Use `MsgStream.subscribe()` in `Context.result()`

The case exists where there is multiple tasks consuming from an open
2-way stream created via `Context.open_stream()` where a sibling task is
pulling from the stream while some other task also calls `.result()`.
Previously the `.result()` call would consume (drain) stream messages
directly from the underlying mem chan which would mean any sibling task
would not receive those same messages. Instead, make `.result()` check
if a stream is open and instead consume (and discard) stream msgs using
a `BroadcastReceiver` (via `MsgStream.subscribe()`) such that all
interested tasks get copies of the same packets.

tractor/_streaming.py

@@ -27,7 +27,8 @@ from typing import (
     Optional,
     Callable,
     AsyncGenerator,
-    AsyncIterator
+    AsyncIterator,
+    TYPE_CHECKING,
 )
 
 import warnings
@@ -41,6 +42,10 @@ from .log import get_logger
 from .trionics import broadcast_receiver, BroadcastReceiver
 
 
+if TYPE_CHECKING:
+    from ._portal import Portal
+
+
 log = get_logger(__name__)
 
 
@@ -365,7 +370,8 @@ class Context:
     _remote_func_type: Optional[str] = None
 
     # only set on the caller side
-    _portal: Optional['Portal'] = None    # type: ignore # noqa
+    _portal: Optional[Portal] = None    # type: ignore # noqa
+    _stream: Optional[MsgStream] = None
     _result: Optional[Any] = False
     _error: Optional[BaseException] = None
 
@@ -473,6 +479,7 @@ class Context:
         log.cancel(f'Cancelling {side} side of context to {self.chan.uid}')
 
         self._cancel_called = True
+        ipc_broken: bool = False
 
         if side == 'caller':
             if not self._portal:
@@ -490,7 +497,14 @@ class Context:
                 # NOTE: we're telling the far end actor to cancel a task
                 # corresponding to *this actor*. The far end local channel
                 # instance is passed to `Actor._cancel_task()` implicitly.
-                await self._portal.run_from_ns('self', '_cancel_task', cid=cid)
+                try:
+                    await self._portal.run_from_ns(
+                        'self',
+                        '_cancel_task',
+                        cid=cid,
+                    )
+                except trio.BrokenResourceError:
+                    ipc_broken = True
 
             if cs.cancelled_caught:
                 # XXX: there's no way to know if the remote task was indeed
@@ -506,7 +520,10 @@ class Context:
                         "Timed out on cancelling remote task "
                         f"{cid} for {self._portal.channel.uid}")
 
-        # callee side remote task
+            elif ipc_broken:
+                log.cancel(
+                    "Transport layer was broken before cancel request "
+                    f"{cid} for {self._portal.channel.uid}")
         else:
             self._cancel_msg = msg
 
@@ -593,10 +610,11 @@ class Context:
         async with MsgStream(
             ctx=self,
             rx_chan=ctx._recv_chan,
-        ) as rchan:
+        ) as stream:
+            self._stream = stream
 
             if self._portal:
-                self._portal._streams.add(rchan)
+                self._portal._streams.add(stream)
 
             try:
                 self._stream_opened = True
@@ -604,7 +622,7 @@ class Context:
                 # ensure we aren't cancelled before delivering
                 # the stream
                 # await trio.lowlevel.checkpoint()
-                yield rchan
+                yield stream
 
                 # XXX: Make the stream "one-shot use".  On exit, signal
                 # ``trio.EndOfChannel``/``StopAsyncIteration`` to the
@@ -635,25 +653,22 @@ class Context:
 
             if not self._recv_chan._closed:  # type: ignore
 
-                # wait for a final context result consuming
-                # and discarding any bi dir stream msgs still
-                # in transit from the far end.
-                while True:
+                def consume(
+                    msg: dict,
 
-                    msg = await self._recv_chan.receive()
+                ) -> Optional[dict]:
                     try:
-                        self._result = msg['return']
-                        break
+                        return msg['return']
                     except KeyError as msgerr:
 
                         if 'yield' in msg:
                             # far end task is still streaming to us so discard
                             log.warning(f'Discarding stream delivered {msg}')
-                            continue
+                            return
 
                         elif 'stop' in msg:
                             log.debug('Remote stream terminated')
-                            continue
+                            return
 
                         # internal error should never get here
                         assert msg.get('cid'), (
@@ -663,6 +678,24 @@ class Context:
                             msg, self._portal.channel
                         ) from msgerr
 
+                # wait for a final context result consuming
+                # and discarding any bi dir stream msgs still
+                # in transit from the far end.
+                if self._stream:
+                    async with self._stream.subscribe() as bstream:
+                        async for msg in bstream:
+                            result = consume(msg)
+                            if result:
+                                self._result = result
+
+                if not self._result:
+                    while True:
+                        msg = await self._recv_chan.receive()
+                        result = consume(msg)
+                        if result:
+                            self._result = result
+                            break
+
         return self._result
 
     async def started(