backend/tests/api/test_engine_domain.py

import pytest
from unittest.mock import patch, MagicMock

def test_start_engine(client):
    """Test starting the engine with a story id, without actually spawning process."""
    with patch("subprocess.Popen") as mock_popen:
        mock_process = MagicMock()
        mock_process.pid = 12345
        mock_popen.return_value = mock_process
        
        response = client.post("/api/engine/start", json={"story_id": "test_story"})
        
        # Depending on if it sees 'asset_generator' in psutil, it might return 400 or 200
        # If no other generator was running during test (which is expected):
        if response.status_code == 200:
            assert response.json()["status"] == "success"
            assert response.json()["pid"] == 12345
            assert mock_popen.called
        else:
            # If the engine was truly running on the host, it skips Popen.
            assert response.status_code == 400
            assert "Engine is already running" in response.json()["error"]