Fix race condition in notes-data.json read-modify-write

[anshul23102]

Add threading lock to protect concurrent access to notes-data.json, preventing silent data loss when multiple uploads/deletes happen simultaneously.

Problem

When two upload or delete requests arrive concurrently:

1. Request A reads notes-data.json
2. Request B reads the same version (before A writes)
3. Request A writes its changes
4. Request B writes its changes (silently overwriting A's data)
5. Result: A's changes are lost with no error returned

Solution

• Add module-level _notes_lock = threading.Lock()
• Wrap read-modify-write operations in both functions with the lock
• File uploads happen outside the lock for performance

Changes

• app.py: Added threading lock protection to critical sections
• Both admin_upload() and delete_material() now use with _notes_lock:
• Lock ensures only one thread reads and writes at a time

Testing

• Concurrent uploads: Each upload's metadata is preserved
• Concurrent deletes: Each delete's JSON update is applied atomically
• File operations succeed without data corruption

Resolves #289

Work completed under GSSoC'26

[github-actions]

📥 Pull Request Received

Thank you for your contribution to College Daddy. Your pull request has been received and is currently under review.

🔗 Linked Issues

• [Bug][Level 2] app.py: concurrent uploads and deletes race on notes-data.json read-modify-write, causing silent data loss #289: [Bug][Level 2] app.py: concurrent uploads and deletes race on notes-data.json read-modify-write, causing silent data loss

✅ Pre-Merge Checklist

Please ensure the following requirements are met:

• Changes adhere to contribution guidelines
• Related issues are referenced in description
• All changes tested and verified locally
• Code follows project standards

👤 Reviewer Assigned

@mugenkyou has been assigned to review this pull request.

---

Our team will review your submission shortly. We appreciate your effort in improving the platform for students.