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Aleksei Sviridkin
db2ddfab2b
The e2e sandbox boots three QEMU management nodes at `-smp 8`, so 24 guest vCPUs are committed before any tenant VM starts. The nightly lane ran that sandbox on a 24-vCPU runner, a 1:1 commit that leaves the host nothing for QEMU's emulation and IO threads, containerd, the kernel and the runner agent. The tag, fork and pull-request lanes already run the same sandbox on the 32-vCPU class, so move nightly there too. This aligns headroom and claims no performance effect. Measured boot data shows the load a workflow puts on the host, not the runner class, dominates guest starvation, and the two are confounded in that data; no red run is attributed to the class here. The case is the arithmetic: a 1:1 commit leaves no headroom, and two lanes running the identical sandbox on different classes is drift nothing was watching. Add hack/sandbox-runner-headroom.bats to keep the two sides paired. It derives the guest vCPU and memory demand from the sandbox bats file, and the runner class of every job that drives the sandbox make targets, then requires the runner to be strictly larger on both axes. Neither side is a literal list, so raising `-smp`, adding a management node or adding a lane on a smaller runner turns it red instead of drifting apart silently. Input the derivation cannot read in full is refused rather than guessed: a guess produces a demand smaller than the real one, which passes a runner that cannot hold the sandbox, while a refusal names the line. That covers a launch written outside any `@test` block, a block left open at end of file, a `for` opener the strict pattern misses, a non-literal loop list, and a `while` loop sharing the block. The derivation runs against the live tree, which passes whether or not those refusals still work, so each one is pinned by a fixture feeding a synthetic file to the same program. Loosening the parser turns a fixture red rather than quietly widening what the check accepts. Record the rule alongside the other environment invariants in docs/agents/e2e-testing.md, so a new lane picks its runner class from the documentation rather than from a failing unit suite. Assisted-By: Claude <noreply@anthropic.com> Signed-off-by: Aleksei Sviridkin <f@lex.la>