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 async external-function execution model and event-loop lifecycle, which can affect concurrency, cancellation, and resource cleanup under load. CI/release workflow tweaks may also impact publishing/release asset behavior if misconfigured.

Overview
Async external functions no longer run on the server event loop. The ASGI dispatcher now offloads UDF execution to a thread that runs the coroutine in a fresh event loop via _run_with_graceful_shutdown, with _cancellable_run polling a threading.Event to propagate disconnect/timeout cancellation.

Improves teardown and cancellation semantics. Pending tasks/async generators/executors are drained/cancelled before loop close to avoid Event loop is closed errors, and the cancel event is always set in finally.

Tests and CI updated. Adds test_udf_event_loop.py coverage for loop cleanup/cancellation/context propagation, makes pandas dtype assertions tolerate string vs object, and updates workflows to publish docs on release, create prerelease GitHub releases for test tags, and upload assets using the release tag name.

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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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