[libcxxabi] Use InitByteFutex for __cxa_guard in WASM Workers

system/lib/libcxxabi/src/cxa_guard_impl.h

@@ -436,6 +436,16 @@ void PlatformFutexWake(int* addr) {
   __tsan_release(addr);
   syscall(SYS_futex, addr, WAKE, INT_MAX);
 }
+#elif defined(__EMSCRIPTEN__) && defined(__EMSCRIPTEN_SHARED_MEMORY__)
+// WASM Workers: pthread stubs are noops, making GlobalMutex broken
+// (non-atomic read-then-write allows double initialization).
+// InitByteFutex uses atomic CAS for correct single initialization.
+// Wait/wake are no-ops — losers spin in the CAS retry loop.
+// Cannot use memory.atomic.wait32 (traps on the main browser thread).
+// In practice, contention on a single guard is rare and the spin is
+// bounded by the static constructor duration (typically sub-microsecond).
+void PlatformFutexWait(int*, int) {}
+void PlatformFutexWake(int*) {}
 #else
 constexpr void (*PlatformFutexWait)(int*, int) = nullptr;
 constexpr void (*PlatformFutexWake)(int*) = nullptr;

tools/system_libs.py

@@ -1648,6 +1648,8 @@ def get_cflags(self):
     cflags = super().get_cflags()
     if not self.is_mt and not self.is_ww:
       cflags.append('-D_LIBCXXABI_HAS_NO_THREADS')
+    elif self.is_ww:
+      cflags.append('-D_LIBCXXABI_USE_FUTEX')
     match self.eh_mode:
       case Exceptions.NONE:
         cflags.append('-D_LIBCXXABI_NO_EXCEPTIONS')