Stop the no-CPU test slice so guarded-runner runs stop leaking systemd units - #229

tests.test_adoption_guarded_runners.BoundedRunCpuQuotaTests.test_scope_with_a_dropped_or_altered_cpu_quota_is_refused_with_78 left one empty systemd user slice on the host every time it ran. Its dropped-CPUQuota stand-in launcher placed the scope in ecosystem-check-nocpu-$$.slice (the launcher's PID) and nothing stopped it. systemd keeps an emptied transient slice loaded, so the units piled up: a read-only host audit on the WSL2 host found 63 empty ecosystem-check-nocpu-*.slice units (Tasks 0, Memory 0B) spanning 21 hours.

Change. The slice name is fixed per test process (NOCPU_SLICE = ecosystem-check-nocpu-<test pid>.slice). The test registers a cleanup stop, then stops the slice itself and asserts its ActiveState is inactive. The slice is still one that no CPUQuota unit touches, so the refusal it exercises (cpu.max is not enforced) is unchanged. manifests/evidence.json re-registers the file's hash.

Checks (WSL2 Ubuntu 24.04, systemd 255, cgroup v2 with cpu delegated, Python 3.13, on 636751a):

• Leak reproduced and fixed: 62 leaked slices before. One run of the original test method: 63. The fixed BoundedRunCpuQuotaTests class (7 tests OK): still 63. The user journal shows Created slice ecosystem-check-nocpu-1273346.slice with no removal for the original, and Created then Removed slice ecosystem-check-nocpu-1273365.slice for the fix.
• python3 -m unittest tests.test_adoption_guarded_runners: 33 tests, OK (skipped=1).
• scripts/validate.py passed (5338 hashed files, 145 receipts), scripts/evidence_manifest.py --check passed, scripts/verdict_review_gate.py --base origin/main reports no verdict rows changed, and the gitleaks 8.30.1 pre-commit hook found no leaks. The full python3 -m unittest suite and the other validate.yml steps were not run locally; CI runs them.
• Independent read-only review (Claude evidence-reviewer): no defects. It noted one residual edge, not a defect: if a run is killed before cleanup and the OS later reuses the same PID for another test run, that run would reuse the stale slice (which still carries no CPUQuota). Cross-family Codex review was skipped because of a usage limit through 2026-09-30.

The 63 leaked slices on the audited host were stopped separately, and only those with 0 tasks.

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