feat(slots): push-driven failure detector for unexpected unit deaths#8
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When a slot's systemd unit died mid-life (OOM, segfault, image-pull failure during warmup), the state machine only flipped to ERROR on the next status() poll or after _await_ready's 180s grace — leaving SSE watchers stuck on "ready" for minutes before any error frame. Add a per-slot async watcher task that polls `systemctl is-active` every 2s while the slot is in READY/SERVING/IDLE. On inactive/failed, it pushes a forced ERROR transition (broadcast over SSE) within ~1s of detection. Watcher is spawned/cancelled automatically from _transition so live-state entry/exit drives lifecycle. Tests in tests/slots/test_fail_watcher.py simulate a unit dying and assert the ERROR transition + SSE frame land within 5s, plus a clean unload cancels the watcher cleanly with no spurious ERROR push. Shared fixtures (systemctl_stub, stub_await_ready, slot_root) moved from test_manager.py to tests/slots/conftest.py so the new test file reuses them. Refs task #11. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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Summary
systemctl is-activeevery 2s while READY/SERVING/IDLE_transitionso live-state entry/exit drives lifecycleTest plan
tests/slots/test_fail_watcher.pygreen (simulates unit dying, asserts ERROR + SSE within 5s; clean unload cancels watcher with no spurious push)🤖 Generated with Claude Code