Bug: Race condition — concurrent requests corrupt MCTS parameters via shared global state #304

Summary

The Flask request handler proxy() mutates a module-level global server_config dict on every request. Under Flask's default threaded mode or any multi-threaded WSGI server (gunicorn, uWSGI, Hypercorn), concurrent requests overwrite each other's MCTS parameters, causing silent result degradation.

Affected Code

File: optillm/server.py, lines 747–749

@app.route('/v1/chat/completions', methods=['POST'])
def proxy():
    # ...
    server_config['mcts_depth'] = data.get('mcts_depth', server_config['mcts_depth'])
    server_config['mcts_exploration'] = data.get('mcts_exploration', server_config['mcts_exploration'])
    server_config['mcts_simulations'] = data.get('mcts_simulations', server_config['mcts_simulations'])

These mutated values are then read back by execute_single_approach() at lines 419–420:

return chat_with_mcts(system_prompt, initial_query, client, model,
    server_config['mcts_simulations'],
    server_config['mcts_exploration'],
    server_config['mcts_depth'], ...)

Reproduction

1. Start optillm with a multi-threaded WSGI server: gunicorn -w 1 --threads 4 optillm.server:app
2. Send two concurrent MCTS requests with different parameters:

# Terminal 1
curl -s -X POST http://localhost:8000/v1/chat/completions \
  -H 'Content-Type: application/json' \
  -d '{"model":"mcts-gpt-4o-mini","messages":[{"role":"user","content":"Solve hard problem"}],"mcts_simulations":20,"mcts_depth":5}' &

# Terminal 2
curl -s -X POST http://localhost:8000/v1/chat/completions \
  -H 'Content-Type: application/json' \
  -d '{"model":"mcts-gpt-4o-mini","messages":[{"role":"user","content":"Solve easy problem"}],"mcts_simulations":2,"mcts_depth":1}' &

One or both requests will use the wrong MCTS parameters.

Impact

• Incorrect AI results: reasoning quality degrades silently when a low-parameter request races a high-parameter request
• Cross-tenant data leakage: one user's request parameters bleed into another user's inference in shared deployments
• Affects any multi-threaded deployment — the standard production deployment model

Suggested Fix

Extract per-request MCTS parameters into a local dict instead of mutating global state:

@app.route('/v1/chat/completions', methods=['POST'])
def proxy():
    # Per-request overrides — do NOT write to server_config
    mcts_config = {
        'mcts_depth': data.get('mcts_depth', server_config['mcts_depth']),
        'mcts_exploration': data.get('mcts_exploration', server_config['mcts_exploration']),
        'mcts_simulations': data.get('mcts_simulations', server_config['mcts_simulations']),
    }
    # Pass mcts_config to execute_single_approach() instead of reading from server_config

Found via automated codebase analysis. Confirmed independently by three reviewers (Claude, Codex, Gemini).