feat: add stop event check

[fatelei]

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Summary

fix #30390

here is a user log
log.txt

2025-12-31 14:05:06 2025-12-31 06:05:06.326 DEBUG [Thread-26 (_generate_worker)] [worker_pool.py:129]  - Stopping worker pool: 1 workers
2025-12-31 14:05:16 2025-12-31 06:05:16.333 DEBUG [Thread-26 (_generate_worker)] [execution_limits.py:110]  - Execution completed: 8 steps in 33.43 seconds

we can see here, wait about 10s

14:05:06.326  WorkerPool.stop()
                ├─ worker.stop() → _stop_event.set()
                └─ worker.join(timeout=10.0) # start wait
                    ↓
                    ↓ 
                    ↓ （possible node.run() iteration）
                    ↓
  14:05:16.333  join() # timeout
                ↓
                ↓ print "Execution completed"

so we need check the stop_event is set or not in the iteration

Screenshots

Before	After
...	...

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[gemini-code-assist]

Summary of Changes

Hello @fatelei, 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 significantly improves the responsiveness and efficiency of worker pool shutdowns. By introducing an immediate check for stop events within the worker's processing loop, it eliminates a previously identified 10-second delay during termination, ensuring that workers can be stopped promptly and predictably.

Highlights

• Addressing Worker Shutdown Delay: This pull request resolves a 10-second delay observed during worker pool shutdown. Previously, worker.join() would wait for node.run() iterations to complete, even after a stop event was issued, causing unnecessary delays.
• Implementing Immediate Stop Event Check: A crucial check for _stop_event.is_set() has been added within the node.run() event iteration in api/core/workflow/graph_engine/worker.py. This ensures that workers immediately cease processing events upon receiving a stop signal, leading to more responsive shutdowns.

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[gemini-code-assist]

Code Review

The pull request modifies api/core/workflow/graph_engine/worker.py by adding a check within the _execute_node method. Specifically, inside the loop that processes node_events, a condition if self._stop_event.is_set(): break was introduced. This change ensures that if a stop event is set, the worker will immediately cease iterating through the remaining events from the current node's execution and will not add them to the _event_queue.