server: Added thread check to protect writedata

Source/cmConnection.cxx

@@ -69,6 +69,12 @@ bool cmEventBasedConnection::IsOpen() const
 
 void cmEventBasedConnection::WriteData(const std::string& _data)
 {
+#ifndef NDEBUG
+  auto curr_thread_id = uv_thread_self();
+  assert(this->Server);
+  assert(uv_thread_equal(&curr_thread_id, &this->Server->ServeThreadId));
+#endif
+
   auto data = _data;
   assert(this->WriteStream);
   if (BufferStrategy) {

Source/cmServer.cxx

@@ -436,6 +436,12 @@ bool cmServerBase::StartServeThread()
 
 bool cmServerBase::Serve(std::string* errorMessage)
 {
+#ifndef NDEBUG
+  uv_thread_t blank_thread_t = {};
+  assert(uv_thread_equal(&blank_thread_t, &ServeThreadId));
+  ServeThreadId = uv_thread_self();
+#endif
+
   errorMessage->clear();
 
   uv_signal_init(&Loop, &this->SIGINTHandler);

Source/cmServer.h

@@ -67,6 +67,15 @@ protected:
   bool ServeThreadRunning = false;
   uv_thread_t ServeThread;
   uv_async_t ShutdownSignal;
+#ifndef NDEBUG
+public:
+  // When the server starts it will mark down it's current thread ID,
+  // which is useful in other contexts to just assert that operations
+  // are performed on that same thread.
+  uv_thread_t ServeThreadId = {};
+
+protected:
+#endif
 
   uv_loop_t Loop;