worker: move receiving_messages_ field to MessagePort

This is a property of the native object associated with the
`MessagePort`, not something that should be set on the
conceptual `MessagePort` that may be transferred around.

src/node_messaging.cc

@@ -588,9 +588,9 @@ void MessagePort::OnMessage() {
       Mutex::ScopedLock lock(data_->mutex_);
 
       Debug(this, "MessagePort has message, receiving = %d",
-            static_cast<int>(data_->receiving_messages_));
+            static_cast<int>(receiving_messages_));
 
-      if (!data_->receiving_messages_)
+      if (!receiving_messages_)
         break;
       if (data_->incoming_messages_.empty())
         break;
@@ -722,17 +722,16 @@ void MessagePort::PostMessage(const FunctionCallbackInfo<Value>& args) {
 }
 
 void MessagePort::Start() {
-  Mutex::ScopedLock lock(data_->mutex_);
   Debug(this, "Start receiving messages");
-  data_->receiving_messages_ = true;
+  receiving_messages_ = true;
+  Mutex::ScopedLock lock(data_->mutex_);
   if (!data_->incoming_messages_.empty())
     TriggerAsync();
 }
 
 void MessagePort::Stop() {
-  Mutex::ScopedLock lock(data_->mutex_);
   Debug(this, "Stop receiving messages");
-  data_->receiving_messages_ = false;
+  receiving_messages_ = false;
 }
 
 void MessagePort::Start(const FunctionCallbackInfo<Value>& args) {

src/node_messaging.h

@@ -116,7 +116,6 @@ class MessagePortData : public MemoryRetainer {
   // This mutex protects all fields below it, with the exception of
   // sibling_.
   mutable Mutex mutex_;
-  bool receiving_messages_ = false;
   std::list<Message> incoming_messages_;
   MessagePort* owner_ = nullptr;
   // This mutex protects the sibling_ field and is shared between two entangled
@@ -205,6 +204,7 @@ class MessagePort : public HandleWrap {
   void TriggerAsync();
 
   std::unique_ptr<MessagePortData> data_ = nullptr;
+  bool receiving_messages_ = false;
   uv_async_t async_;
 
   friend class MessagePortData;