Throw internal errors into the main coroutine

If an internal error is bubbled up from some sub-actor throw that error
into the `MainProcess` "main" async function / coro in order to trigger
nursery teardowns (i.e. cancellations) that need to be done.

I'll likely change this shortly back to where we run a "main task"
inside `actor._async_main()`...

tractor/_actor.py

@@ -19,7 +19,6 @@ from ._portal import (
     open_portal,
     _do_handshake,
     LocalPortal,
-    maybe_open_nursery
 )
 from . import _state
 from ._state import current_actor
@@ -189,9 +188,10 @@ class Actor:
             self._mods[path] = importlib.import_module(path)
 
             # XXX: triggers an internal error which causes a hanging
-            # problem on teardown (root nursery tears down thus killing all
-            # channels before sending cancels to subactors during actor
-            # nursery teardown - has to do with await main() in MainProcess)
+            # problem (without the recently added .throw()) on teardown
+            # (root nursery tears down thus killing all channels before
+            # sending cancels to subactors during actor nursery teardown
+            # - has to do with await main() in MainProcess)
             # if self.name == 'gretchen':
             #     self._mods.pop('test_discovery')
 
@@ -355,13 +355,16 @@ class Actor:
             else:  # channel disconnect
                 log.debug(f"{chan} from {chan.uid} disconnected")
 
-        except InternalActorError:
+        except InternalActorError as err:
             # ship internal errors upwards
             log.exception("Received internal error:")
             if self._parent_chan:
                 await self._parent_chan.send(
                     {'error': traceback.format_exc(), 'cid': 'internal'})
                 raise
+            else:
+                assert self._main_coro
+                self._main_coro.throw(err)
         except trio.ClosedResourceError:
             log.error(f"{chan} form {chan.uid} broke")
         except Exception:
@@ -395,7 +398,7 @@ class Actor:
         accept_addr,
         arbiter_addr=None,
         parent_addr=None,
-        nursery=None,
+        _main_coro=None,
         task_status=trio.TASK_STATUS_IGNORED,
     ):
         """Start the channel server, maybe connect back to the parent, and
@@ -404,10 +407,14 @@ class Actor:
         A "root-most" (or "top-level") nursery for this actor is opened here
         and when cancelled effectively cancels the actor.
         """
+        # if this is the `MainProcess` then we get a ref to the main
+        # task's coroutine object for tossing errors into
+        self._main_coro = _main_coro
+
         arbiter_addr = arbiter_addr or self._arb_addr
         registered_with_arbiter = False
         try:
-            async with maybe_open_nursery(nursery) as nursery:
+            async with trio.open_nursery() as nursery:
                 self._root_nursery = nursery
 
                 # load allowed RPC module
@@ -659,17 +666,22 @@ async def _start_actor(actor, main, host, port, arbiter_addr, nursery=None):
     log.info(f"Starting local {actor} @ {host}:{port}")
 
     async with trio.open_nursery() as nursery:
+
+        if main is not None:
+            main_coro = main()
+
         await nursery.start(
             partial(
                 actor._async_main,
                 accept_addr=(host, port),
                 parent_addr=None,
                 arbiter_addr=arbiter_addr,
-                # nursery=nursery
+                _main_coro=main_coro
             )
         )
         if main is not None:
-            result = await main()
+            result = await main_coro
+
         # XXX: If spawned with a dedicated "main function",
         # the actor is cancelled when this context is complete
         # given that there are no more active peer channels connected