fix: enforce thread-safety during reranker initialization #815

[suhaniiz]

📋 PR Checklist

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🔗 Related Issue

Closes #815

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📝 What does this PR do?

This PR resolves concurrency and race condition vulnerabilities during model initialization in app/reranker.py:

• It implements two global mutual exclusion locks (_model_lock and _instance_lock) using Python's built-in threading.Lock.
• It applies a highly efficient Double-Check Locking Pattern inside both the global get_reranker() singleton accessor and the internal lazy-loading _load_model() method.
• This guarantees that even under heavy, concurrent multi-threaded or asynchronous web traffic (e.g., instant high-volume startup requests via FastAPI/Uvicorn), the heavy cross-encoder model weights are loaded into CPU/GPU memory exactly once.

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🗂️ Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🔧 Refactor / code cleanup
• 📝 Documentation update
• 🎨 UI / styling change
• ⚙️ CI / tooling / config change
• 🧪 Tests

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🧪 How was this tested?

• Tested the affected API endpoints manually
• Ran the backend locally (uvicorn app.main:app --reload)
• Ran the frontend locally (npm run dev inside frontend/)
• Added / updated tests

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📸 Screenshots (if UI change)

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⚠️ Anything to flag for reviewers?

The double-check locking mechanic keeps performance optimal because threads only acquire the block lock during the very first initialization window. Once _reranker_instance and _model are populated, subsequent access bypasses the lock entirely, ensuring zero overhead for concurrent production pipelines.

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✅ Self-Review Checklist

• My branch is based on dev, not main
• I have not added any secrets / API keys
• I have not modified main branch or any HuggingFace deployment config
• My code follows the existing style (no unnecessary formatting changes)
• I have updated relevant docs / comments if needed

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