Merge pull request #229 from goodboy/live_on_air_from_tokio

`tokio`-style broadcast channels

newsfragments/229.feature.rst

@@ -0,0 +1,12 @@
+Add `tokio-style broadcast channels
+<https://docs.rs/tokio/1.11.0/tokio/sync/broadcast/index.html>`_ as
+a solution for `#204 <https://github.com/goodboy/tractor/pull/204>`_ and
+discussed thoroughly in `trio/#987
+<https://github.com/python-trio/trio/issues/987>`_.
+
+This gives us local task broadcast functionality using a new
+``BroadcastReceiver`` type which can wrap ``trio.ReceiveChannel``  and
+provide fan-out copies of a stream of data to every subscribed consumer.
+We use this new machinery to provide a ``ReceiveMsgStream.subscribe()``
+async context manager which can be used by actor-local concumers tasks
+to easily pull from a shared and dynamic IPC stream.

tests/test_task_broadcasting.py

@@ -0,0 +1,459 @@
+"""
+Broadcast channels for fan-out to local tasks.
+"""
+from contextlib import asynccontextmanager
+from functools import partial
+from itertools import cycle
+import time
+from typing import Optional, List, Tuple
+
+import pytest
+import trio
+from trio.lowlevel import current_task
+import tractor
+from tractor._broadcast import broadcast_receiver, Lagged
+
+
+@tractor.context
+async def echo_sequences(
+
+    ctx:  tractor.Context,
+
+) -> None:
+    '''Bidir streaming endpoint which will stream
+    back any sequence it is sent item-wise.
+
+    '''
+    await ctx.started()
+
+    async with ctx.open_stream() as stream:
+        async for sequence in stream:
+            seq = list(sequence)
+            for value in seq:
+                await stream.send(value)
+                print(f'producer sent {value}')
+
+
+async def ensure_sequence(
+
+    stream: tractor.ReceiveMsgStream,
+    sequence: list,
+    delay: Optional[float] = None,
+
+) -> None:
+
+    name = current_task().name
+    async with stream.subscribe() as bcaster:
+        assert not isinstance(bcaster, type(stream))
+        async for value in bcaster:
+            print(f'{name} rx: {value}')
+            assert value == sequence[0]
+            sequence.remove(value)
+
+            if delay:
+                await trio.sleep(delay)
+
+            if not sequence:
+                # fully consumed
+                break
+
+
+@asynccontextmanager
+async def open_sequence_streamer(
+
+    sequence: List[int],
+    arb_addr: Tuple[str, int],
+    start_method: str,
+
+) -> tractor.MsgStream:
+
+    async with tractor.open_nursery(
+        arbiter_addr=arb_addr,
+        start_method=start_method,
+    ) as tn:
+
+        portal = await tn.start_actor(
+            'sequence_echoer',
+            enable_modules=[__name__],
+        )
+
+        async with portal.open_context(
+            echo_sequences,
+        ) as (ctx, first):
+
+            assert first is None
+            async with ctx.open_stream() as stream:
+                yield stream
+
+        await portal.cancel_actor()
+
+
+def test_stream_fan_out_to_local_subscriptions(
+    arb_addr,
+    start_method,
+):
+
+    sequence = list(range(1000))
+
+    async def main():
+
+        async with open_sequence_streamer(
+            sequence,
+            arb_addr,
+            start_method,
+        ) as stream:
+
+            async with trio.open_nursery() as n:
+                for i in range(10):
+                    n.start_soon(
+                        ensure_sequence,
+                        stream,
+                        sequence.copy(),
+                        name=f'consumer_{i}',
+                    )
+
+                await stream.send(tuple(sequence))
+
+                async for value in stream:
+                    print(f'source stream rx: {value}')
+                    assert value == sequence[0]
+                    sequence.remove(value)
+
+                    if not sequence:
+                        # fully consumed
+                        break
+
+    trio.run(main)
+
+
+@pytest.mark.parametrize(
+    'task_delays',
+    [
+        (0.01, 0.001),
+        (0.001, 0.01),
+    ]
+)
+def test_consumer_and_parent_maybe_lag(
+    arb_addr,
+    start_method,
+    task_delays,
+):
+
+    async def main():
+
+        sequence = list(range(300))
+        parent_delay, sub_delay = task_delays
+
+        async with open_sequence_streamer(
+            sequence,
+            arb_addr,
+            start_method,
+        ) as stream:
+
+            try:
+                async with trio.open_nursery() as n:
+
+                    n.start_soon(
+                        ensure_sequence,
+                        stream,
+                        sequence.copy(),
+                        sub_delay,
+                        name='consumer_task',
+                    )
+
+                    await stream.send(tuple(sequence))
+
+                    # async for value in stream:
+                    lagged = False
+                    lag_count = 0
+
+                    while True:
+                        try:
+                            value = await stream.receive()
+                            print(f'source stream rx: {value}')
+
+                            if lagged:
+                                # re set the sequence starting at our last
+                                # value
+                                sequence = sequence[sequence.index(value) + 1:]
+                            else:
+                                assert value == sequence[0]
+                                sequence.remove(value)
+
+                            lagged = False
+
+                        except Lagged:
+                            lagged = True
+                            print(f'source stream lagged after {value}')
+                            lag_count += 1
+                            continue
+
+                        # lag the parent
+                        await trio.sleep(parent_delay)
+
+                        if not sequence:
+                            # fully consumed
+                            break
+                    print(f'parent + source stream lagged: {lag_count}')
+
+                    if parent_delay > sub_delay:
+                        assert lag_count > 0
+
+            except Lagged:
+                # child was lagged
+                assert parent_delay < sub_delay
+
+    trio.run(main)
+
+
+def test_faster_task_to_recv_is_cancelled_by_slower(
+    arb_addr,
+    start_method,
+):
+    '''Ensure that if a faster task consuming from a stream is cancelled
+    the slower task can continue to receive all expected values.
+
+    '''
+    async def main():
+
+        sequence = list(range(1000))
+
+        async with open_sequence_streamer(
+            sequence,
+            arb_addr,
+            start_method,
+
+        ) as stream:
+
+            async with trio.open_nursery() as n:
+                n.start_soon(
+                    ensure_sequence,
+                    stream,
+                    sequence.copy(),
+                    0,
+                    name='consumer_task',
+                )
+
+                await stream.send(tuple(sequence))
+
+                # pull 3 values, cancel the subtask, then
+                # expect to be able to pull all values still
+                for i in range(20):
+                    try:
+                        value = await stream.receive()
+                        print(f'source stream rx: {value}')
+                        await trio.sleep(0.01)
+                    except Lagged:
+                        print(f'parent overrun after {value}')
+                        continue
+
+                print('cancelling faster subtask')
+                n.cancel_scope.cancel()
+
+            try:
+                value = await stream.receive()
+                print(f'source stream after cancel: {value}')
+            except Lagged:
+                print(f'parent overrun after {value}')
+
+            # expect to see all remaining values
+            with trio.fail_after(0.5):
+                async for value in stream:
+                    assert stream._broadcaster._state.recv_ready is None
+                    print(f'source stream rx: {value}')
+                    if value == 999:
+                        # fully consumed and we missed no values once
+                        # the faster subtask was cancelled
+                        break
+
+                # await tractor.breakpoint()
+                # await stream.receive()
+                print(f'final value: {value}')
+
+    trio.run(main)
+
+
+def test_subscribe_errors_after_close():
+
+    async def main():
+
+        size = 1
+        tx, rx = trio.open_memory_channel(size)
+        async with broadcast_receiver(rx, size) as brx:
+            pass
+
+        try:
+            # open and close
+            async with brx.subscribe():
+                pass
+
+        except trio.ClosedResourceError:
+            assert brx.key not in brx._state.subs
+
+        else:
+            assert 0
+
+    trio.run(main)
+
+
+def test_ensure_slow_consumers_lag_out(
+    arb_addr,
+    start_method,
+):
+    '''This is a pure local task test; no tractor
+    machinery is really required.
+
+    '''
+    async def main():
+
+        # make sure it all works within the runtime
+        async with tractor.open_root_actor():
+
+            num_laggers = 4
+            laggers: dict[str, int] = {}
+            retries = 3
+            size = 100
+            tx, rx = trio.open_memory_channel(size)
+            brx = broadcast_receiver(rx, size)
+
+            async def sub_and_print(
+                delay: float,
+            ) -> None:
+
+                task = current_task()
+                start = time.time()
+
+                async with brx.subscribe() as lbrx:
+                    while True:
+                        print(f'{task.name}: starting consume loop')
+                        try:
+                            async for value in lbrx:
+                                print(f'{task.name}: {value}')
+                                await trio.sleep(delay)
+
+                            if task.name == 'sub_1':
+                                # the non-lagger got
+                                # a ``trio.EndOfChannel``
+                                # because the ``tx`` below was closed
+                                assert len(lbrx._state.subs) == 1
+
+                                await lbrx.aclose()
+
+                                assert len(lbrx._state.subs) == 0
+
+                        except trio.ClosedResourceError:
+                            # only the fast sub will try to re-enter
+                            # iteration on the now closed bcaster
+                            assert task.name == 'sub_1'
+                            return
+
+                        except Lagged:
+                            lag_time = time.time() - start
+                            lags = laggers[task.name]
+                            print(
+                                f'restarting slow task {task.name} '
+                                f'that bailed out on {lags}:{value} '
+                                f'after {lag_time:.3f}')
+                            if lags <= retries:
+                                laggers[task.name] += 1
+                                continue
+                            else:
+                                print(
+                                    f'{task.name} was too slow and terminated '
+                                    f'on {lags}:{value}')
+                                return
+
+            async with trio.open_nursery() as nursery:
+
+                for i in range(1, num_laggers):
+
+                    task_name = f'sub_{i}'
+                    laggers[task_name] = 0
+                    nursery.start_soon(
+                        partial(
+                            sub_and_print,
+                            delay=i*0.001,
+                        ),
+                        name=task_name,
+                    )
+
+                # allow subs to sched
+                await trio.sleep(0.1)
+
+                async with tx:
+                    for i in cycle(range(size)):
+                        await tx.send(i)
+                        if len(brx._state.subs) == 2:
+                            # only one, the non lagger, sub is left
+                            break
+
+                # the non-lagger
+                assert laggers.pop('sub_1') == 0
+
+                for n, v in laggers.items():
+                    assert v == 4
+
+                assert tx._closed
+                assert not tx._state.open_send_channels
+
+                # check that "first" bcaster that we created
+                # above, never wass iterated and is thus overrun
+                try:
+                    await brx.receive()
+                except Lagged:
+                    # expect tokio style index truncation
+                    seq = brx._state.subs[brx.key]
+                    assert seq == len(brx._state.queue) - 1
+
+                # all backpressured entries in the underlying
+                # channel should have been copied into the caster
+                # queue trailing-window
+                async for i in rx:
+                    print(f'bped: {i}')
+                    assert i in brx._state.queue
+
+                # should be noop
+                await brx.aclose()
+
+    trio.run(main)
+
+
+def test_first_recver_is_cancelled():
+
+    async def main():
+
+        # make sure it all works within the runtime
+        async with tractor.open_root_actor():
+
+            tx, rx = trio.open_memory_channel(1)
+            brx = broadcast_receiver(rx, 1)
+            cs = trio.CancelScope()
+            sequence = list(range(3))
+
+            async def sub_and_recv():
+                with cs:
+                    async with brx.subscribe() as bc:
+                        async for value in bc:
+                            print(value)
+
+            async def cancel_and_send():
+                await trio.sleep(0.2)
+                cs.cancel()
+                await tx.send(1)
+
+            async with trio.open_nursery() as n:
+
+                n.start_soon(sub_and_recv)
+                await trio.sleep(0.1)
+                assert brx._state.recv_ready
+
+                n.start_soon(cancel_and_send)
+
+                # ensure that we don't hang because no-task is now
+                # waiting on the underlying receive..
+                with trio.fail_after(0.5):
+                    value = await brx.receive()
+                    print(f'parent: {value}')
+                    assert value == 1
+
+    trio.run(main)

tractor/_actor.py

@@ -567,7 +567,7 @@ class Actor:
         try:
             send_chan, recv_chan = self._cids2qs[(actorid, cid)]
         except KeyError:
-            send_chan, recv_chan = trio.open_memory_channel(1000)
+            send_chan, recv_chan = trio.open_memory_channel(2**6)
             send_chan.cid = cid  # type: ignore
             recv_chan.cid = cid  # type: ignore
             self._cids2qs[(actorid, cid)] = send_chan, recv_chan

tractor/_broadcast.py

@@ -0,0 +1,315 @@
+'''
+``tokio`` style broadcast channel.
+https://docs.rs/tokio/1.11.0/tokio/sync/broadcast/index.html
+
+'''
+from __future__ import annotations
+from abc import abstractmethod
+from collections import deque
+from contextlib import asynccontextmanager
+from dataclasses import dataclass
+from functools import partial
+from operator import ne
+from typing import Optional, Callable, Awaitable, Any, AsyncIterator, Protocol
+from typing import Generic, TypeVar
+
+import trio
+from trio._core._run import Task
+from trio.abc import ReceiveChannel
+from trio.lowlevel import current_task
+
+
+# A regular invariant generic type
+T = TypeVar("T")
+
+# covariant because AsyncReceiver[Derived] can be passed to someone
+# expecting AsyncReceiver[Base])
+ReceiveType = TypeVar("ReceiveType", covariant=True)
+
+
+class AsyncReceiver(
+    Protocol,
+    Generic[ReceiveType],
+):
+    '''An async receivable duck-type that quacks much like trio's
+    ``trio.abc.ReceieveChannel``.
+
+    '''
+    @abstractmethod
+    async def receive(self) -> ReceiveType:
+        ...
+
+    @abstractmethod
+    def __aiter__(self) -> AsyncIterator[ReceiveType]:
+        ...
+
+    @abstractmethod
+    async def __anext__(self) -> ReceiveType:
+        ...
+
+    # ``trio.abc.AsyncResource`` methods
+    @abstractmethod
+    async def aclose(self):
+        ...
+
+    @abstractmethod
+    async def __aenter__(self) -> AsyncReceiver[ReceiveType]:
+        ...
+
+    @abstractmethod
+    async def __aexit__(self, *args) -> None:
+        ...
+
+
+class Lagged(trio.TooSlowError):
+    '''Subscribed consumer task was too slow and was overrun
+    by the fastest consumer-producer pair.
+
+    '''
+
+
+@dataclass
+class BroadcastState:
+    '''Common state to all receivers of a broadcast.
+
+    '''
+    queue: deque
+    maxlen: int
+
+    # map of underlying instance id keys to receiver instances which
+    # must be provided as a singleton per broadcaster set.
+    subs: dict[int, int]
+
+    # broadcast event to wake up all sleeping consumer tasks
+    # on a newly produced value from the sender.
+    recv_ready: Optional[tuple[int, trio.Event]] = None
+
+
+class BroadcastReceiver(ReceiveChannel):
+    '''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()`` and receiving from the new instance it delivers.
+
+    '''
+    def __init__(
+        self,
+
+        rx_chan: AsyncReceiver,
+        state: BroadcastState,
+        receive_afunc: Optional[Callable[[], Awaitable[Any]]] = None,
+
+    ) -> None:
+
+        # register the original underlying (clone)
+        self.key = id(self)
+        self._state = state
+        state.subs[self.key] = -1
+
+        # underlying for this receiver
+        self._rx = rx_chan
+        self._recv = receive_afunc or rx_chan.receive
+        self._closed: bool = False
+
+    async def receive(self) -> ReceiveType:
+
+        key = self.key
+        state = self._state
+
+        # TODO: ideally we can make some way to "lock out" the
+        # underlying receive channel in some way such that if some task
+        # tries to pull from it directly (i.e. one we're unaware of)
+        # then it errors out.
+
+        # only tasks which have entered ``.subscribe()`` can
+        # receive on this broadcaster.
+        try:
+            seq = state.subs[key]
+        except KeyError:
+            if self._closed:
+                raise trio.ClosedResourceError
+
+            raise RuntimeError(
+                f'{self} is not registerd as subscriber')
+
+        # check that task does not already have a value it can receive
+        # immediately and/or that it has lagged.
+        if seq > -1:
+            # get the oldest value we haven't received immediately
+            try:
+                value = state.queue[seq]
+            except IndexError:
+
+                # adhere to ``tokio`` style "lagging":
+                # "Once RecvError::Lagged is returned, the lagging
+                # receiver's position is updated to the oldest value
+                # contained by the channel. The next call to recv will
+                # return this value."
+                # https://docs.rs/tokio/1.11.0/tokio/sync/broadcast/index.html#lagging
+
+                # 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.maxlen - 1
+
+                # this task was overrun by the producer side
+                task: Task = current_task()
+                raise Lagged(f'Task {task.name} was overrun')
+
+            state.subs[key] -= 1
+            return value
+
+        # current task already has the latest value **and** is the
+        # first task to begin waiting for a new one
+        if state.recv_ready is None:
+
+            if self._closed:
+                raise trio.ClosedResourceError
+
+            event = trio.Event()
+            state.recv_ready = key, event
+
+            # if we're cancelled here it should be
+            # fine to bail without affecting any other consumers
+            # 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
+                # all sequence numbers that will point in the queue to
+                # their latest available value.
+
+                # don't decrement the sequence for this task since we
+                # already retreived the last value
+
+                # XXX: which of these impls is fastest?
+
+                # subs = state.subs.copy()
+                # subs.pop(key)
+
+                for sub_key in filter(
+                    # lambda k: k != key, state.subs,
+                    partial(ne, key), state.subs,
+                ):
+                    state.subs[sub_key] += 1
+
+                # NOTE: this should ONLY be set if the above task was *NOT*
+                # cancelled on the `._recv()` call.
+                event.set()
+                return value
+
+            except trio.Cancelled:
+                # handle cancelled specially otherwise sibling
+                # consumers will be awoken with a sequence of -1
+                # state.recv_ready = trio.Cancelled
+                if event.statistics().tasks_waiting:
+                    event.set()
+                raise
+
+            finally:
+
+                # Reset receiver waiter task event for next blocking condition.
+                # this MUST be reset even if the above ``.recv()`` call
+                # was cancelled to avoid the next consumer from blocking on
+                # an event that won't be set!
+                state.recv_ready = None
+
+        # This task is all caught up and ready to receive the latest
+        # value, so queue sched it on the internal event.
+        else:
+            seq = state.subs[key]
+            assert seq == -1  # sanity
+            _, ev = state.recv_ready
+            await ev.wait()
+
+            # NOTE: if we ever would like the behaviour where if the
+            # first task to recv on the underlying is cancelled but it
+            # still DOES trigger the ``.recv_ready``, event we'll likely need
+            # this logic:
+
+            if seq > -1:
+                # stuff from above..
+                seq = state.subs[key]
+
+                value = state.queue[seq]
+                state.subs[key] -= 1
+                return value
+
+            elif seq == -1:
+                # XXX: In the case where the first task to allocate the
+                # ``.recv_ready`` event is cancelled we will be woken with
+                # a non-incremented sequence number and thus will read the
+                # oldest value if we use that. Instead we need to detect if
+                # we have not been incremented and then receive again.
+                return await self.receive()
+
+            else:
+                raise ValueError(f'Invalid sequence {seq}!?')
+
+    @asynccontextmanager
+    async def subscribe(
+        self,
+    ) -> AsyncIterator[BroadcastReceiver]:
+        '''Subscribe for values from this broadcast receiver.
+
+        Returns a new ``BroadCastReceiver`` which is registered for and
+        pulls data from a clone of the original ``trio.abc.ReceiveChannel``
+        provided at creation.
+
+        '''
+        if self._closed:
+            raise trio.ClosedResourceError
+
+        state = self._state
+        br = BroadcastReceiver(
+            rx_chan=self._rx,
+            state=state,
+            receive_afunc=self._recv,
+        )
+        # assert clone in state.subs
+        assert br.key in state.subs
+
+        try:
+            yield br
+        finally:
+            await br.aclose()
+
+    async def aclose(
+        self,
+    ) -> None:
+
+        if self._closed:
+            return
+
+        # XXX: leaving it like this consumers can still get values
+        # up to the last received that still reside in the queue.
+        self._state.subs.pop(self.key)
+
+        self._closed = True
+
+
+def broadcast_receiver(
+
+    recv_chan: AsyncReceiver,
+    max_buffer_size: int,
+    **kwargs,
+
+) -> BroadcastReceiver:
+
+    return BroadcastReceiver(
+        recv_chan,
+        state=BroadcastState(
+            queue=deque(maxlen=max_buffer_size),
+            maxlen=max_buffer_size,
+            subs={},
+        ),
+        **kwargs,
+    )

tractor/_portal.py

@@ -294,6 +294,7 @@ class Portal:
     async def open_stream_from(
         self,
         async_gen_func: Callable,  # typing: ignore
+        shield: bool = False,
         **kwargs,
 
     ) -> AsyncGenerator[ReceiveMsgStream, None]:
@@ -320,7 +321,9 @@ class Portal:
         ctx = Context(self.channel, cid, _portal=self)
         try:
             # deliver receive only stream
-            async with ReceiveMsgStream(ctx, recv_chan) as rchan:
+            async with ReceiveMsgStream(
+                ctx, recv_chan, shield=shield
+            ) as rchan:
                 self._streams.add(rchan)
                 yield rchan
 

tractor/_streaming.py

@@ -2,12 +2,14 @@
 Message stream types and APIs.
 
 """
+from __future__ import annotations
 import inspect
 from contextlib import contextmanager, asynccontextmanager
 from dataclasses import dataclass
 from typing import (
     Any, Iterator, Optional, Callable,
     AsyncGenerator, Dict,
+    AsyncIterator
 )
 
 import warnings
@@ -17,6 +19,7 @@ import trio
 from ._ipc import Channel
 from ._exceptions import unpack_error, ContextCancelled
 from ._state import current_actor
+from ._broadcast import broadcast_receiver, BroadcastReceiver
 from .log import get_logger
 
 
@@ -47,11 +50,14 @@ class ReceiveMsgStream(trio.abc.ReceiveChannel):
     def __init__(
         self,
         ctx: 'Context',  # typing: ignore # noqa
-        rx_chan: trio.abc.ReceiveChannel,
+        rx_chan: trio.MemoryReceiveChannel,
+        shield: bool = False,
+        _broadcaster: Optional[BroadcastReceiver] = None,
 
     ) -> None:
         self._ctx = ctx
         self._rx_chan = rx_chan
+        self._broadcaster = _broadcaster
 
         # flag to denote end of stream
         self._eoc: bool = False
@@ -231,6 +237,50 @@ class ReceiveMsgStream(trio.abc.ReceiveChannel):
         # still need to consume msgs that are "in transit" from the far
         # end (eg. for ``Context.result()``).
 
+    @asynccontextmanager
+    async def subscribe(
+        self,
+
+    ) -> AsyncIterator[BroadcastReceiver]:
+        '''Allocate and return a ``BroadcastReceiver`` which delegates
+        to this message stream.
+
+        This allows multiple local tasks to receive each their own copy
+        of this message stream.
+
+        This operation is indempotent and and mutates this stream's
+        receive machinery to copy and window-length-store each received
+        value from the far end via the internally created broudcast
+        receiver wrapper.
+
+        '''
+        # NOTE: This operation is indempotent and non-reversible, so be
+        # sure you can deal with any (theoretical) overhead of the the
+        # allocated ``BroadcastReceiver`` before calling this method for
+        # the first time.
+        if self._broadcaster is None:
+
+            bcast = self._broadcaster = broadcast_receiver(
+                self,
+                # use memory channel size by default
+                self._rx_chan._state.max_buffer_size,  # type: ignore
+                receive_afunc=self.receive,
+            )
+
+            # NOTE: we override the original stream instance's receive
+            # method to now delegate to the broadcaster's ``.receive()``
+            # such that new subscribers will be copied received values
+            # and this stream doesn't have to expect it's original
+            # consumer(s) to get a new broadcast rx handle.
+            self.receive = bcast.receive  # type: ignore
+            # seems there's no graceful way to type this with ``mypy``?
+            # https://github.com/python/mypy/issues/708
+
+        async with self._broadcaster.subscribe() as bstream:
+            assert bstream.key != self._broadcaster.key
+            assert bstream._recv == self._broadcaster._recv
+            yield bstream
+
 
 class MsgStream(ReceiveMsgStream, trio.abc.Channel):
     """
@@ -247,17 +297,6 @@ class MsgStream(ReceiveMsgStream, trio.abc.Channel):
         '''
         await self._ctx.chan.send({'yield': data, 'cid': self._ctx.cid})
 
-    # TODO: but make it broadcasting to consumers
-    def clone(self):
-        """Clone this receive channel allowing for multi-task
-        consumption from the same channel.
-
-        """
-        return MsgStream(
-            self._ctx,
-            self._rx_chan.clone(),
-        )
-
 
 @dataclass
 class Context: