Fix process hanging when global onmessage is set in main thread

src/bun.js/bindings/BunWorkerGlobalScope.cpp

@@ -17,24 +17,29 @@ void WorkerGlobalScope::onDidChangeListenerImpl(EventTarget& self, const AtomStr
 {
     if (eventType == eventNames().messageEvent) {
         auto& global = static_cast<WorkerGlobalScope&>(self);
+        auto* context = global.scriptExecutionContext();
+        // Only ref/unref the event loop if we're in a worker thread, not the main thread.
+        // In the main thread, onmessage handlers shouldn't keep the process alive.
+        bool shouldRefEventLoop = context && !context->isMainThread();
+
         switch (kind) {
         case Add:
-            if (global.m_messageEventCount == 0) {
-                global.scriptExecutionContext()->refEventLoop();
+            if (global.m_messageEventCount == 0 && shouldRefEventLoop) {
+                context->refEventLoop();
             }
             global.m_messageEventCount++;
             break;
         case Remove:
             global.m_messageEventCount--;
-            if (global.m_messageEventCount == 0) {
-                global.scriptExecutionContext()->unrefEventLoop();
+            if (global.m_messageEventCount == 0 && shouldRefEventLoop) {
+                context->unrefEventLoop();
             }
             break;
         // I dont think clear in this context is ever called. If it is (search OnDidChangeListenerKind::Clear for the impl),
         // it may actually call once per event, in a way the Remove code above would suffice.
         case Clear:
-            if (global.m_messageEventCount > 0) {
-                global.scriptExecutionContext()->unrefEventLoop();
+            if (global.m_messageEventCount > 0 && shouldRefEventLoop) {
+                context->unrefEventLoop();
             }
             global.m_messageEventCount = 0;
             break;

test/js/web/workers/onmessage-main-thread.test.ts

@@ -0,0 +1,70 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, tempDir } from "harness";
+
+test("setting global onmessage in main thread should not prevent process exit", async () => {
+  // This test verifies that setting a global onmessage handler in the main thread
+  // doesn't keep the event loop alive and prevent the process from exiting.
+  // This was a bug where packages like 'lzma' that detect Web Worker environments
+  // by checking `typeof onmessage !== 'undefined'` would inadvertently keep the
+  // process alive.
+
+  using dir = tempDir("onmessage-test", {
+    "test.js": `
+      // Set a global onmessage handler (simulating what the lzma package does)
+      onmessage = function(e) {
+        console.log('received message:', e);
+      };
+      console.log('OK');
+      // Process should exit here, not hang
+    `,
+  });
+
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "test.js"],
+    env: bunEnv,
+    cwd: String(dir),
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  expect(stdout.trim()).toBe("OK");
+  expect(stderr).toBe("");
+  expect(exitCode).toBe(0);
+}, 5000); // 5 second timeout - should exit quickly
+
+test("setting global onmessage in worker thread should work normally", async () => {
+  // This test verifies that onmessage in a worker thread still works correctly
+  // and doesn't exit prematurely.
+
+  using dir = tempDir("onmessage-worker-test", {
+    "worker.js": `
+      onmessage = function(e) {
+        postMessage('received: ' + e.data);
+      };
+    `,
+    "main.js": `
+      const worker = new Worker(new URL('worker.js', import.meta.url).href);
+      worker.postMessage('hello');
+      worker.onmessage = (e) => {
+        console.log(e.data);
+        worker.terminate();
+      };
+    `,
+  });
+
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "main.js"],
+    env: bunEnv,
+    cwd: String(dir),
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  expect(stdout.trim()).toBe("received: hello");
+  expect(stderr).toBe("");
+  expect(exitCode).toBe(0);
+}, 5000);