Chore: dispatch write to workers

Build/lib/rules/output-worker-farm.ts

@@ -0,0 +1,30 @@
+import type { JestWorkerFarm } from 'jest-worker';
+import { createWorker } from '../worker';
+
+type OutputWorkerModule = typeof import('./output.worker');
+type OutputWorkerFarm = JestWorkerFarm<Pick<OutputWorkerModule, 'writeOutput'>>;
+
+let farm: OutputWorkerFarm | null = null;
+
+/**
+ * Lazily boot the output worker farm. Only FileOutput#write dispatches here, and
+ * only when an output crosses the offload threshold -- today that is exclusively
+ * the reject domainsets / adguardhome outputs of build-reject-domainset.
+ *
+ * IMPORTANT: whoever triggers the lazy boot is responsible for a matching
+ * endOutputWorkerFarm() (idempotent, safe to call unconditionally), otherwise the
+ * worker threads keep a standalone task run alive.
+ */
+export function getOutputWorkerFarm(): OutputWorkerFarm {
+  // 3 workers: reject, reject_extra and reject_phishing format & write in parallel
+  farm ??= createWorker<OutputWorkerModule>(require.resolve('./output.worker'), 3)(['writeOutput']);
+  return farm;
+}
+
+export async function endOutputWorkerFarm(): Promise<void> {
+  if (farm) {
+    const f = farm;
+    farm = null;
+    await f.end();
+  }
+}