[embind] Initialize embind on wasm workers.

Fixes #24189

Author: brendandahl

src/lib/libwasm_worker.js

@@ -135,6 +135,11 @@ addToLibrary({
     writeStackCookie();
 #endif
 
+#if EMBIND
+    // Embind must initialize itself on all threads, as it generates support JS.
+    __embind_initialize_bindings();
+#endif
+
 #if AUDIO_WORKLET
     // Audio Worklets do not have postMessage()ing capabilities.
     if (typeof AudioWorkletGlobalScope === 'undefined') {

test/embind/test_embind_wasm_workers.cpp

@@ -0,0 +1,34 @@
+#include <stdio.h>
+#include <emscripten.h>
+#include <emscripten/console.h>
+#include <emscripten/bind.h>
+#include <emscripten/val.h>
+#include <emscripten/wasm_worker.h>
+
+using namespace emscripten;
+
+int foo() {
+  return 42;
+}
+
+void do_exit() {
+  emscripten_out("do_exit");
+  emscripten_terminate_all_wasm_workers();
+}
+
+void run_in_worker() {
+  emscripten_out("Hello from Wasm Worker!");
+  int result = val::module_property("foo")().as<int>();
+  assert(result == 42);
+  emscripten_wasm_worker_post_function_v(EMSCRIPTEN_WASM_WORKER_ID_PARENT, do_exit);
+}
+
+int main() {
+  emscripten_wasm_worker_t worker = emscripten_malloc_wasm_worker(/*stackSize: */1024);
+  emscripten_wasm_worker_post_function_v(worker, run_in_worker);
+  emscripten_exit_with_live_runtime();
+}
+
+EMSCRIPTEN_BINDINGS(xxx) {
+  function("foo", &foo);
+}

test/embind/test_embind_wasm_workers.out

@@ -0,0 +1,2 @@
+Hello from Wasm Worker!
+do_exit

test/test_core.py

@@ -7588,6 +7588,10 @@ def test_embind_no_rtti_followed_by_rtti(self):
     self.emcc_args += ['-lembind', '-fno-rtti', '-frtti']
     self.do_run(src, '418\ndotest returned: 42\n')
 
+  @no_asan('ASan does not support WASM_WORKERS')
+  def test_embind_wasm_workers(self):
+    self.do_run_in_out_file_test('embind/test_embind_wasm_workers.cpp', emcc_args=['-lembind', '-sWASM_WORKERS'])
+
   @parameterized({
     '': ('DEFAULT', False),
     'all': ('ALL', False),

tools/link.py

@@ -1405,6 +1405,8 @@ def limit_incoming_module_api():
 
   if settings.WASM_WORKERS:
     settings.DEFAULT_LIBRARY_FUNCS_TO_INCLUDE += ['$_wasmWorkerInitializeRuntime']
+    if settings.EMBIND:
+      settings.REQUIRED_EXPORTS.append('_embind_initialize_bindings')
     add_system_js_lib('libwasm_worker.js')
 
   # Set min browser versions based on certain settings such as WASM_BIGINT,