fix(py): Handle stream task cancellation without crashing callback

[huangjeff5]

When the stream task is cancelled (e.g., client disconnects during streaming), the channel.closed done callback previously called result() on a cancelled future, which raised CancelledError and crashed the callback.

Fix: Add _propagate_closed_to_result that checks cancelled/exception/result before acting. When channel.closed is cancelled, cancel result_future instead of calling result(). Propagates cancellation cleanly to consumers.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request primarily focuses on enhancing the robustness of asynchronous stream handling within the genkit library. While the pull request title and description refer to a JSON serialization issue, the actual code changes address a different concern: preventing crashes in stream channel callbacks when an asyncio.Future is cancelled. The updated implementation ensures that cancellations and exceptions are gracefully propagated, improving the stability of streaming operations. A new test case has been added to validate this improved cancellation handling.

Highlights

• Stream Cancellation Handling: Improved the handling of stream task cancellations in _action.py to prevent callbacks from crashing when an asyncio.Future is cancelled, ensuring more robust asynchronous operations.
• Robust Callback Logic: Implemented a new callback function, _propagate_closed_to_result, which correctly propagates cancellation, exceptions, or results from the channel.closed future to the result_future.
• New Test Case: Added a dedicated asynchronous test, test_stream_cancellation_does_not_crash_callback, to validate the fix for stream cancellation scenarios and confirm that the system behaves as expected under cancellation.

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[huangjeff5]

Fix comment format

[gemini-code-assist]

Code Review

This pull request addresses a potential crash when a streaming action is cancelled. The fix correctly propagates the cancellation status from the underlying task to the result future, preventing hangs or unhandled exceptions. A new asynchronous test case has been added to verify this fix by simulating a stream cancellation and asserting the correct CancelledError is raised. The changes appear correct and improve the robustness of streaming actions.

It's worth noting that the pull request title and description refer to a JSON serialization issue in FastAPI/Flask, which does not seem to match the content of the changes. It would be beneficial to update them to accurately reflect the work done on stream cancellation handling.