[WASM_WORKERS] Replace call_once usage in libc++ with atomics

system/lib/libcxx/include/__locale

@@ -146,7 +146,9 @@ private:
 };
 
 class _LIBCPP_EXPORTED_FROM_ABI locale::id {
+#if !defined(__EMSCRIPTEN__) || !defined(__EMSCRIPTEN_WASM_WORKERS__)
   once_flag __flag_;
+#endif
   int32_t __id_;
 
   static int32_t __next_id;

system/lib/libcxx/src/locale.cpp

@@ -589,7 +589,24 @@ void locale::facet::__on_zero_shared() noexcept { delete this; }
 constinit int32_t locale::id::__next_id = 0;
 
 long locale::id::__get() {
+#if defined(__EMSCRIPTEN__) && defined(__EMSCRIPTEN_WASM_WORKERS__)
+  // Avoid `call_once` under Emscripten since we want to avoid the pthread
+  // dependenency that it comes with.
+  // TODO(https://github.com/emscripten-core/emscripten/issues/26426):
+  // Remove this patch once we have some kind of locking primitive
+  // in Wasm Worker that can be used to implement call_once (or
+  // __libcpp_mutex_t/__libcpp_condvar_t).
+  if (__libcpp_atomic_load(&__id_) == 0) {
+    int32_t proposed_id = __libcpp_atomic_add(&__next_id, 1);
+    int32_t expected = 0;
+    // If we race with another thread here the CAS will fail and
+    // the proposed_id will be leaked, but __id_ will be non-zero
+    // in either case.
+    __libcpp_atomic_compare_exchange(&__id_, &expected, proposed_id);
+  }
+#else
   call_once(__flag_, [&] { __id_ = __libcpp_atomic_add(&__next_id, 1); });
+#endif
   return __id_ - 1;
 }
 

test/codesize/hello_libcxx.cpp

@@ -9,4 +9,3 @@ int main() {
   std::cout << "hello, world!" << std::endl;
   return 0;
 }
-

test/test_other.py

@@ -13537,6 +13537,9 @@ def test_wasm_worker_pthread_api_usage(self):
   def test_wasm_worker_cxx_init(self):
     self.do_run_in_out_file_test('wasm_worker/wasm_worker_cxx_init.cpp', cflags=['-sWASM_WORKERS'])
 
+  def test_wasm_worker_hello_libcxx(self):
+    self.do_runf('codesize/hello_libcxx.cpp', 'hello, world!\n', cflags=['-sWASM_WORKERS'])
+
   @parameterized({
     # we will warn here since -O2 runs the optimizer and -g enables DWARF
     'O2_g': (True, ['-O2', '-g']),