fix(runtime): run user code on a Proactor loop on Windows in run and script mode

[BurakErdemci]

This pull request was authored by a coding agent.

📝 Summary

Closes #9182

On Windows, a cell that awaits asyncio.create_subprocess_exec raises NotImplementedError in marimo run and in script mode (python notebook.py). #9194 fixed edit mode only.

marimo sets WindowsSelectorEventLoopPolicy for the whole process in two places: when marimo._islands._island_generator is imported, and in initialize_asyncio() at server start. A Selector loop on Windows cannot create subprocesses. Edit and IPC kernels run in child processes, and _bootstrap_subprocess switches them back to Proactor. A run-mode kernel runs in a thread inside the server, and AppScriptRunner runs in the caller's process, so both call plain asyncio.run and get a Selector loop.

This PR adds run_on_subprocess_capable_loop to marimo/_utils/asyncio_utils.py. On Windows it runs the coroutine on a ProactorEventLoop that it creates itself, and it leaves the global policy alone because the server and other kernel threads create loops at the same time. It uses asyncio.run(..., loop_factory=...) on 3.12+, asyncio.Runner on 3.11, and a copy of asyncio.run's shutdown steps on 3.10. On other platforms it calls asyncio.run.

• launch_kernel uses the helper for in-process kernels (run mode and app host). The edit and IPC branches keep their current code.
• AppScriptRunner uses it for async notebooks.
• The server keeps its Selector loop. I did not touch the islands import or initialize_asyncio.

test_run_mode_on_windows_does_not_touch_event_loop_policy stated that run mode needs Selector for ConnectionDistributor.add_reader(). Run mode uses QueueDistributor (see NotificationListenerExtension._create_distributor), and ConnectionDistributor.start holds the only add_reader call in the codebase. I replaced that test with one that checks run mode calls the helper and leaves the policy unchanged. If run mode should stay on Selector for a reason I missed, let me know.

Before and after (Windows 11, Python 3.13, a notebook with one async cell that runs python -c "print('ok')"):

Mode	Before	After
python notebook.py	NotImplementedError on _WindowsSelectorEventLoop	prints ok on ProactorEventLoop
marimo run notebook.py	NotImplementedError	prints ok

Tests

• tests/_utils/test_asyncio_utils.py: return value, exception propagation, pending task cancellation, closed loop, RuntimeError inside a running loop, and a Windows test that spawns a subprocess under the Selector policy and checks the policy object stays the same.
• tests/_ast/test_app.py::TestApp::test_run_async_subprocess (Windows only) fails with NotImplementedError on main and passes with this change.
• TestLaunchKernelEventLoop: updated run-mode tests for Windows and other platforms.
• I ran these on Windows with Python 3.10, 3.11 and 3.13. test_session_manager_file_rename fails on Windows on main too, because of the error message text.

Not covered: a synchronous cell that calls asyncio.run(...) in script mode still gets a Selector loop from the policy that the islands import sets. Changing that import-time policy looks like it would affect islands build(), so I left that decision to you.

📋 Pre-Review Checklist

• For large changes, or changes that affect the public API: this change was discussed or approved through an issue, on Discord, or the community discussions (Please provide a link if applicable).
• Any AI generated code has been reviewed line-by-line by the human PR author, who stands by it.
• Video or media evidence is provided for any visual changes (optional).

✅ Merge Checklist

• I have read the contributor guidelines.
• Documentation has been updated where applicable, including docstrings for API changes.
• Tests have been added for the changes made.

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated
marimo-docs	Ready	Preview	Sep 13, 2026 2:28pm UTC

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[BurakErdemci]

I have read the CLA Document and I hereby sign the CLA