More work on Trio background loop

Revert changes to _async_helpers.py. Add a new _trio_runner property to
the `InteractiveShell` class for use with a PR on the `ipykernel` repo
that populates this property.

Co-authored-by: Brian Mackintosh <[e-mail]>

IPython/core/async_helpers.py

@@ -42,49 +42,9 b' def _curio_runner(coroutine):'
     return curio.run(coroutine)
 
 
-_TRIO_TOKEN = None
-
-
-def _init_trio(trio):
-    global _TRIO_TOKEN
-    import traceback
-    import builtins
-    import threading
-    # We use an Event to avoid a race condition between starting the Trio thread
-    # and running Trio code.
-    thread_start = threading.Event()
-
-    def log_nursery_exc(exc):
-        import logging
-        import traceback
-        exc = '\n'.join(traceback.format_exception(type(exc), exc,
-            exc.__traceback__))
-        logging.error('An exception occurred in a global nursery task.\n%s',
-            exc)
-
-    async def trio_entry():
-        global _TRIO_TOKEN, _TRIO_NURSERY
-        _TRIO_TOKEN = trio.hazmat.current_trio_token()
-        async with trio.open_nursery() as nursery:
-            # TODO This hack prevents the nursery from cancelling all child
-            # tasks when but it's ugly.
-            nursery._add_exc = log_nursery_exc
-            builtins.GLOBAL_NURSERY = nursery
-            thread_start.set()
-            await trio.sleep_forever()
-
-    threading.Thread(target=trio.run, args=(trio_entry,)).start()
-    thread_start.wait()
-
-
 def _trio_runner(async_fn):
-    global _TRIO_TOKEN
-    import logging
     import trio
 
-    if not _TRIO_TOKEN:
-        _init_trio(trio)
-
     async def loc(coro):
         """
         We need the dummy no-op async def to protect from
@@ -92,7 +52,7 b' def _trio_runner(async_fn):'
         """
         return await coro
 
-    return trio.from_thread.run(loc, async_fn, trio_token=_TRIO_TOKEN)
+    return trio.run(loc, async_fn)
 
 
 def _pseudo_sync_runner(coro):

IPython/core/interactiveshell.py

@@ -695,6 +695,13 b' class InteractiveShell(SingletonConfigurable):'
         self.events.trigger('shell_initialized', self)
         atexit.register(self.atexit_operations)
 
+        # The trio runner is used for running Trio in the foreground thread. It
+        # is different from `_trio_runner(async_fn)` in `async_helpers.py`
+        # which calls `trio.run()` for every cell. This runner runs all cells
+        # inside a single Trio event loop. If used, it is set from
+        # `ipykernel.kernelapp`.
+        self.trio_runner = None
+
     def get_ipython(self):
         """Return the currently running IPython instance."""
         return self
@@ -715,6 +722,9 b' class InteractiveShell(SingletonConfigurable):'
         else:
             self.autoindent = value
 
+    def set_trio_runner(self, tr):
+        self.trio_runner = tr
+
     #-------------------------------------------------------------------------
     # init_* methods called by __init__
     #-------------------------------------------------------------------------
@@ -2865,7 +2875,9 b' class InteractiveShell(SingletonConfigurable):'
         # when this is the case, we want to run it using the pseudo_sync_runner
         # so that code can invoke eventloops (for example via the %run , and
         # `%paste` magic.
-        if self.should_run_async(raw_cell) or self.loop_runner is _trio_runner:
+        if self.trio_runner:
+            runner = self.trio_runner
+        elif self.should_run_async(raw_cell):
             runner = self.loop_runner
         else:
             runner = _pseudo_sync_runner