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 enters a wait loop using WaitForSingleOject(), this is triggered by a different C++ app (An X-Plane plugin) writing to a bit of shared memory. I'm trying to get my Node plugin to wake up when the Windows shared event is signaled then call a JavaScript function that I've registered from the node app, which will in turn pass the data which originated in X-Plane back to Node and the web world.

I've managed to work out how to register a JavaScript function and call it from C++, but only in the main V8 thread. I can't seem to find a way of calling the function from the std::thread.

I've tried various approaches, Locker objects (variable success), Persistent functions (didn't work), saving the main isolate object, entering/exiting the isolate, but if/when the code eventually reaches the function object it's not valid.

I get different results, ranging from crashing to freezing depending on whether I create various locker and unlocker objects.

I'm totally new to V8, so I'm not really sure I'm doing anything right. The code in question as follows:

If anyone could help at all I'll be eternally grateful!.

float* mem = 0;
HANDLE event = NULL;
Isolate* thisIsolate;

void readSharedMemory()
{
    //Isolate* isolate = Isolate::GetCurrent();
    //HandleScope scope(isolate);

    thisIsolate->Enter();
    v8::Locker locker(thisIsolate);
    v8::Isolate::Scope isolateScope(thisIsolate);
    //HandleScope scope(thisIsolate);        

    //v8::Local<Value> myVal = v8::String::NewFromUtf8(isolate, "Plugin world");
    v8::Local<Value> myVal = v8::Number::New(thisIsolate, *mem);

    // If it get's this far 'myFunction' is not valid
    bool isFun = myFunction->IsFunction();
    isFun = callbackFunction->IsFunction();

    v8::Context *thisContext = *(thisIsolate->GetCurrentContext());
    myFunction->Call(thisContext->Global(), 1, &(Handle<Value>(myVal)));
}

void registerCallback(const FunctionCallbackInfo<Value>& args)
{
    Isolate* isolate = Isolate::GetCurrent();
    v8::Locker locker(isolate);
    HandleScope scope(isolate);

    /** Standard parameter checking code removed **/

    // Various attempts at saving a function object
    v8::Local<v8::Value> func = args[0];
    bool isFun = func->IsFunction();

    Handle<Object> callbackObject = args[0]->ToObject();

    callbackFunction = Handle<Function>::Cast(callbackObject);
    isFun = callbackFunction->IsFunction();

    // save the function call object - This appears to work
    myFunction = v8::Function::Cast(*callbackObject);
    isFun = myFunction->IsFunction();


    // Test the function - this works *without* the Unlocker object below
    v8::Local<Value> myVal = v8::String::NewFromUtf8(isolate, "Plugin world");   
    myFunction->Call(isolate->GetCurrentContext()->Global(), 1, &(Handle<Value>(myVal)));
}

void threadFunc()
{
    thisIsolate->Exit();
    // If I include this unlocker, the function call test above fails.
    // If I don't include it, the app hangs trying to create the locker in 'readSharedMemory()'
    //v8::Unlocker unlocker(thisIsolate); 

    event = OpenEventW(EVENT_ALL_ACCESS, FALSE, L"Global\\myEventObject");
    DWORD err = GetLastError();

    //thisIsolate = v8::Isolate::New();

    std::cout << "Hello from thread" << std::endl;
    bool runThread = true;

    while (runThread)
    {
        DWORD dwWaitResult;
        DWORD waitTime = 60000;
        dwWaitResult = WaitForSingleObject(event, waitTime);

        err = GetLastError();

        if (dwWaitResult == WAIT_TIMEOUT)
            runThread = false;

        // event has been signaled - continue
        readSharedMemory();    
    }
}

void init(Handle<Object> exports) 
{
    /** NODE INITILISATION STUFF REMOVED **/

    // save the isolate - Is this a safe thing to do?
    thisIsolate = Isolate::GetCurrent();
    //Launch a thread
    eventThread = std::thread(threadFunc);
}

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)