Raise explicitly on missing `greenback` portal

When `.pause_from_sync()` is called from an `asyncio.Task` which was
never bestowed a portal we want to be mega pedantic about it; indicate
that the task was NOT spawned from our `.to_asyncio` API and likely by
some out-of-our-control code (normally using
`asyncio.ensure_future()/.create_task()`). Though `greenback` already
errors on such usage, it's not always clear why no portal exists;
explaining the situation of a 3rd-party-bg-spawned-task should avoid
dev confusion for most cases.

Impl deats,
- distinguish between an actor in infected mode versus the actual caller
  of `.pause_from_sync()` being an `asyncio.Task` with more explicit
  `asyncio_task` and `is_infected_aio` vars.
- ONLY in the case of being both an infected-mode-actor AND detecting
  that the caller is an `asyncio.Task`, check `greenback.has_portal()`
  such that when not bestowed we presume the aforementioned
  3rd-party-bg-task case above and raise a new explicit RTE with
  a detailed explanatory message.
- add some masked draft code for handling the speical case of a root
  actor `asyncio.Task` caller which could (in theory) not actually
  require gb portal use since the `Lock` can be acquired directly
  without IPC.
 |_this will likely require factoring of various pause machinery funcs
   into a `_pause_from_root_task()` to mk the impl sane XD

Other,
- expose a new `debug_filter: Callable` which can be provided by the
  caller of `_maybe_enter_pm()` to predicate whether to enter the
  debugger REPL based on the caught `BaseException|BaseExceptionGroup`;
  this is handy for customizing the meaning of "graceful cancellations"
  so as to avoid crash handling on expected egs of more then
  `trioCancelled`.
|_ make the default as it was implemented: `not is_multi_cancelled(err)`
- pass-through a new `ignore: set[BaseException]` as
  `open_crash_handler(ignore_nested=ignore)` to allow for the same
  silent-cancellation-egs-swallowing as desired from outside the actor
  runtime.

tractor/devx/_debug.py

@@ -75,6 +75,7 @@ from tractor import _state
 from tractor._exceptions import (
     InternalError,
     NoRuntime,
+    is_multi_cancelled,
 )
 from tractor._state import (
     current_actor,
@@ -1743,7 +1744,7 @@ async def _pause(
     ] = trio.TASK_STATUS_IGNORED,
     **debug_func_kwargs,
 
-) -> tuple[PdbREPL, Task]|None:
+) -> tuple[Task, PdbREPL]|None:
     '''
     Inner impl for `pause()` to avoid the `trio.CancelScope.__exit__()`
     stack frame when not shielded (since apparently i can't figure out
@@ -1929,7 +1930,7 @@ async def _pause(
                 )
                 with trio.CancelScope(shield=shield):
                     await trio.lowlevel.checkpoint()
-                return repl, task
+                return (repl, task)
 
             # elif repl_task:
             #     log.warning(
@@ -2530,26 +2531,17 @@ def pause_from_sync(
             f'{actor.uid} task called `tractor.pause_from_sync()`\n'
         )
 
-        # TODO: once supported, remove this AND the one
-        # inside `._pause()`!
-        # outstanding impl fixes:
-        # -[ ] need to make `.shield_sigint()` below work here!
-        # -[ ] how to handle `asyncio`'s new SIGINT-handler
-        #     injection?
-        # -[ ] should `breakpoint()` work and what does it normally
-        #     do in `asyncio` ctxs?
-        # if actor.is_infected_aio():
-        #     raise RuntimeError(
-        #         '`tractor.pause[_from_sync]()` not yet supported '
-        #         'for infected `asyncio` mode!'
-        #     )
-
         repl: PdbREPL = mk_pdb()
 
         # message += f'-> created local REPL {repl}\n'
         is_trio_thread: bool = DebugStatus.is_main_trio_thread()
         is_root: bool = is_root_process()
-        is_aio: bool = actor.is_infected_aio()
+        is_infected_aio: bool = actor.is_infected_aio()
+        thread: Thread = threading.current_thread()
+
+        asyncio_task: asyncio.Task|None = None
+        if is_infected_aio:
+            asyncio_task = asyncio.current_task()
 
         # TODO: we could also check for a non-`.to_thread` context
         # using `trio.from_thread.check_cancelled()` (says
@@ -2565,20 +2557,14 @@ def pause_from_sync(
         if (
             not is_trio_thread
             and
-            not is_aio  # see below for this usage
+            not asyncio_task
         ):
             # TODO: `threading.Lock()` this so we don't get races in
             # multi-thr cases where they're acquiring/releasing the
             # REPL and setting request/`Lock` state, etc..
-            thread: threading.Thread = threading.current_thread()
+            repl_owner: Thread = thread
-            repl_owner = thread
 
             # TODO: make root-actor bg thread usage work!
-            if (
-                is_root
-                # or
-                # is_aio
-            ):
             if is_root:
                 message += (
                     f'-> called from a root-actor bg {thread}\n'
@@ -2637,13 +2623,32 @@ def pause_from_sync(
                 DebugStatus.shield_sigint()
                 assert bg_task is not DebugStatus.repl_task
 
+        # TODO: once supported, remove this AND the one
+        # inside `._pause()`!
+        # outstanding impl fixes:
+        # -[ ] need to make `.shield_sigint()` below work here!
+        # -[ ] how to handle `asyncio`'s new SIGINT-handler
+        #     injection?
+        # -[ ] should `breakpoint()` work and what does it normally
+        #     do in `asyncio` ctxs?
+        # if actor.is_infected_aio():
+        #     raise RuntimeError(
+        #         '`tractor.pause[_from_sync]()` not yet supported '
+        #         'for infected `asyncio` mode!'
+        #     )
         elif (
             not is_trio_thread
             and
-            is_aio
+            is_infected_aio  # as in, the special actor-runtime mode
+            # ^NOTE XXX, that doesn't mean the caller is necessarily
+            # an `asyncio.Task` just that `trio` has been embedded on
+            # the `asyncio` event loop!
+            and
+            asyncio_task  # transitive caller is an actual `asyncio.Task`
         ):
             greenback: ModuleType = maybe_import_greenback()
-            repl_owner: Task = asyncio.current_task()
+
+            if greenback.has_portal():
                 DebugStatus.shield_sigint()
                 fute: asyncio.Future = run_trio_task_in_future(
                     partial(
@@ -2660,11 +2665,53 @@ def pause_from_sync(
                         **_pause_kwargs
                     )
                 )
-
+                repl_owner = asyncio_task
-            # TODO: for async version -> `.pause_from_aio()`?
-            # bg_task, _ = await fute
                 bg_task, _ = greenback.await_(fute)
-            bg_task: asyncio.Task = asyncio.current_task()
+                # TODO: ASYNC version -> `.pause_from_aio()`?
+                # bg_task, _ = await fute
+
+            # handle the case where an `asyncio` task has been
+            # spawned WITHOUT enabling a `greenback` portal..
+            # => can often happen in 3rd party libs.
+            else:
+                bg_task = repl_owner
+
+                # TODO, ostensibly we can just acquire the
+                # debug lock directly presuming we're the
+                # root actor running in infected asyncio
+                # mode?
+                #
+                # TODO, this would be a special case where
+                # a `_pause_from_root()` would come in very
+                # handy!
+                # if is_root:
+                #     import pdbp; pdbp.set_trace()
+                #     log.warning(
+                #         'Allowing `asyncio` task to acquire debug-lock in root-actor..\n'
+                #         'This is not fully implemented yet; there may be teardown hangs!\n\n'
+                #     )
+                # else:
+
+                # simply unsupported, since there exists no hack (i
+                # can think of) to workaround this in a subactor
+                # which needs to lock the root's REPL ow we're sure
+                # to get prompt stdstreams clobbering..
+                cf_repr: str = ''
+                if api_frame:
+                    caller_frame: FrameType = api_frame.f_back
+                    cf_repr: str = f'caller_frame: {caller_frame!r}\n'
+
+                raise RuntimeError(
+                    f"CAN'T USE `greenback._await()` without a portal !?\n\n"
+                    f'Likely this task was NOT spawned via the `tractor.to_asyncio` API..\n'
+                    f'{asyncio_task}\n'
+                    f'{cf_repr}\n'
+
+                    f'Prolly the task was started out-of-band (from some lib?)\n'
+                    f'AND one of the below was never called ??\n'
+                    f'- greenback.ensure_portal()\n'
+                    f'- greenback.bestow_portal(<task>)\n'
+                )
 
         else:  # we are presumably the `trio.run()` + main thread
             # raises on not-found by default
@@ -2915,8 +2962,14 @@ async def _maybe_enter_pm(
     tb: TracebackType|None = None,
     api_frame: FrameType|None = None,
     hide_tb: bool = False,
+
+    # only enter debugger REPL when returns `True`
+    debug_filter: Callable[
+        [BaseException|BaseExceptionGroup],
+        bool,
+    ] = lambda err: not is_multi_cancelled(err),
+
 ):
-    from tractor._exceptions import is_multi_cancelled
     if (
         debug_mode()
 
@@ -2933,7 +2986,8 @@ async def _maybe_enter_pm(
 
         # Really we just want to mostly avoid catching KBIs here so there
         # might be a simpler check we can do?
-        and not is_multi_cancelled(err)
+        and
+        debug_filter(err)
     ):
         api_frame: FrameType = api_frame or inspect.currentframe()
         tb: TracebackType = tb or sys.exc_info()[2]
@@ -3139,10 +3193,16 @@ def open_crash_handler(
     try:
         yield
     except tuple(catch) as err:
-        if type(err) not in ignore:
+        if (
-
+            type(err) not in ignore
-            # use our re-impl-ed version
+            and
+            not is_multi_cancelled(
+                err,
+                ignore_nested=ignore
+            )
+        ):
             try:
+                # use our re-impl-ed version
                 _post_mortem(
                     repl=mk_pdb(),
                     tb=sys.exc_info()[2],