Factor error translation into a ctx mngr

Pull the common `asyncio` -> `trio` error translation logic into
a common context manager and don't expect a final result to be captured
when using `open_channel_from()` since it's a manager interface and it
would be clunky to try and deliver some "final result" after exit.

tractor/to_asyncio.py

@@ -92,14 +92,28 @@ def _run_asyncio_task(
             raise
 
         finally:
-            aio_task_complete.set()
-            if result != orig and aio_err is None:
+            if (
+                result != orig and
+                aio_err is None and
+
+                # in the ``open_channel_from()`` case we don't
+                # relay through the "return value".
+                not provide_channels
+            ):
                 to_trio.send_nowait(result)
 
+            to_trio.close()
+            from_aio.close()
+            aio_task_complete.set()
+
     # start the asyncio task we submitted from trio
     if inspect.isawaitable(coro):
         task = asyncio.create_task(
-            wait_on_coro_final_result(to_trio, coro, aio_task_complete)
+            wait_on_coro_final_result(
+                to_trio,
+                coro,
+                aio_task_complete
+            )
         )
 
     else:
@@ -120,7 +134,7 @@ def _run_asyncio_task(
             cancel_scope.cancel()
         else:
             if aio_err is not None:
-                log.exception(f"infected task errorred:")
+                log.exception("infected task errorred:")
                 from_aio._err = aio_err
                 # order is opposite here
                 cancel_scope.cancel()
@@ -131,41 +145,20 @@ def _run_asyncio_task(
     return task, from_aio, to_trio, cancel_scope, aio_task_complete
 
 
-async def run_task(
-    func: Callable,
-    *,
+@acm
+async def translate_aio_errors(
 
-    qsize: int = 2**10,
-    **kwargs,
+    from_aio: trio.MemoryReceiveChannel,
+    task: asyncio.Task,
 
-) -> Any:
+) -> None:
     '''
-    Run an ``asyncio`` async function or generator in a task, return
-    or stream the result back to ``trio``.
+    Error handling context around ``asyncio`` task spawns which
+    appropriately translates errors and cancels into ``trio`` land.
 
     '''
-    # simple async func
     try:
-        task, from_aio, to_trio, cs, _ = _run_asyncio_task(
-            func,
-            qsize=1,
-            **kwargs,
-        )
-
-        # return single value
-        with cs:
-            # naively expect the mem chan api to do the job
-            # of handling cross-framework cancellations / errors
-            return await from_aio.receive()
-
-        if cs.cancelled_caught:
-            aio_err = from_aio._err
-
-            # always raise from any captured asyncio error
-            if aio_err:
-                raise aio_err
-
-    # Do we need this?
+        yield
     except (
         Exception,
         CancelledError,
@@ -190,6 +183,41 @@ async def run_task(
         #     ... do what ..
 
 
+async def run_task(
+    func: Callable,
+    *,
+
+    qsize: int = 2**10,
+    **kwargs,
+
+) -> Any:
+    '''
+    Run an ``asyncio`` async function or generator in a task, return
+    or stream the result back to ``trio``.
+
+    '''
+    # simple async func
+    task, from_aio, to_trio, cs, _ = _run_asyncio_task(
+        func,
+        qsize=1,
+        **kwargs,
+    )
+    async with translate_aio_errors(from_aio, task):
+
+        # return single value
+        with cs:
+            # naively expect the mem chan api to do the job
+            # of handling cross-framework cancellations / errors
+            return await from_aio.receive()
+
+        if cs.cancelled_caught:
+            aio_err = from_aio._err
+
+            # always raise from any captured asyncio error
+            if aio_err:
+                raise aio_err
+
+
 # TODO: explicitly api for the streaming case where
 # we pull from the mem chan in an async generator?
 # This ends up looking more like our ``Portal.open_stream_from()``
@@ -203,40 +231,21 @@ async def open_channel_from(
 
 ) -> AsyncIterator[Any]:
 
-    try:
-        task, from_aio, to_trio, cs, aio_task_complete = _run_asyncio_task(
-            target,
-            qsize=2**8,
-            provide_channels=True,
-            **kwargs,
-        )
-
+    task, from_aio, to_trio, cs, aio_task_complete = _run_asyncio_task(
+        target,
+        qsize=2**8,
+        provide_channels=True,
+        **kwargs,
+    )
+    async with translate_aio_errors(from_aio, task):
         with cs:
             # sync to "started()" call.
             first = await from_aio.receive()
+
             # stream values upward
             async with from_aio:
                 yield first, from_aio
-                # await aio_task_complete.wait()
-
-    except BaseException as err:
-
-        aio_err = from_aio._err
-
-        if aio_err is not None:
-            # always raise from any captured asyncio error
-            raise err from aio_err
-        else:
-            raise
-
-    finally:
-        if cs.cancelled_caught:
-            # always raise from any captured asyncio error
-            if from_aio._err:
-                raise from_aio._err
-
-        if not task.done():
-            task.cancel()
+                await aio_task_complete.wait()
 
 
 def run_as_asyncio_guest(
@@ -284,7 +293,7 @@ def run_as_asyncio_guest(
             run_sync_soon_threadsafe=loop.call_soon_threadsafe,
             done_callback=trio_done_callback,
         )
-        (await trio_done_fut).unwrap()
+        return (await trio_done_fut).unwrap()
 
     # might as well if it's installed.
     try: