Well then, I guess it just needed, a checkpoint XD

Here I was thinking the bcaster (usage) maybe required a rework but,
NOPE it's just bc a checkpoint was needed in the parent task owning the
`tn` which spawns `get_sub_and_pull()` tasks to ensure the bg allocated
`an`/portal is eventually cancel-called..

Ah well, at least i started a patch for `MsgStream.subscribe()` to make
it multicast revertible.. XD

Anyway, I tossed in some checks & notes related to all that unnecessary
effort since I do think i'll move forward implementing it:
- for the `cache_hit` case always verify that the `bcast` clone is
  unregistered from the common state subs after
  `.subscribe().__aexit__()`.
- do a light check that the implicit `MsgStream._broadcaster` is always
  the only bcrx instance left-leaked into that state.. that is until
  i get the proper de-allocation/reversion from multicast -> unicast
  working.
- put in mega detailed note about the required parent-task checkpoint.

tests/test_resource_cache.py

@@ -120,13 +120,13 @@ async def open_stream() -> Awaitable[
                     print('Entered open_stream() caller')
                     yield an, stream
                     print('Exited open_stream() caller')
-                    # await tractor.pause(shield=True)
 
             finally:
-                print('Cancelling streamer')
-                # await tractor.pause(shield=True)
-                with trio.CancelScope(shield=True):
-                    await portal.cancel_actor()
+                print(
+                    'Cancelling streamer with,\n'
+                    '=> `Portal.cancel_actor()`'
+                )
+                await portal.cancel_actor()
                 print('Cancelled streamer')
 
     except Exception as err:
@@ -147,6 +147,8 @@ async def maybe_open_stream(taskname: str):
         cache_hit,
         (an, stream)
     ):
+        # when the actor + portal + ctx + stream has already been
+        # allocated we want to just bcast to this task.
         if cache_hit:
             print(f'{taskname} loaded from cache')
 
@@ -160,6 +162,11 @@ async def maybe_open_stream(taskname: str):
                     f')>\n'
                     f' |_{taskname}\n'
                 )
+
+            # we should always unreg the "cloned" bcrc for this
+            # consumer-task
+            assert id(bstream) not in bstream._state.subs
+
         else:
             # yield the actual stream
             try:
@@ -171,15 +178,21 @@ async def maybe_open_stream(taskname: str):
                     f' |_{taskname}\n'
                 )
 
-        bstream = stream._broadcaster
-        if (
-            not bstream._state.subs
-            and
-            not cache_hit
-        ):
-            await tractor.pause(shield=True)
-            # await an.cancel()
-
+        first_bstream = stream._broadcaster
+        bcrx_state = first_bstream._state
+        subs: dict[int, int] = bcrx_state.subs
+        if len(subs) == 1:
+            assert id(first_bstream) in subs
+            # ^^TODO! the bcrx should always de-allocate all subs,
+            # including the implicit first one allocated on entry
+            # by the first subscribing peer task, no?
+            #
+            # -[ ] adjust `MsgStream.subscribe()` to do this mgmt!
+            #  |_ allows reverting `MsgStream.receive()` to the
+            #    non-bcaster method.
+            #  |_ we can decide whether to reset `._broadcaster`?
+            #
+            # await tractor.pause(shield=True)
 
 
 def test_open_local_sub_to_stream(
@@ -261,13 +274,25 @@ def test_open_local_sub_to_stream(
                         )
                         await trio.sleep(0.001)
 
-                print('all consumer tasks finished.')
+                print('all consumer tasks finished!')
 
-                # ensure actor-nursery is shutdown or we might
-                # hang here..?
-                # if root.ipc_server.has_peers():
-                #     await tractor.pause()
-                #     await an.cancel()
+                # ?XXX, ensure actor-nursery is shutdown or we might
+                # hang here due to a minor task deadlock/race-condition?
+                #
+                # - seems that all we need is a checkpoint to ensure
+                #   the last suspended task, which is inside
+                #   `.maybe_open_context()`, can do the
+                #   `Portal.cancel_actor()` call?
+                #
+                # - if that bg task isn't resumed, then this blocks
+                #   timeout might hit before that?
+                #
+                if root.ipc_server.has_peers():
+                    await trio.lowlevel.checkpoint()
+
+                    # alt approach, cancel the entire `an`
+                    # await tractor.pause()
+                    # await an.cancel()
 
             # end of runtime scope
             print('root actor terminated.')