test(server): probe config timing under CPU contention

packages/server/src/utils/paseo-config-file.test.ts

@@ -1,6 +1,8 @@
 import { mkdirSync, mkdtempSync, readFileSync, realpathSync, rmSync, writeFileSync } from "node:fs";
 import { execFile } from "node:child_process";
 import { promisify } from "node:util";
+import { Worker } from "node:worker_threads";
+import { once } from "node:events";
 import { fileURLToPath } from "node:url";
 import { writeFile, truncate, rm } from "node:fs/promises";
 import { tmpdir } from "node:os";
@@ -62,6 +64,25 @@ async function measureConfigRead(directory: string, profile = false) {
   };
 }
 
+// Temporary diagnostic: keep scheduling pressure present throughout the paired
+// cold reads, rather than depending on which other CI files are still running.
+async function withCpuContention<T>(action: () => Promise<T>): Promise<T> {
+  const workers = Array.from(
+    { length: 3 },
+    () =>
+      new Worker(
+        'require("node:worker_threads").parentPort.postMessage("ready"); while (true) { Math.sqrt(Math.random()); }',
+        { eval: true },
+      ),
+  );
+  try {
+    await Promise.all(workers.map((worker) => once(worker, "message")));
+    return await action();
+  } finally {
+    await Promise.all(workers.map((worker) => worker.terminate()));
+  }
+}
+
 describe("paseo config file substrate", () => {
   let tempDir: string;
 
@@ -138,7 +159,7 @@ describe("paseo config file substrate", () => {
   });
 
   it.each(
-    Array.from({ length: 500 }, (_, repetition) =>
+    Array.from({ length: 100 }, (_, repetition) =>
       [false, true].map((profile) => ({ repetition, profile })),
     ).flat(),
   )(
@@ -152,7 +173,7 @@ describe("paseo config file substrate", () => {
       };
       await writeFile(join(tempDir, "paseo.json"), JSON.stringify(config));
       const { result, blockingMs, filesystemRequests, yields, workerMessages } =
-        await measureConfigRead(tempDir, profile);
+        await withCpuContention(() => measureConfigRead(tempDir, profile));
       expect(result).toEqual({ ok: true, config, revision: await statPaseoConfigPath(tempDir) });
       expect(blockingMs).toBeLessThan(20);
       expect(filesystemRequests).toBeGreaterThan(0);

packages/server/src/utils/test-fixtures/config-responsiveness.ts

@@ -51,6 +51,11 @@ activity.disable();
 process.stderr.write(`Config profiling: ${Boolean(profiler)}\n`);
 const profile = profiler ? (await profiler.post("Profiler.stop")).profile : null;
 profiler?.disconnect();
+if (spans.length > 0) {
+  process.stderr.write(
+    `Config long callbacks: ${JSON.stringify(spans.map(({ start, end, cpuMs }) => ({ wallMs: (end - start) / 1000, cpuMs })))}\n`,
+  );
+}
 if (profile && spans.length > 0) {
   const nodes = new Map(profile.nodes.map((node) => [node.id, node]));
   const parents = new Map<number, number>();
@@ -62,13 +67,15 @@ if (profile && spans.length > 0) {
   for (const [index, id] of (profile.samples ?? []).entries()) {
     const delta = profile.timeDeltas?.[index] ?? 0;
     sampleTime += delta;
-    if (spans.some((span) => sampleTime >= span.start && sampleTime <= span.end)) {
-      samples.set(id, (samples.get(id) ?? 0) + delta);
-    }
+    const overlap = spans.reduce(
+      (total, span) =>
+        total +
+        Math.max(0, Math.min(sampleTime, span.end) - Math.max(sampleTime - delta, span.start)),
+      0,
+    );
+    if (overlap > 0) samples.set(id, (samples.get(id) ?? 0) + overlap);
   }
-  process.stderr.write(
-    `Config long callbacks: ${JSON.stringify(spans.map(({ start, end, cpuMs }) => ({ wallMs: (end - start) / 1000, cpuMs })))}\n`,
-  );
+
   const hottest = [...samples]
     .sort((a, b) => b[1] - a[1])
     .slice(0, 20)