Propagate any spawned asyncio task error upwards

This should mostly maintain top level SC principles for any task spawned
using `tractor.to_asyncio.run()`. When the `asyncio` task completes make
sure to cancel the pertaining `trio` cancel scope and raise any error
that may have resulted.

Resolves #120

tractor/to_asyncio.py

@@ -0,0 +1,99 @@
+"""
+Infection apis for ``asyncio`` loops running ``trio`` using guest mode.
+"""
+import asyncio
+import inspect
+from typing import (
+    Any,
+    Callable,
+    AsyncGenerator,
+    Awaitable,
+    Union,
+)
+
+import trio
+
+from ._state import current_actor
+
+
+__all__ = ['run']
+
+
+async def _invoke(
+    from_trio: trio.abc.ReceiveChannel,
+    to_trio: asyncio.Queue,
+    coro: Awaitable,
+) -> Union[AsyncGenerator, Awaitable]:
+    """Await or stream awaiable object based on type into
+    ``trio`` memory channel.
+    """
+    async def stream_from_gen(c):
+        async for item in c:
+            to_trio.send_nowait(item)
+
+    async def just_return(c):
+        to_trio.send_nowait(await c)
+
+    if inspect.isasyncgen(coro):
+        return await stream_from_gen(coro)
+    elif inspect.iscoroutine(coro):
+        return await coro
+
+
+async def run(
+    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``.
+    """
+    assert current_actor()._infected_aio
+
+    # ITC (inter task comms)
+    from_trio = asyncio.Queue(qsize)
+    to_trio, from_aio = trio.open_memory_channel(qsize)
+
+    # allow target func to accept/stream results manually
+    kwargs['to_trio'] = to_trio
+    kwargs['from_trio'] = to_trio
+
+    coro = func(**kwargs)
+
+    cancel_scope = trio.CancelScope()
+
+    # start the asyncio task we submitted from trio
+    # TODO: try out ``anyio`` asyncio based tg here
+    task = asyncio.create_task(_invoke(from_trio, to_trio, coro))
+    err = None
+
+    # XXX: I'm not sure this actually does anything...
+    def cancel_trio(task):
+        """Cancel the calling ``trio`` task on error.
+        """
+        nonlocal err
+        err = task.exception()
+        cancel_scope.cancel()
+
+    task.add_done_callback(cancel_trio)
+
+    # determine return type async func vs. gen
+    if inspect.isasyncgen(coro):
+        # simple async func
+        async def result():
+            with cancel_scope:
+                return await from_aio.get()
+            if cancel_scope.cancelled_caught and err:
+                raise err
+
+    elif inspect.iscoroutine(coro):
+        # asycn gen
+        async def result():
+            with cancel_scope:
+                async with from_aio:
+                    async for item in from_aio:
+                        yield item
+            if cancel_scope.cancelled_caught and err:
+                raise err
+
+    return result()