Backend Hang Debug

Purpose

• Detect and resolve event-loop hangs where the FastAPI app stops responding (e.g., curl http://localhost:8000/ times out) due to synchronous executor shutdown in the SSE news stream.
• Provide a repeatable triage flow using py-spy to capture live stacks and pinpoint blocking code.

Scope

• Backend: backend/app/api/routes/stream.py (news stream), backend/app/services/rss_ingestion.py (RSS workers), startup processes.
• Tooling: py-spy for live stack dumps; curl with timeouts for smoke tests.

Quick Triage

1. Reproduce hang: curl -m 5 http://localhost:8000/ and curl -m 5 http://localhost:8000/health; note timeouts.
2. Process check: ss -tlnp | grep 8000 to confirm listener; ls /proc/$(pgrep -f "uvicorn app.main")/fd | wc -l to rule out FD leak.
3. Stack capture (inside backend venv): uv pip install py-spy then sudo /home/bender/classwork/Thesis/backend/.venv/bin/py-spy dump --pid $(pgrep -f "uvicorn app.main") (and worker pid if multiprocess). Look for ThreadPoolExecutor.shutdown in api/routes/stream.py frames.