Toss in masked `.set_trace()` for unshielded `.pause()` debug

Such that we handle them despite a cancellation condition. This is
almost always the case, that `root_tn.cancel_scope.cancel_called` is
set, by the time the `debug._maybe_enter_pm()` hits. Previous I guess we
just weren't actually ever REPL-debugging such cases? Still needs
a test obvi.

tractor/_root.py

@@ -478,7 +478,10 @@ async def open_root_actor(
 
             # start runtime in a bg sub-task, yield to caller.
             async with (
-                collapse_eg(),
+                collapse_eg(
+                    bp=True,
+                    hide_tb=False,
+                ),
                 trio.open_nursery() as root_tn,
 
                 # XXX, finally-footgun below?
@@ -523,6 +526,12 @@ async def open_root_actor(
                         err,
                         api_frame=inspect.currentframe(),
                         debug_filter=debug_filter,
+
+                        # XXX NOTE, required to debug root-actor
+                        # crashes under cancellation conditions; so
+                        # most of them!
+                        shield=root_tn.cancel_scope.cancel_called,
+                        # ^TODO? write a (debugger) test for this ya?
                     )
 
                     if (

tractor/_supervise.py

@@ -468,7 +468,11 @@ async def _open_and_supervise_one_cancels_all_nursery(
 
     # errors from this daemon actor nursery bubble up to caller
     async with (
-        collapse_eg(),
+        # collapse_eg(),
+        collapse_eg(
+            bp=True,
+            hide_tb=False,
+        ),
         trio.open_nursery() as da_nursery,
     ):
         try:
@@ -481,7 +485,11 @@ async def _open_and_supervise_one_cancels_all_nursery(
             # As such if the strategy propagates any error(s) upwards
             # the above "daemon actor" nursery will be notified.
             async with (
-                collapse_eg(),
+                # collapse_eg(),
+                collapse_eg(
+                    bp=True,
+                    hide_tb=False,
+                ),
                 trio.open_nursery() as ria_nursery,
             ):
                 an = ActorNursery(
@@ -621,11 +629,17 @@ async def _open_and_supervise_one_cancels_all_nursery(
 
                 # use `BaseExceptionGroup` as needed
                 if len(errors) > 1:
-                    raise BaseExceptionGroup(
+                    beg = BaseExceptionGroup(
                         'tractor.ActorNursery errored with',
                         tuple(errors.values()),
                     )
+                    beg.add_note(
+                        'This beg was created from an actor-nursery!\n'
+                    )
+                    await debug.pause(shield=True)
+                    raise beg
                 else:
+                    # await debug.pause(shield=True)
                     raise list(errors.values())[0]
 
             # show frame on any (likely) internal error
@@ -683,16 +697,30 @@ async def open_nursery(
             # mark us for teardown on exit
             implicit_runtime: bool = True
 
-            async with open_root_actor(
-                hide_tb=hide_tb,
-                **kwargs,
-            ) as actor:
+            async with (
+                # collapse_eg(),
+                collapse_eg(
+                    bp=True,
+                    hide_tb=False,
+                ),
+                open_root_actor(
+                    hide_tb=hide_tb,
+                    **kwargs,
+                ) as actor
+            ):
                 assert actor is current_actor()
 
                 try:
-                    async with _open_and_supervise_one_cancels_all_nursery(
-                        actor
-                    ) as an:
+                    async with (
+                        # collapse_eg(),
+                        collapse_eg(
+                            bp=True,
+                            hide_tb=False,
+                        ),
+                        _open_and_supervise_one_cancels_all_nursery(
+                            actor
+                        ) as an,
+                    ):
 
                         # NOTE: mark this nursery as having
                         # implicitly started the root actor so

tractor/devx/debug/_trace.py

@@ -561,6 +561,9 @@ async def _pause(
             return
 
         elif isinstance(pause_err, trio.Cancelled):
+            __tracebackhide__: bool = False
+            # XXX, unmask to REPL it.
+            # mk_pdb().set_trace(frame=inspect.currentframe())
             _repl_fail_report += (
                 'You called `tractor.pause()` from an already cancelled scope!\n\n'
                 'Consider `await tractor.pause(shield=True)` to make it work B)\n'