Fix async UDF event loop starvation in Jupyter

[kesmit13]

Summary

• Async UDFs running under uvicorn in Jupyter notebooks become unresponsive under heavy load due to event loop starvation
• Root cause: async UDFs ran directly in uvicorn's event loop via asyncio.create_task, competing with HTTP connection handling
• Fix: dedicated event loop in a background thread for async UDF execution, submitted via run_coroutine_threadsafe() and awaited from the server loop
• Sync UDFs are unchanged (already offloaded to threads)

Test plan

• Existing test_ext_func.py tests pass
• Manual: launch uvicorn from Jupyter, fire 50+ concurrent async UDF calls, confirm responsiveness
• Verify cancellation: disconnect mid-execution, confirm UDF stops
• Verify sync UDFs still work identically (regression)

🤖 Generated with Claude Code

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Note

Medium Risk
Changes the external-function ASGI invoke path (threading, cancellation, teardown); CI/release behavior is lower risk but affects how packages ship.

Overview
Async external UDFs no longer run on the uvicorn event loop (or via nested asyncio.run in-thread). Every invoke is scheduled with to_thread and runs inside _run_with_graceful_shutdown, which uses a dedicated event loop per call, drains pending tasks/callbacks before close (avoids “Event loop is closed” from httpx/anyio), and wraps the coroutine in _cancellable_run so a threading.Event can cancel work on disconnect/timeout. cancel_event is set when the function task loses the race or in finally.

Release automation: publish.yml triggers on pre-release tag pushes, rewrites pyproject.toml version for test/alpha/beta/rc tags, creates a prerelease GitHub release with built wheels on tag push, uploads assets on release: published, and tightens when PyPI publish runs. fusion-docs.yml runs on release publish and uploads docs with github.event.release.tag_name.

Tests: new test_udf_event_loop.py for shutdown/cancellation; pandas alltypes expectations use runtime string dtype where applicable.

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[mgiannakopoulos]

🚢 Ship IT!

[cursor]

Cursor Bugbot has reviewed your changes using default mode and found 1 potential issue.

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[cursor]

Successful UDF result discarded when cancel races completion

Medium Severity

_cancellable_run unconditionally prioritizes cancellation over a successful result when both tasks complete in the same event loop iteration. asyncio.wait(return_when=FIRST_COMPLETED) can return both task and cancel_check in done if they finish simultaneously. Because if cancel_check in done is checked without first checking whether task also succeeded, a completed UDF result is silently discarded and replaced with CancelledError. Checking task in done first (and returning its result) would preserve the successfully-computed value.

 

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