Fix Node.js exit for library builds with pthreads

src/lib/libpthread.js

@@ -208,11 +208,11 @@ var LibraryPThread = {
       // worker pool as an unused worker.
       worker.pthread_ptr = 0;
 
-#if ENVIRONMENT_MAY_BE_NODE && PROXY_TO_PTHREAD
+#if ENVIRONMENT_MAY_BE_NODE && (PROXY_TO_PTHREAD || !HAS_MAIN)
       if (ENVIRONMENT_IS_NODE) {
-        // Once the proxied main thread has finished, mark it as weakly
+        // Once the worker is returned to the pool, mark it as weakly
         // referenced so that its existence does not prevent Node.js from
-        // exiting.  This has no effect if the worker is already weakly
+        // exiting. This has no effect if the worker is already weakly
         // referenced.
         worker.unref();
       }
@@ -687,7 +687,7 @@ var LibraryPThread = {
     msg.moduleCanvasId = threadParams.moduleCanvasId;
     msg.offscreenCanvases = threadParams.offscreenCanvases;
 #endif
-#if ENVIRONMENT_MAY_BE_NODE
+#if ENVIRONMENT_MAY_BE_NODE && HAS_MAIN
     if (ENVIRONMENT_IS_NODE) {
       // Mark worker as weakly referenced once we start executing a pthread,
       // so that its existence does not prevent Node.js from exiting.  This

test/core/pthread/test_pthread_exit_library.c

@@ -0,0 +1,26 @@
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <assert.h>
+#include <emscripten.h>
+#include <emscripten/threading.h>
+
+void* worker(void* arg) {
+    printf("worker starting\n");
+    fflush(stdout);
+    emscripten_thread_sleep(100);
+
+    // proxy back to the main thread
+    MAIN_THREAD_ASYNC_EM_ASM({
+        resolve();
+    });
+    return NULL;
+}
+
+EMSCRIPTEN_KEEPALIVE
+void create_thread_async() {
+    pthread_t thread;
+    int rc = pthread_create(&thread, NULL, worker, NULL);
+    assert(rc == 0);
+    pthread_detach(thread);
+}

test/core/pthread/test_pthread_exit_library.out

@@ -0,0 +1,3 @@
+initialized
+worker starting
+exiting

test/core/pthread/test_pthread_exit_library.pre.js

@@ -0,0 +1,7 @@
+let { promise, resolve } = Promise.withResolvers();
+promise.then(() => console.log('exiting'));
+
+Module['onRuntimeInitialized'] = function() {
+  console.log('initialized');
+  _create_thread_async();
+};

test/test_core.py

@@ -9164,6 +9164,13 @@ def test_pthread_keepalive(self):
   def test_pthread_weak_ref(self):
     self.do_core_test('pthread/test_pthread_weak_ref.c')
 
+  @no_asan('asan exits the runtime')
+  @requires_pthreads
+  def test_pthread_exit_library(self):
+    # Test that Node.js doesn't exit while there are still pthreads running when there is no main function.
+    self.cflags += ['--pre-js', test_file('core/pthread/test_pthread_exit_library.pre.js')]
+    self.do_core_test('pthread/test_pthread_exit_library.c')
+
   @requires_pthreads
   def test_pthread_exit_main(self):
     self.do_core_test('pthread/test_pthread_exit_main.c')