Perf: worker farm

Build/lib/build-worker-farm.ts

@@ -0,0 +1,49 @@
+import type { JestWorkerFarm } from 'jest-worker';
+import { createWorker } from './worker';
+import { once } from 'foxts/once';
+
+type BuildWorkerModule = typeof import('./build.worker');
+type BuildWorkerFarm = JestWorkerFarm<BuildWorkerModule>;
+
+/**
+ * Every thread costs ~250ms of CPU to boot (Node + @swc-node/register + the module
+ * graph), and CI has 4 cores. One farm of 3 threads plus the main thread fills
+ * them exactly, and 3 is enough because the two phases that use the farm do not
+ * overlap and each needs at most 3 threads at once:
+ *
+ *   early:  getRejectSources | getPhishingDomains | buildMicrosoftCdn -> buildCdnDownloadConf
+ *   late:   writeOutput reject | reject_extra | reject_phishing -> reject-adguardhome
+ *
+ * jest-worker holds a lock per in-flight call, so a 4th concurrent call queues
+ * behind the first thread to free up; in the early phase that is the ~0.3s
+ * cdn-download-conf job waiting for the ~0.75s microsoft-cdn job, both well
+ * inside the ~2.4s the reject sources take on the critical path.
+ */
+const NUM_WORKERS = 3;
+
+/**
+ * IMPORTANT: whoever triggers the lazy boot is responsible for a matching
+ * endBuildWorkerFarm() (idempotent, safe to call unconditionally), otherwise the
+ * worker threads keep a standalone task run alive.
+ */
+export const getBuildWorkerFarm = once((): BuildWorkerFarm => createWorker<BuildWorkerModule>(require.resolve('./build.worker'), NUM_WORKERS)([
+  'writeOutput',
+  'getPhishingDomains',
+  'getRejectSources',
+  'buildMicrosoftCdn',
+  'buildCdnDownloadConf'
+]), false /* warm manually */);
+
+/**
+ * Boot the farm ahead of time so the thread spin-up overlaps with whatever the
+ * main thread is doing first, instead of landing on the critical path of the
+ * first job dispatched to it.
+ */
+export function warmBuildWorkerFarm(): void {
+  getBuildWorkerFarm();
+}
+
+export async function endBuildWorkerFarm(): Promise<void> {
+  const farm = getBuildWorkerFarm();
+  await farm.end();
+}