Repair FastAPI Lifespan Shutdown Task Leak

Orphaned background workers that survive a FastAPI shutdown prevent clean redeploys and exhaust database connection pools. This skill identifies why tasks persist after listeners close, tracing the breakdown between lifespan hooks and asyncio task cancellation.

What it does

• Map lifespan hooks by identifying where shutdown signals fail to propagate to background workers.
• Trace cancellation paths to find blocking calls or shielded tasks that ignore CancelledError.
• Identify task ownership to locate unmanaged global tasks that lack a parent lifecycle controller.
• Inventory resource leaks by listing database handles and file descriptors left open by surviving tasks.
• Generate regression tests to ensure future deployments validate full task cleanup before exiting.

How it works

1. Analyze logs and source to establish the timeline of the shutdown sequence and task persistence.
2. Execute four-check matrix covering lifespan execution, cancellation paths, task ownership, and cleanup logic.
3. Verify findings against supplied service traces and dependency versions, labeling evidence as VERIFIED or INFERRED.
4. Deliver remediation proposal including a task shutdown timeline and a resource leak inventory.

Frameworks & tools

This skill is designed for FastAPI applications using asyncio or trio, specifically focusing on lifespan handlers and database drivers like asyncpg or SQLAlchemy.

Why this beats prompting it yourself

General prompts often miss the nuances of asyncio.shield or the difference between the legacy @app.on_event("shutdown") and the modern lifespan context manager. This skill enforces a strict evidence matrix that prevents false positives in complex race conditions.

Use cases

• Debugging DB connection limit errors during rolling updates.
• Fixing hanging CI/CD pipelines that timeout during service stop.
• Auditing third-party library background tasks for shutdown compliance.
• Developing robust integration tests for graceful service termination.

Known limitations

Requires specific source code revisions and reproduction logs to function. It proposes code changes but does not execute migrations or live rollbacks.