V8 Multithreaded function

Question

I\'m writing a Node plugin and I\'m having problems trying to call a V8 function object from a C++ worker thread.

My plugin basically starts a C++ std::thread and en

Answer 1

You probably need a bit of libuv magic to get the main node.js/v8 thread to execute your callback from another thread. This will involve:

• A uv_async_t handle which acts as the wake-up call for the main v8 thread:

  extern uv_async_t       async;
  

• A uv_async_init call which binds the uv_async_t to the V8 default loop:

  uv_async_init(uv_default_loop(), &async, async_cb_handler);
  

• And a event handler which will act on the uvasync_t event on the v8 main thread:

  void async_cb_handler(uv_async_t *handle) {
      NotifInfo *notif;
      mutex::scoped_lock sl(zqueue_mutex);
      while (!zqueue.empty()) {
          notif = zqueue.front();
          handleNotification(notif);
          delete notif;
          zqueue.pop();
      }
  }
  

• Finally, you'll also probably need a mutex-protected queue to be able to pass some data from the c++ addon thread to Node/V8:

  extern mutex                   zqueue_mutex;
  extern std::queue<NotifInfo *> zqueue;
  

• When something happens in your C++ thread, just push a new item to the mutex-protected queue, and call uv_async_send to wake up V8's default event loop (the so called 'main thread') to process the item (which can then call your Javascript callbacks)

  void ozw_watcher_callback(OpenZWave::Notification const *cb, void *ctx) {
      NotifInfo *notif = new NotifInfo();
      notif->type   = cb->GetType();
      notif->homeid = cb->GetHomeId();
      ...
      mutex::scoped_lock sl(zqueue_mutex);
      zqueue.push(notif);
      uv_async_send(&async);
  }
  

(Code snippets taken from the official Node.JS addon for OpenZWave)