, Channel]:
@@ -618,6 +636,7 @@ class Actor:
         # worked out we'll likely want to use that!
         msg = None
         nursery_cancelled_before_task: bool = False
+        uid = chan.uid
 
         log.runtime(f"Entering msg loop for {chan} from {chan.uid}")
         try:
@@ -641,7 +660,7 @@ class Actor:
 
                         log.runtime(
                                 f"Msg loop signalled to terminate for"
-                                f" {chan} from {chan.uid}")
+                                f" {chan} from {uid}")
 
                         break
 
@@ -676,38 +695,46 @@ class Actor:
                         f"{ns}.{funcname}({kwargs})")
                     if ns == 'self':
                         func = getattr(self, funcname)
-                        if funcname == 'cancel':
 
-                            # don't start entire actor runtime cancellation if this
-                            # actor is in debug mode
+                        if funcname == 'cancel':
+                            # self.cancel() was called so kill this
+                            # msg loop and break out into
+                            # ``_async_main()``
+
+                            log.cancel(
+                                f"{self.uid} remote cancel msg from {uid}")
+
+                            # don't start entire actor runtime
+                            # cancellation if this actor is in debug
+                            # mode
                             pdb_complete = _debug._local_pdb_complete
                             if pdb_complete:
+                                log.cancel(
+                                    f'{self.uid} is in debug, wait for unlock')
                                 await pdb_complete.wait()
 
-                            # we immediately start the runtime machinery shutdown
+                            # we immediately start the runtime machinery
+                            # shutdown
                             with trio.CancelScope(shield=True):
-                                # self.cancel() was called so kill this msg loop
-                                # and break out into ``_async_main()``
-                                log.cancel(
-                                    f"Actor {self.uid} was remotely cancelled; "
-                                    "waiting on cancellation completion..")
-                                await _invoke(self, cid, chan, func, kwargs, is_rpc=False)
-                                # await self._cancel_complete.wait()
+                                await _invoke(
+                                    self, cid, chan, func, kwargs, is_rpc=False
+                                )
 
                             loop_cs.cancel()
-                            break
+                            continue
 
                         if funcname == '_cancel_task':
-
-                            # we immediately start the runtime machinery shutdown
-                            with trio.CancelScope(shield=True):
-                                # self.cancel() was called so kill this msg loop
-                                # and break out into ``_async_main()``
-                                kwargs['chan'] = chan
+                            task_cid = kwargs['cid']
                             log.cancel(
-                                    f"Actor {self.uid} was remotely cancelled; "
-                                    "waiting on cancellation completion..")
-                                await _invoke(self, cid, chan, func, kwargs, is_rpc=False)
+                                f'Actor {uid} requests cancel for {task_cid}')
+
+                            # we immediately start the runtime machinery
+                            # shutdown
+                            with trio.CancelScope(shield=True):
+                                kwargs['chan'] = chan
+                                await _invoke(
+                                    self, cid, chan, func, kwargs, is_rpc=False
+                                )
                                 continue
                     else:
                         # complain to client about restricted modules
@@ -752,7 +779,8 @@ class Actor:
                     log.runtime(
                         f"Waiting on next msg for {chan} from {chan.uid}")
 
-                # end of async for, channel disconnect vis ``trio.EndOfChannel``
+                # end of async for, channel disconnect vis
+                # ``trio.EndOfChannel``
                 log.runtime(
                     f"{chan} for {chan.uid} disconnected, cancelling tasks"
                 )
@@ -1193,7 +1221,10 @@ class Actor:
         tasks = self._rpc_tasks
         if tasks:
             log.cancel(f"Cancelling all {len(tasks)} rpc tasks:\n{tasks} ")
-            for (chan, cid), (scope, func, is_complete) in tasks.copy().items():
+            for (
+                (chan, cid),
+                (scope, func, is_complete),
+            ) in tasks.copy().items():
                 if only_chan is not None:
                     if only_chan != chan:
                         continue
@@ -1202,6 +1233,7 @@ class Actor:
                 if func != self._cancel_task:
                     await self._cancel_task(cid, chan)
 
+            if tasks:
                 log.cancel(
                     f"Waiting for remaining rpc tasks to complete {tasks}")
             await self._ongoing_rpc_tasks.wait()