Avoid mutate during interate error

tests/test_2way.py

@@ -262,7 +262,7 @@ async def test_caller_closes_ctx_after_callee_opens_stream(
             async with ctx.open_stream() as stream:
                 async for msg in stream:
                     pass
-        except trio.ClosedResourceError:
+        except tractor.ContextCancelled:
             pass
         else:
             assert 0, "Should have received closed resource error?"

tests/test_advanced_streaming.py

@@ -30,7 +30,7 @@ async def publisher(
 
         sub = 'even' if is_even(val) else 'odd'
 
-        for sub_stream in _registry[sub]:
+        for sub_stream in _registry[sub].copy():
             await sub_stream.send(val)
 
         # throttle send rate to ~1kHz

tractor/_actor.py

@@ -46,6 +46,7 @@ class ActorFailure(Exception):
 
 
 async def _invoke(
+
     actor: 'Actor',
     cid: str,
     chan: Channel,
@@ -58,10 +59,15 @@ async def _invoke(
     """Invoke local func and deliver result(s) over provided channel.
     """
     treat_as_gen = False
-    cs = None
+
+    # possible a traceback (not sure what typing is for this..)
+    tb = None
+
     cancel_scope = trio.CancelScope()
-    ctx = Context(chan, cid, _cancel_scope=cancel_scope)
-    context = False
+    cs: trio.CancelScope = None
+
+    ctx = Context(chan, cid)
+    context: bool = False
 
     if getattr(func, '_tractor_stream_function', False):
         # handle decorated ``@tractor.stream`` async functions
@@ -149,14 +155,34 @@ async def _invoke(
             # context func with support for bi-dir streaming
             await chan.send({'functype': 'context', 'cid': cid})
 
-            with cancel_scope as cs:
+            async with trio.open_nursery() as scope_nursery:
+                ctx._scope_nursery = scope_nursery
+                cs = scope_nursery.cancel_scope
                 task_status.started(cs)
+                try:
                     await chan.send({'return': await coro, 'cid': cid})
+                except trio.Cancelled as err:
+                    tb = err.__traceback__
 
             if cs.cancelled_caught:
+
+                # TODO: pack in ``trio.Cancelled.__traceback__`` here
+                # so they can be unwrapped and displayed on the caller
+                # side!
+
+                fname = func.__name__
+                if ctx._cancel_called:
+                    msg = f'{fname} cancelled itself'
+
+                elif cs.cancel_called:
+                    msg = (
+                        f'{fname} was remotely cancelled by its caller '
+                        f'{ctx.chan.uid}'
+                    )
+
                 # task-contex was cancelled so relay to the cancel to caller
                 raise ContextCancelled(
-                    f'{func.__name__} cancelled itself',
+                    msg,
                     suberror_type=trio.Cancelled,
                 )
 
@@ -185,14 +211,16 @@ async def _invoke(
                 log.exception("Actor crashed:")
 
         # always ship errors back to caller
-        err_msg = pack_error(err)
+        err_msg = pack_error(err, tb=tb)
         err_msg['cid'] = cid
         try:
             await chan.send(err_msg)
 
         except trio.ClosedResourceError:
-            log.warning(
-                f"Failed to ship error to caller @ {chan.uid}")
+            # if we can't propagate the error that's a big boo boo
+            log.error(
+                f"Failed to ship error to caller @ {chan.uid} !?"
+            )
 
         if cs is None:
             # error is from above code not from rpc invocation
@@ -210,7 +238,7 @@ async def _invoke(
                 f"Task {func} likely errored or cancelled before it started")
         finally:
             if not actor._rpc_tasks:
-                log.info("All RPC tasks have completed")
+                log.runtime("All RPC tasks have completed")
                 actor._ongoing_rpc_tasks.set()
 
 
@@ -225,10 +253,10 @@ _lifetime_stack: ExitStack = ExitStack()
 class Actor:
     """The fundamental concurrency primitive.
 
-    An *actor* is the combination of a regular Python or
-    ``multiprocessing.Process`` executing a ``trio`` task tree, communicating
+    An *actor* is the combination of a regular Python process
+    executing a ``trio`` task tree, communicating
     with other actors through "portals" which provide a native async API
-    around "channels".
+    around various IPC transport "channels".
     """
     is_arbiter: bool = False
 
@@ -372,14 +400,18 @@ class Actor:
             raise mne
 
     async def _stream_handler(
+
         self,
         stream: trio.SocketStream,
+
     ) -> None:
         """Entry point for new inbound connections to the channel server.
+
         """
         self._no_more_peers = trio.Event()  # unset
+
         chan = Channel(stream=stream)
-        log.info(f"New connection to us {chan}")
+        log.runtime(f"New connection to us {chan}")
 
         # send/receive initial handshake response
         try:
@@ -410,8 +442,12 @@ class Actor:
             event.set()
 
         chans = self._peers[uid]
+
+        # TODO: re-use channels for new connections instead 
+        # of always new ones; will require changing all the
+        # discovery funcs
         if chans:
-            log.warning(
+            log.runtime(
                 f"already have channel(s) for {uid}:{chans}?"
             )
         log.trace(f"Registered {chan} for {uid}")  # type: ignore
@@ -423,10 +459,24 @@ class Actor:
         try:
             await self._process_messages(chan)
         finally:
+
+            # channel cleanup sequence
+
+            # for (channel, cid) in self._rpc_tasks.copy():
+            #     if channel is chan:
+            #         with trio.CancelScope(shield=True):
+            #             await self._cancel_task(cid, channel)
+
+            #             # close all consumer side task mem chans
+            #             send_chan, _ = self._cids2qs[(chan.uid, cid)]
+            #             assert send_chan.cid == cid  # type: ignore
+            #             await send_chan.aclose()
+
             # Drop ref to channel so it can be gc-ed and disconnected
             log.debug(f"Releasing channel {chan} from {chan.uid}")
             chans = self._peers.get(chan.uid)
             chans.remove(chan)
+
             if not chans:
                 log.debug(f"No more channels for {chan.uid}")
                 self._peers.pop(chan.uid, None)
@@ -439,14 +489,22 @@ class Actor:
 
             # # XXX: is this necessary (GC should do it?)
             if chan.connected():
+                # if the channel is still connected it may mean the far
+                # end has not closed and we may have gotten here due to
+                # an error and so we should at least try to terminate
+                # the channel from this end gracefully.
+
                 log.debug(f"Disconnecting channel {chan}")
                 try:
-                    # send our msg loop terminate sentinel
+                    # send a msg loop terminate sentinel
                     await chan.send(None)
+
+                    # XXX: do we want this?
+                    # causes "[104] connection reset by peer" on other end
                     # await chan.aclose()
+
                 except trio.BrokenResourceError:
-                    log.exception(
-                        f"Channel for {chan.uid} was already zonked..")
+                    log.warning(f"Channel for {chan.uid} was already closed")
 
     async def _push_result(
         self,
@@ -456,18 +514,22 @@ class Actor:
     ) -> None:
         """Push an RPC result to the local consumer's queue.
         """
-        actorid = chan.uid
-        assert actorid, f"`actorid` can't be {actorid}"
-        send_chan, recv_chan = self._cids2qs[(actorid, cid)]
+        # actorid = chan.uid
+        assert chan.uid, f"`chan.uid` can't be {chan.uid}"
+        send_chan, recv_chan = self._cids2qs[(chan.uid, cid)]
         assert send_chan.cid == cid  # type: ignore
 
-        # if 'stop' in msg:
+        if 'error' in msg:
+            ctx = getattr(recv_chan, '_ctx', None)
+            # if ctx:
+            #     ctx._error_from_remote_msg(msg)
+
         #     log.debug(f"{send_chan} was terminated at remote end")
         #     # indicate to consumer that far end has stopped
         #     return await send_chan.aclose()
 
         try:
-            log.debug(f"Delivering {msg} from {actorid} to caller {cid}")
+            log.debug(f"Delivering {msg} from {chan.uid} to caller {cid}")
             # maintain backpressure
             await send_chan.send(msg)
 
@@ -486,7 +548,9 @@ class Actor:
         self,
         actorid: Tuple[str, str],
         cid: str
+
     ) -> Tuple[trio.abc.SendChannel, trio.abc.ReceiveChannel]:
+
         log.debug(f"Getting result queue for {actorid} cid {cid}")
         try:
             send_chan, recv_chan = self._cids2qs[(actorid, cid)]
@@ -548,9 +612,15 @@ class Actor:
                             if channel is chan:
                                 await self._cancel_task(cid, channel)
 
+                                # close all consumer side task mem chans
+                                # send_chan, _ = self._cids2qs[(chan.uid, cid)]
+                                # assert send_chan.cid == cid  # type: ignore
+                                # await send_chan.aclose()
+
                         log.debug(
                                 f"Msg loop signalled to terminate for"
                                 f" {chan} from {chan.uid}")
+
                         break
 
                     log.trace(   # type: ignore
@@ -621,7 +691,7 @@ class Actor:
                         else:
                             # mark that we have ongoing rpc tasks
                             self._ongoing_rpc_tasks = trio.Event()
-                            log.info(f"RPC func is {func}")
+                            log.runtime(f"RPC func is {func}")
                             # store cancel scope such that the rpc task can be
                             # cancelled gracefully if requested
                             self._rpc_tasks[(chan, cid)] = (
@@ -630,7 +700,7 @@ class Actor:
                         # self.cancel() was called so kill this msg loop
                         # and break out into ``_async_main()``
                         log.warning(
-                            f"{self.uid} was remotely cancelled; "
+                            f"Actor {self.uid} was remotely cancelled; "
                             "waiting on cancellation completion..")
                         await self._cancel_complete.wait()
                         loop_cs.cancel()
@@ -648,17 +718,13 @@ class Actor:
         except (
             TransportClosed,
             trio.BrokenResourceError,
-            trio.ClosedResourceError
+            # trio.ClosedResourceError
         ):
             # channels "breaking" is ok since we don't have a teardown
             # handshake for them (yet) and instead we simply bail out
-            # of the message loop and expect the teardown sequence
-            # to clean up.
-            log.error(f"{chan} form {chan.uid} closed abruptly")
-            # raise
-
-        except trio.ClosedResourceError:
-            log.error(f"{chan} form {chan.uid} broke")
+            # of the message loop and expect the surrounding
+            # caller's teardown sequence to clean up.
+            log.warning(f"Channel from {chan.uid} closed abruptly")
 
         except (Exception, trio.MultiError) as err:
             # ship any "internal" exception (i.e. one from internal machinery
@@ -1102,7 +1168,7 @@ class Actor:
             raise ValueError(f"{uid} is not a valid uid?!")
 
         chan.uid = uid
-        log.info(f"Handshake with actor {uid}@{chan.raddr} complete")
+        log.runtime(f"Handshake with actor {uid}@{chan.raddr} complete")
         return uid
 
 

tractor/_debug.py

@@ -207,11 +207,24 @@ async def _hijack_stdin_relay_to_child(
     return "pdb_unlock_complete"
 
 
-async def _breakpoint(debug_func) -> None:
-    """``tractor`` breakpoint entry for engaging pdb machinery
-    in subactors.
+async def _breakpoint(
+
+    debug_func,
+
+    # TODO:
+    # shield: bool = False
+
+) -> None:
+    '''``tractor`` breakpoint entry for engaging pdb machinery
+    in the root or a subactor.
+
+    '''
+    # TODO: is it possible to debug a trio.Cancelled except block?
+    # right now it seems like we can kinda do with by shielding
+    # around ``tractor.breakpoint()`` but not if we move the shielded
+    # scope here???
+    # with trio.CancelScope(shield=shield):
 
-    """
     actor = tractor.current_actor()
     task_name = trio.lowlevel.current_task().name
 

tractor/_discovery.py

@@ -16,12 +16,14 @@ from ._state import current_actor, _runtime_vars
 
 @asynccontextmanager
 async def get_arbiter(
+
     host: str,
     port: int,
+
 ) -> typing.AsyncGenerator[Union[Portal, LocalPortal], None]:
-    """Return a portal instance connected to a local or remote
+    '''Return a portal instance connected to a local or remote
     arbiter.
-    """
+    '''
     actor = current_actor()
 
     if not actor:
@@ -33,7 +35,9 @@ async def get_arbiter(
         yield LocalPortal(actor, Channel((host, port)))
     else:
         async with _connect_chan(host, port) as chan:
+
             async with open_portal(chan) as arb_portal:
+
                 yield arb_portal
 
 
@@ -41,8 +45,10 @@ async def get_arbiter(
 async def get_root(
     **kwargs,
 ) -> typing.AsyncGenerator[Union[Portal, LocalPortal], None]:
+
     host, port = _runtime_vars['_root_mailbox']
     assert host is not None
+
     async with _connect_chan(host, port) as chan:
         async with open_portal(chan, **kwargs) as portal:
             yield portal
@@ -60,12 +66,16 @@ async def find_actor(
     """
     actor = current_actor()
     async with get_arbiter(*arbiter_sockaddr or actor._arb_addr) as arb_portal:
+
         sockaddr = await arb_portal.run_from_ns('self', 'find_actor', name=name)
+
         # TODO: return portals to all available actors - for now just
         # the last one that registered
         if name == 'arbiter' and actor.is_arbiter:
             raise RuntimeError("The current actor is the arbiter")
+
         elif sockaddr:
+
             async with _connect_chan(*sockaddr) as chan:
                 async with open_portal(chan) as portal:
                     yield portal
@@ -83,9 +93,12 @@ async def wait_for_actor(
     A portal to the first registered actor is returned.
     """
     actor = current_actor()
+
     async with get_arbiter(*arbiter_sockaddr or actor._arb_addr) as arb_portal:
+
         sockaddrs = await arb_portal.run_from_ns('self', 'wait_for_actor', name=name)
         sockaddr = sockaddrs[-1]
+
         async with _connect_chan(*sockaddr) as chan:
             async with open_portal(chan) as portal:
                 yield portal

tractor/_exceptions.py

@@ -56,13 +56,22 @@ class NoRuntime(RuntimeError):
     "The root actor has not been initialized yet"
 
 
-def pack_error(exc: BaseException) -> Dict[str, Any]:
+def pack_error(
+    exc: BaseException,
+    tb = None,
+
+) -> Dict[str, Any]:
     """Create an "error message" for tranmission over
     a channel (aka the wire).
     """
+    if tb:
+        tb_str = ''.join(traceback.format_tb(tb))
+    else:
+        tb_str = traceback.format_exc()
+
     return {
         'error': {
-            'tb_str': traceback.format_exc(),
+            'tb_str': tb_str,
             'type_str': type(exc).__name__,
         }
     }

tractor/_portal.py

@@ -177,6 +177,7 @@ class Portal:
                 f"Cancelling all streams with {self.channel.uid}")
             for stream in self._streams.copy():
                 try:
+                    # with trio.CancelScope(shield=True):
                     await stream.aclose()
                 except trio.ClosedResourceError:
                     # don't error the stream having already been closed
@@ -294,6 +295,7 @@ class Portal:
         self,
         async_gen_func: Callable,  # typing: ignore
         **kwargs,
+
     ) -> AsyncGenerator[ReceiveMsgStream, None]:
 
         if not inspect.isasyncgenfunction(async_gen_func):
@@ -346,7 +348,6 @@ class Portal:
 
         self,
         func: Callable,
-        cancel_on_exit: bool = False,
         **kwargs,
 
     ) -> AsyncGenerator[Tuple[Context, Any], None]:
@@ -358,6 +359,7 @@ class Portal:
         and synchronized final result collection. See ``tractor.Context``.
 
         '''
+
         # conduct target func method structural checks
         if not inspect.iscoroutinefunction(func) and (
             getattr(func, '_tractor_contex_function', False)
@@ -369,7 +371,6 @@ class Portal:
 
         recv_chan: Optional[trio.MemoryReceiveChannel] = None
 
-        try:
         cid, recv_chan, functype, first_msg = await self._submit(
             fn_mod_path, fn_name, kwargs)
 
@@ -390,44 +391,81 @@ class Portal:
             else:
                 raise
 
+        _err = None
         # deliver context instance and .started() msg value in open
         # tuple.
+        try:
+            async with trio.open_nursery() as scope_nursery:
                 ctx = Context(
                     self.channel,
                     cid,
                     _portal=self,
                     _recv_chan=recv_chan,
+                    _scope_nursery=scope_nursery,
                 )
+                recv_chan._ctx = ctx
 
-            try:
+                # await trio.lowlevel.checkpoint()
                 yield ctx, first
 
-                if cancel_on_exit:
-                    await ctx.cancel()
+                # if not ctx._cancel_called:
+                #     await ctx.result()
 
-                else:
+            # await recv_chan.aclose()
+
+        except ContextCancelled as err:
+            _err = err
             if not ctx._cancel_called:
-                        await ctx.result()
+                # context was cancelled at the far end but was
+                # not part of this end requesting that cancel
+                # so raise for the local task to respond and handle.
+                raise
 
-            except ContextCancelled:
             # if the context was cancelled by client code
             # then we don't need to raise since user code
-                # is expecting this.
-                if not ctx._cancel_called:
-                    raise
+            # is expecting this and the block should exit.
+            else:
+                log.debug(f'Context {ctx} cancelled gracefully')
 
-            except BaseException:
-                # the context cancels itself on any deviation
+        except (
+            trio.Cancelled,
+            trio.MultiError,
+            Exception,
+        ) as err:
+            _err = err
+            # the context cancels itself on any cancel
+            # causing error.
+            log.error(f'Context {ctx} sending cancel to far end')
+            with trio.CancelScope(shield=True):
                 await ctx.cancel()
             raise
 
         finally:
-                log.info(f'Context for {func.__name__} completed')
+            result = await ctx.result()
 
-        finally:
+            # though it should be impossible for any tasks
+            # operating *in* this scope to have survived
+            # we tear down the runtime feeder chan last
+            # to avoid premature stream clobbers.
             if recv_chan is not None:
                 await recv_chan.aclose()
 
+            if _err:
+                if ctx._cancel_called:
+                    log.warning(
+                        f'Context {fn_name} cancelled by caller with\n{_err}'
+                    )
+                elif _err is not None:
+                    log.warning(
+                        f'Context {fn_name} cancelled by callee with\n{_err}'
+                    )
+            else:
+                log.info(
+                    f'Context {fn_name} returned '
+                    f'value from callee `{self._result}`'
+                )
+
+
 @dataclass
 class LocalPortal:
     """A 'portal' to a local ``Actor``.
@@ -450,10 +488,12 @@ class LocalPortal:
 
 @asynccontextmanager
 async def open_portal(
+
     channel: Channel,
     nursery: Optional[trio.Nursery] = None,
     start_msg_loop: bool = True,
     shield: bool = False,
+
 ) -> AsyncGenerator[Portal, None]:
     """Open a ``Portal`` through the provided ``channel``.
 
@@ -464,6 +504,7 @@ async def open_portal(
     was_connected = False
 
     async with maybe_open_nursery(nursery, shield=shield) as nursery:
+
         if not channel.connected():
             await channel.connect()
             was_connected = True
@@ -485,12 +526,14 @@ async def open_portal(
         portal = Portal(channel)
         try:
             yield portal
+
         finally:
             await portal.aclose()
 
             if was_connected:
-                # cancel remote channel-msg loop
+                # gracefully signal remote channel-msg loop
                 await channel.send(None)
+                # await channel.aclose()
 
             # cancel background msg loop task
             if msg_loop_cs:

tractor/_streaming.py

@@ -7,7 +7,7 @@ from contextlib import contextmanager, asynccontextmanager
 from dataclasses import dataclass
 from typing import (
     Any, Iterator, Optional, Callable,
-    AsyncGenerator,
+    AsyncGenerator, Dict,
 )
 
 import warnings
@@ -15,7 +15,7 @@ import warnings
 import trio
 
 from ._ipc import Channel
-from ._exceptions import unpack_error
+from ._exceptions import unpack_error, ContextCancelled
 from ._state import current_actor
 from .log import get_logger
 
@@ -67,6 +67,7 @@ class ReceiveMsgStream(trio.abc.ReceiveChannel):
             raise trio.EndOfChannel
 
         try:
+
             msg = await self._rx_chan.receive()
             return msg['yield']
 
@@ -134,11 +135,6 @@ class ReceiveMsgStream(trio.abc.ReceiveChannel):
 
             raise  # propagate
 
-        # except trio.Cancelled:
-        #     # relay cancels to the remote task
-        #     await self.aclose()
-        #     raise
-
     @contextmanager
     def shield(
         self
@@ -212,7 +208,10 @@ class ReceiveMsgStream(trio.abc.ReceiveChannel):
                 # stop from the caller side
                 await self._ctx.send_stop()
 
-            except trio.BrokenResourceError:
+            except (
+                trio.BrokenResourceError,
+                trio.ClosedResourceError
+            ):
                 # the underlying channel may already have been pulled
                 # in which case our stop message is meaningless since
                 # it can't traverse the transport.
@@ -254,18 +253,6 @@ class ReceiveMsgStream(trio.abc.ReceiveChannel):
         # still need to consume msgs that are "in transit" from the far
         # end (eg. for ``Context.result()``).
 
-    # TODO: but make it broadcasting to consumers
-    # def clone(self):
-    #     """Clone this receive channel allowing for multi-task
-    #     consumption from the same channel.
-
-    #     """
-    #     return ReceiveStream(
-    #         self._cid,
-    #         self._rx_chan.clone(),
-    #         self._portal,
-    #     )
-
 
 class MsgStream(ReceiveMsgStream, trio.abc.Channel):
     """
@@ -282,6 +269,17 @@ 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:
@@ -308,7 +306,7 @@ class Context:
     _cancel_called: bool = False
 
     # only set on the callee side
-    _cancel_scope: Optional[trio.CancelScope] = None
+    _scope_nursery: Optional[trio.Nursery] = None
 
     async def send_yield(self, data: Any) -> None:
 
@@ -323,6 +321,22 @@ class Context:
     async def send_stop(self) -> None:
         await self.chan.send({'stop': True, 'cid': self.cid})
 
+    def _error_from_remote_msg(
+        self,
+        msg: Dict[str, Any],
+
+    ) -> None:
+        '''Unpack and raise a msg error into the local scope
+        nursery for this context.
+
+        Acts as a form of "relay" for a remote error raised
+        in the corresponding remote callee task.
+        '''
+        async def raiser():
+            raise unpack_error(msg, self.chan)
+
+        self._scope_nursery.start_soon(raiser)
+
     async def cancel(self) -> None:
         '''Cancel this inter-actor-task context.
 
@@ -330,9 +344,13 @@ class Context:
         Timeout quickly in an attempt to sidestep 2-generals...
 
         '''
+        side = 'caller' if self._portal else 'callee'
+
+        log.warning(f'Cancelling {side} side of context to {self.chan}')
+
         self._cancel_called = True
 
-        if self._portal:  # caller side:
+        if side == 'caller':
             if not self._portal:
                 raise RuntimeError(
                     "No portal found, this is likely a callee side context"
@@ -360,14 +378,17 @@ class Context:
                         "May have failed to cancel remote task "
                         f"{cid} for {self._portal.channel.uid}")
         else:
-            # ensure callee side
-            assert self._cancel_scope
+            # callee side remote task
+
             # TODO: should we have an explicit cancel message
             # or is relaying the local `trio.Cancelled` as an
             # {'error': trio.Cancelled, cid: "blah"} enough?
             # This probably gets into the discussion in
             # https://github.com/goodboy/tractor/issues/36
-            self._cancel_scope.cancel()
+            self._scope_nursery.cancel_scope.cancel()
+
+        if self._recv_chan:
+            await self._recv_chan.aclose()
 
     @asynccontextmanager
     async def open_stream(
@@ -409,19 +430,25 @@ class Context:
             self.cid
         )
 
-        # XXX: If the underlying receive mem chan has been closed then
-        # likely client code has already exited a ``.open_stream()``
-        # block prior. we error here until such a time that we decide
-        # allowing streams to be "re-connected" is supported and/or
-        # a good idea.
-        if recv_chan._closed:
+        # Likewise if the surrounding context has been cancelled we error here
+        # since it likely means the surrounding block was exited or
+        # killed
+
+        if self._cancel_called:
             task = trio.lowlevel.current_task().name
-            raise trio.ClosedResourceError(
-                f'stream for {actor.uid[0]}:{task} has already been closed.'
-                '\nRe-opening a closed stream is not yet supported!'
-                '\nConsider re-calling the containing `@tractor.context` func'
+            raise ContextCancelled(
+                f'Context around {actor.uid[0]}:{task} was already cancelled!'
             )
 
+        # XXX: If the underlying channel feeder receive mem chan has
+        # been closed then likely client code has already exited
+        # a ``.open_stream()`` block prior or there was some other
+        # unanticipated error or cancellation from ``trio``.
+
+        if recv_chan._closed:
+            raise trio.ClosedResourceError(
+                'The underlying channel for this stream was already closed!?')
+
         async with MsgStream(
             ctx=self,
             rx_chan=recv_chan,