Five Hidden Bugs in a Polling Architecture and How Each Was Fixed

A code audit of a Python polling engine uncovered five critical bugs that could silently cripple system observability and error handling. Key failures included a missing PollStats.snapshot() method causing AttributeError on every metrics read, and a PollResult.down() factory that raised TypeError when error handlers passed an unexpected keyword argument. Backoff logic was effectively dead code for rate-limited and timeout scenarios, causing the engine to retry at full speed and worsen rate-limiting conditions. Ten consecutive timeouts failed to trigger a STUCK alarm because only empty-result counters advanced the threshold, leaving the system reporting healthy while broken. The hardened implementation addresses all five issues with explicit method signatures, bounded memory via deque and __slots__, and unified failure-count checks for stuck-state detection.