test: time out on CPU time, not wall time

test/budget.test.ts

@@ -0,0 +1,14 @@
+import { spawnSync } from "node:child_process";
+
+import { expect, test } from "vitest";
+
+import { spent } from "./budget";
+
+test.runIf(process.platform === "linux")("a test's CPU counts a child it waited for", () => {
+	const before = spent();
+	// The child spins until it has spent 400 ms of CPU, however long a loaded host takes to give it that.
+	const spin = "while (process.cpuUsage().user + process.cpuUsage().system < 400_000);";
+	expect(spawnSync(process.execPath, ["-e", spin]).status).toBe(0);
+	// Each of the four tick counters may round down by one tick.
+	expect(spent() - before).toBeGreaterThanOrEqual(400 - 40);
+});

test/budget.ts

@@ -0,0 +1,35 @@
+// Fails a test that spends more than CPU_BUDGET_MS of CPU, counting its hooks and the processes it
+// spawned and waited for. A wall-clock limit would instead measure the host's load, so the wall
+// timeout in vitest.config.ts only catches a test awaiting what never comes.
+
+import { readFileSync } from "node:fs";
+
+import { afterEach, beforeEach } from "vitest";
+
+/** The CPU one test may spend, five times the most any test spent on Linux (1.9 s, 2026-11-11). */
+const CPU_BUDGET_MS = 10_000;
+
+/**
+ * This process's CPU time and that of every child it waited for, in milliseconds. A child counts
+ * once reaped, as when a test awaits it. Node reports only its own time (`process.cpuUsage`).
+ */
+export function spent(): number {
+	const stat = readFileSync("/proc/self/stat", "utf8");
+	// The command name, in parentheses, may hold spaces; utime, stime, cutime, and cstime follow it
+	// as fields 14 to 17.
+	const fields = stat.slice(stat.lastIndexOf(")") + 2).split(" ");
+	// Linux reports these in USER_HZ, 100 per second on every architecture Node supports.
+	return fields.slice(11, 15).reduce((sum, ticks) => sum + Number(ticks), 0) * 10;
+}
+
+// Only Linux exposes its children's CPU (`/proc`), so elsewhere the wall timeout alone applies.
+if (process.platform === "linux") {
+	let start = 0;
+	beforeEach(() => {
+		start = spent();
+	});
+	afterEach(() => {
+		const used = spent() - start;
+		if (used > CPU_BUDGET_MS) throw new Error(`test spent ${used} ms of CPU, over its ${CPU_BUDGET_MS} ms budget`);
+	});
+}