inspector: make sure all messages are dispatched

This fixes a race condition when messages are coming while V8 is still
dispatching the previous batch.

PR-URL: https://github.com/nodejs/node/pull/8264
Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>

src/inspector_agent.cc

@@ -197,7 +197,7 @@ class AgentImpl {
   void SetConnected(bool connected);
   void DispatchMessages();
   void Write(int session_id, const String16& message);
-  void AppendMessage(MessageQueue* vector, int session_id,
+  bool AppendMessage(MessageQueue* vector, int session_id,
                      const String16& message);
   void SwapBehindLock(MessageQueue* vector1, MessageQueue* vector2);
   void PostIncomingMessage(const String16& message);
@@ -666,10 +666,12 @@ void AgentImpl::WorkerRunIO() {
   CHECK_EQ(err, 0);
 }
 
-void AgentImpl::AppendMessage(MessageQueue* queue, int session_id,
+bool AgentImpl::AppendMessage(MessageQueue* queue, int session_id,
                               const String16& message) {
   Mutex::ScopedLock scoped_lock(queue_lock_);
+  bool trigger_pumping = queue->empty();
   queue->push_back(std::make_pair(session_id, message));
+  return trigger_pumping;
 }
 
 void AgentImpl::SwapBehindLock(MessageQueue* vector1, MessageQueue* vector2) {
@@ -678,13 +680,14 @@ void AgentImpl::SwapBehindLock(MessageQueue* vector1, MessageQueue* vector2) {
 }
 
 void AgentImpl::PostIncomingMessage(const String16& message) {
-  AppendMessage(&incoming_message_queue_, frontend_session_id_, message);
+  if (AppendMessage(&incoming_message_queue_, frontend_session_id_, message)) {
     v8::Isolate* isolate = parent_env_->isolate();
     platform_->CallOnForegroundThread(isolate,
                                       new DispatchOnInspectorBackendTask(this));
     isolate->RequestInterrupt(InterruptCallback, this);
     uv_async_send(data_written_);
   }
+}
 
 void AgentImpl::OnInspectorConnectionIO(inspector_socket_t* socket) {
   if (client_socket_) {
@@ -698,10 +701,16 @@ void AgentImpl::OnInspectorConnectionIO(inspector_socket_t* socket) {
 }
 
 void AgentImpl::DispatchMessages() {
+  // This function can be reentered if there was an incoming message while
+  // V8 was processing another inspector request (e.g. if the user is
+  // evaluating a long-running JS code snippet). This can happen only at
+  // specific points (e.g. the lines that call inspector_ methods)
   if (dispatching_messages_)
     return;
   dispatching_messages_ = true;
   MessageQueue tasks;
+  do {
+    tasks.clear();
     SwapBehindLock(&incoming_message_queue_, &tasks);
     for (const MessageQueue::value_type& pair : tasks) {
       const String16& message = pair.second;
@@ -725,6 +734,7 @@ void AgentImpl::DispatchMessages() {
         inspector_->dispatchMessageFromFrontend(message);
       }
     }
+  } while (!tasks.empty());
   uv_async_send(data_written_);
   dispatching_messages_ = false;
 }