Refer to kernel laucnhing thread instead of assuming the main thread (#1455)

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

Author: dfalbel

ipykernel/ipkernel.py

@@ -439,7 +439,7 @@ async def run_cell(*args, **kwargs):
 
  cm = (
  self._cancel_on_sigint
- if threading.current_thread() == threading.main_thread()
+ if threading.current_thread() == self.shell_channel_thread.parent_thread
  else self._dummy_context_manager
  )
  with cm(coro_future):

ipykernel/kernelbase.py

@@ -681,19 +681,19 @@ async def shell_main(self, subshell_id: str | None, msg):
  """Handler of shell messages for a single subshell"""
  if self._supports_kernel_subshells:
  if subshell_id is None:
- assert threading.current_thread() == threading.main_thread()
+ assert threading.current_thread() == self.shell_channel_thread.parent_thread
  asyncio_lock = self._main_asyncio_lock
  else:
  assert threading.current_thread() not in (
  self.shell_channel_thread,
- threading.main_thread(),
+ self.shell_channel_thread.parent_thread,
  )
  asyncio_lock = self.shell_channel_thread.manager.get_subshell_asyncio_lock(
  subshell_id
  )
  else:
  assert subshell_id is None
- assert threading.current_thread() == threading.main_thread()
+ assert threading.current_thread() == self.shell_channel_thread.parent_thread
  asyncio_lock = self._main_asyncio_lock
 
  # Whilst executing a shell message, do not accept any other shell messages on the

ipykernel/shellchannel.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import asyncio
+from threading import current_thread
 from typing import Any
 
 import zmq
@@ -28,13 +29,16 @@ def __init__(
  self._manager: SubshellManager | None = None
  self._zmq_context = context # Avoid use of self._context
  self._shell_socket = shell_socket
+ # Record the parent thread - the thread that started the app (usually the main thread)
+ self.parent_thread = current_thread()
 
  self.asyncio_lock = asyncio.Lock()
 
  @property
  def manager(self) -> SubshellManager:
  # Lazy initialisation.
  if self._manager is None:
+ assert current_thread() == self.parent_thread
  self._manager = SubshellManager(
  self._zmq_context,
  self.io_loop,

ipykernel/subshell_manager.py

@@ -7,7 +7,7 @@
 import typing as t
 import uuid
 from functools import partial
-from threading import Lock, current_thread, main_thread
+from threading import Lock, current_thread
 
 import zmq
 from tornado.ioloop import IOLoop
@@ -41,7 +41,7 @@ def __init__(
  shell_socket: zmq.Socket[t.Any],
  ):
  """Initialize the subshell manager."""
- assert current_thread() == main_thread()
+ self._parent_thread = current_thread()
 
  self._context: zmq.Context[t.Any] = context
  self._shell_channel_io_loop = shell_channel_io_loop
@@ -127,7 +127,7 @@ def set_on_recv_callback(self, on_recv_callback):
  """Set the callback used by the main shell and all subshells to receive
  messages sent from the shell channel thread.
  """
- assert current_thread() == main_thread()
+ assert current_thread() == self._parent_thread
  self._on_recv_callback = on_recv_callback
  self._shell_channel_to_main.on_recv(IOLoop.current(), partial(self._on_recv_callback, None))
 
@@ -144,7 +144,7 @@ def subshell_id_from_thread_id(self, thread_id: int) -> str | None:
  Only used by %subshell magic so does not have to be fast/cached.
  """
  with self._lock_cache:
- if thread_id == main_thread().ident:
+ if thread_id == self._parent_thread.ident:
  return None
  for id, subshell in self._cache.items():
  if subshell.ident == thread_id:

tests/test_start_kernel.py

@@ -50,6 +50,40 @@ def test_ipython_start_kernel_userns():
  assert EXPECTED in text
 
 
+def test_start_kernel_background_thread():
+ cmd = dedent(
+ """
+ import threading
+ import asyncio
+ from ipykernel.kernelapp import launch_new_instance
+
+ def launch():
+ # Threads don't always have a default event loop so we need to
+ # create and set a default
+ loop = asyncio.new_event_loop()
+ asyncio.set_event_loop(loop)
+ launch_new_instance()
+
+ thread = threading.Thread(target=launch)
+ thread.start()
+ thread.join()
+ """
+ )
+
+ with setup_kernel(cmd) as client:
+ client.execute("a = 1")
+ msg = client.get_shell_msg(timeout=TIMEOUT)
+ content = msg["content"]
+ assert content["status"] == "ok"
+
+ client.inspect("a")
+ msg = client.get_shell_msg(timeout=TIMEOUT)
+ content = msg["content"]
+ assert content["found"]
+ text = content["data"]["text/plain"]
+ assert "1" in text
+
+
 @pytest.mark.flaky(max_runs=3)
 def test_ipython_start_kernel_no_userns():
  # Issue #4188 - user_ns should be passed to shell as None, not {}