DittoPglite: prevent starting PGlite instances in worker threads

src/utils/worker.test.ts

@@ -0,0 +1,30 @@
+import { assertEquals } from '@std/assert';
+
+import { isWorker } from '@/utils/worker.ts';
+
+Deno.test('isWorker from the main thread returns false', () => {
+  assertEquals(isWorker(), false);
+});
+
+Deno.test('isWorker from a worker thread returns true', async () => {
+  const script = `
+    import { isWorker } from '@/utils/worker.ts';
+    postMessage(isWorker());
+    self.close();
+  `;
+
+  const worker = new Worker(
+    URL.createObjectURL(new Blob([script], { type: 'application/javascript' })),
+    { type: 'module' },
+  );
+
+  const result = await new Promise<boolean>((resolve) => {
+    worker.onmessage = (e) => {
+      resolve(e.data);
+    };
+  });
+
+  worker.terminate();
+
+  assertEquals(result, true);
+});

src/utils/worker.ts

@@ -0,0 +1,9 @@
+/**
+ * Detect if this thread is running in a Worker context.
+ *
+ * https://stackoverflow.com/a/18002694
+ */
+export function isWorker(): boolean {
+  // @ts-ignore This is fine.
+  return typeof WorkerGlobalScope !== 'undefined' && self instanceof WorkerGlobalScope;
+}