Fix memory error in threaded engine (apache#6084)

src/engine/threaded_engine.cc

@@ -349,6 +349,7 @@ void ThreadedEngine::WaitForAll() {
 }
 
 inline void ThreadedEngine::OnComplete(ThreadedOpr* threaded_opr) {
+  bool is_temporary_opr = threaded_opr->temporary;
   // Mark complete for read variables
   for (auto&& i : threaded_opr->const_vars) {
     i->CompleteReadDependency([this](OprBlock* opr) {
@@ -376,6 +377,10 @@ inline void ThreadedEngine::OnComplete(ThreadedOpr* threaded_opr) {
       ThreadedVar::Delete(i);
     }
   }
+  // The function been pushed from `ThreadedEngine::DeleteOperator`
+  // could execute right after we mark all vars as complete, so if
+  // threaded_opr is not temporary, its value is not reliable
+  // anymore start from here.
   int npending;
   {
     std::unique_lock<std::mutex> lock{finished_m_};
@@ -388,7 +393,7 @@ inline void ThreadedEngine::OnComplete(ThreadedOpr* threaded_opr) {
   }
 
   // delte operator if it is temperory
-  if (threaded_opr->temporary) {
+  if (is_temporary_opr) {
     ThreadedOpr::Delete(threaded_opr);
   }
 }