feat: allow fail fast

[fatelei]

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Summary

fix #30225

allow fail fast, not wait all

Screenshots

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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 enhances the system's resilience and responsiveness by introducing a 'fail-fast' strategy for concurrent retrieval operations. Instead of waiting for all parallel tasks to finish, the system will now immediately halt and report an error if any individual task fails. This prevents unnecessary resource consumption and provides quicker feedback in case of issues during data retrieval.

Highlights

• Fail-Fast Mechanism for Concurrent Operations: Implemented a 'fail-fast' approach for concurrent tasks, ensuring that if any task encounters an exception, remaining tasks are immediately cancelled, and the error is propagated without waiting for all tasks to complete.
• Concurrent Futures Update: Modified retrieval_service.py to switch from concurrent.futures.wait(..., return_when=ALL_COMPLETED) to iterating through concurrent.futures.as_completed, allowing for early detection of exceptions and cancellation of outstanding futures.
• Thread-Based Cancellation and Exception Handling: Introduced threading.Event and a shared exception list in dataset_retrieval.py to manage cancellation signals and collect exceptions across multiple worker threads, enabling fail-fast behavior for thread-based operations.
• Test Suite Alignment: Updated unit tests to mock concurrent.futures.as_completed to accurately reflect the new fail-fast behavior in the retrieval_service module.

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[gemini-code-assist]

Code Review

This pull request introduces fail-fast behavior for concurrent retrieval tasks, which is a great improvement for efficiency and error handling. When one task fails, it attempts to cancel the others and stop waiting. I've reviewed the changes and found a critical issue in retrieval_service.py where the fail-fast condition is incorrect and will never trigger. Additionally, I've identified a couple of places in dataset_retrieval.py where the implementation could be improved to make the fail-fast behavior more responsive. My detailed comments provide suggestions for these points.