Simplify api around receive channel

Buncha improvements:
- pass in the queue via constructor
- tracking over all underlying memory channel closure using cloning
- do it like `tokio` and set lagged consumers to the last sequence
  before raising
- copy the subs on first receiver wakeup for iteration instead of
  iterating the table directly (and being forced to skip the current
  tasks sequence increment)
- implement `.aclose()` to close the underlying clone for this task
- make `broadcast_receiver()` just take the recv chan since it doesn't
  need anything on the send side.

tractor/_live_from_tokio.py

@@ -1,25 +1,22 @@
 '''
-``tokio`` style broadcast channels.
+``tokio`` style broadcast channel.
+https://tokio-rs.github.io/tokio/doc/tokio/sync/broadcast/index.html
 
 '''
 from __future__ import annotations
-# from math import inf
 from itertools import cycle
 from collections import deque
-from contextlib import contextmanager  # , asynccontextmanager
+from contextlib import contextmanager
 from functools import partial
 from typing import Optional
 
 import trio
 import tractor
 from trio.lowlevel import current_task
-from trio.abc import ReceiveChannel  # , SendChannel
-# from trio._core import enable_ki_protection
+from trio.abc import ReceiveChannel
 from trio._core._run import Task
 from trio._channel import (
-    MemorySendChannel,
     MemoryReceiveChannel,
-    # MemoryChannelState,
 )
 
 
@@ -28,20 +25,25 @@ class Lagged(trio.TooSlowError):
 
 
 class BroadcastReceiver(ReceiveChannel):
-    '''This isn't Paris, not Berlin, nor Honk Kong..
+    '''A memory receive channel broadcaster which is non-lossy for the
+    fastest consumer.
+
+    Additional consumer tasks can receive all produced values by registering
+    with ``.subscribe()``.
 
     '''
     def __init__(
         self,
+
         rx_chan: MemoryReceiveChannel,
-        buffer_size: int = 100,
+        queue: deque,
 
     ) -> None:
 
         self._rx = rx_chan
-        self._len = buffer_size
-        self._queue = deque(maxlen=buffer_size)
-        self._subs = {id(current_task()): -1}
+        self._queue = queue
+        self._subs: dict[Task, int] = {}  # {id(current_task()): -1}
+        self._clones: dict[Task, MemoryReceiveChannel] = {}
         self._value_received: Optional[trio.Event] = None
 
     async def receive(self):
@@ -56,26 +58,30 @@ class BroadcastReceiver(ReceiveChannel):
         try:
             seq = self._subs[key]
         except KeyError:
-            self._subs.pop(key)
             raise RuntimeError(
                 f'Task {task.name} is not registerd as subscriber')
 
         if seq > -1:
             # get the oldest value we haven't received immediately
-
             try:
                 value = self._queue[seq]
             except IndexError:
+                # decrement to the last value and expect
+                # consumer to either handle the ``Lagged`` and come back
+                # or bail out on it's own (thus un-subscribing)
+                self._subs[key] = self._queue.maxlen - 1
+
+                # this task was overrun by the producer side
                 raise Lagged(f'Task {task.name} was overrun')
 
             self._subs[key] -= 1
             return value
 
         if self._value_received is None:
-            # we already have the latest value **and** are the first
-            # task to begin waiting for a new one
+            # current task already has the latest value **and** is the
+            # first task to begin waiting for a new one
 
-            # sanity checks with underlying chan ?
+            # what sanity checks might we use for the underlying chan ?
             # assert not self._rx._state.data
 
             event = self._value_received = trio.Event()
@@ -87,20 +93,15 @@ class BroadcastReceiver(ReceiveChannel):
             # broadcast new value to all subscribers by increasing
             # all sequence numbers that will point in the queue to
             # their latest available value.
-            for sub_key, seq in self._subs.items():
-
-                if key == sub_key:
-                    # we don't need to increase **this** task's
-                    # sequence number since we just consumed the latest
-                    # value
-                    continue
-
-                # # except TypeError:
-                # #     # already lagged
-                # #     seq = Lagged
 
+            subs = self._subs.copy()
+            # don't decerement the sequence # for this task since we
+            # already retreived the last value
+            subs.pop(key)
+            for sub_key, seq in subs.items():
                 self._subs[sub_key] += 1
 
+            # reset receiver waiter task event for next blocking condition
             self._value_received = None
             event.set()
             return value
@@ -109,7 +110,7 @@ class BroadcastReceiver(ReceiveChannel):
             await self._value_received.wait()
 
             seq = self._subs[key]
-            assert seq > -1, 'Uhhhh'
+            assert seq > -1, 'Internal error?'
 
             self._subs[key] -= 1
             return self._queue[0]
@@ -118,30 +119,37 @@ class BroadcastReceiver(ReceiveChannel):
     @contextmanager
     def subscribe(
         self,
-
     ) -> BroadcastReceiver:
         key = id(current_task())
         self._subs[key] = -1
+        # XXX: we only use this clone for closure tracking
+        clone = self._clones[key] = self._rx.clone()
         try:
             yield self
         finally:
             self._subs.pop(key)
+            clone.close()
 
+    # TODO: do we need anything here?
+    # if we're the last sub to close then close
+    # the underlying rx channel, but couldn't we just
+    # use ``.clone()``s trackign then?
     async def aclose(self) -> None:
-        # TODO: wtf should we do here?
-        # if we're the last sub to close then close
-        # the underlying rx channel
-        pass
+        key = id(current_task())
+        await self._clones[key].aclose()
 
 
-def broadcast_channel(
+def broadcast_receiver(
 
+    recv_chan: MemoryReceiveChannel,
     max_buffer_size: int,
 
-) -> (MemorySendChannel, BroadcastReceiver):
+) -> BroadcastReceiver:
 
-    tx, rx = trio.open_memory_channel(max_buffer_size)
-    return tx, BroadcastReceiver(rx)
+    return BroadcastReceiver(
+        recv_chan,
+        queue=deque(maxlen=max_buffer_size),
+    )
 
 
 if __name__ == '__main__':
@@ -153,7 +161,9 @@ if __name__ == '__main__':
             # loglevel='info',
         ):
 
-            tx, rx = broadcast_channel(100)
+            size = 100
+            tx, rx = trio.open_memory_channel(size)
+            rx = broadcast_receiver(rx, size)
 
             async def sub_and_print(
                 delay: float,