Make kernelbase._eventloop_set event thread-safe (#1366)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

ipykernel/kernelapp.py

@@ -20,7 +20,7 @@
 
 import zmq
 import zmq.asyncio
-from anyio import create_task_group, run
+from anyio import create_task_group, run, to_thread
 from IPython.core.application import (  # type:ignore[attr-defined]
     BaseIPythonApplication,
     base_aliases,
@@ -791,7 +791,7 @@ def start(self) -> None:
         run(self._start, backend=backend)
 
     async def _wait_to_enter_eventloop(self) -> None:
-        await self.kernel._eventloop_set.wait()
+        await to_thread.run_sync(self.kernel._eventloop_set.wait)
         await self.kernel.enter_eventloop()
 
     async def main(self) -> None:

ipykernel/kernelbase.py

@@ -36,7 +36,7 @@
 
 import psutil
 import zmq
-from anyio import TASK_STATUS_IGNORED, Event, create_task_group, sleep, to_thread
+from anyio import TASK_STATUS_IGNORED, create_task_group, sleep, to_thread
 from anyio.abc import TaskStatus
 from IPython.core.error import StdinNotImplementedError
 from jupyter_client.session import Session
@@ -235,12 +235,14 @@ def _parent_header(self):
         "list_subshell_request",
     ]
 
-    _eventloop_set: Event = Event()
+    _eventloop_set: threading.Event
 
     def __init__(self, **kwargs):
         """Initialize the kernel."""
         super().__init__(**kwargs)
 
+        self._eventloop_set = threading.Event()
+
         # Kernel application may swap stdout and stderr to OutStream,
         # which is the case in `IPKernelApp.init_io`, hence `sys.stdout`
         # can already by different from TextIO at initialization time.