Store array `maxlen` in state singleton

The `collections.deque` takes care of array length truncation of values
for us implicitly but in the future we'll likely want this value exposed
to alternate array implementations. This patch is to provide for that as
well as make `mypy` happy since the `dequeu.maxlen` can also be `None`.

tractor/_broadcast.py

@@ -74,6 +74,7 @@ class BroadcastState:
 
     '''
     queue: deque
+    maxlen: int
 
     # map of underlying instance id keys to receiver instances which
     # must be provided as a singleton per broadcaster set.
@@ -81,7 +82,7 @@ class BroadcastState:
 
     # broadcast event to wake up all sleeping consumer tasks
     # on a newly produced value from the sender.
-    recv_ready: Optional[tuple[str, trio.Event]] = None
+    recv_ready: Optional[tuple[int, trio.Event]] = None
 
 
 class BroadcastReceiver(ReceiveChannel):
@@ -150,7 +151,7 @@ class BroadcastReceiver(ReceiveChannel):
                 # decrement to the last value and expect
                 # consumer to either handle the ``Lagged`` and come back
                 # or bail out on its own (thus un-subscribing)
-                state.subs[key] = state.queue.maxlen - 1
+                state.subs[key] = state.maxlen - 1
 
                 # this task was overrun by the producer side
                 task: Task = current_task()
@@ -174,7 +175,12 @@ class BroadcastReceiver(ReceiveChannel):
             # right?
             try:
                 value = await self._recv()
+
                 # items with lower indices are "newer"
+                # NOTE: ``collections.deque`` implicitly takes care of
+                # trucating values outside our ``state.maxlen``. In the
+                # alt-backend-array-case we'll need to make sure this is
+                # implemented in similar ringer-buffer-ish style.
                 state.queue.appendleft(value)
 
                 # broadcast new value to all subscribers by increasing
@@ -295,6 +301,7 @@ def broadcast_receiver(
         recv_chan,
         state=BroadcastState(
             queue=deque(maxlen=max_buffer_size),
+            maxlen=max_buffer_size,
             subs={},
         ),
         **kwargs,