Fix the potential memory leak issue caused by query_result_memory not being properly released

Author: wudidapaopao

src/Client/ClientBase.cpp

@@ -523,7 +523,14 @@ try
         else
         {
             // The underlying vector<char> will be freed by `free_result` from Python side.
-            query_result_memory = new std::vector<char>(4096);
+            // When the input query contains multiple statements, such as "SELECT 1; SELECT 2". When executing "SELECT 2",
+            // the query_result_memory corresponding to SELECT 1 has not been "stolen" by function stealQueryOutputVector,
+            // so it remains non-empty.
+            if (query_result_memory)
+                query_result_memory->resize(4096);
+            else
+                query_result_memory = new std::vector<char>(4096);
+
             query_result_buf = std::make_shared<WriteBufferFromVector<std::vector<char>>>(*query_result_memory);
 
             out_buf = query_result_buf.get();

src/Client/ClientBase.h

@@ -107,13 +107,13 @@ class ClientBase
         return query_result_memory;
     }
 
-    /// Steals and returns the query output vector, replacing it with a new one
+    /// Steals and returns the query output vector.
+    /// Afterward, the content of query_result_memory is released by the Python side.
     std::vector<char> * stealQueryOutputVector()
     {
         auto * result = query_result_memory;
-        query_result_memory = new std::vector<char>(4096);
-        // WriteBufferFromVector takes a reference to the vector but doesn't own it
-        query_result_buf = std::make_shared<WriteBufferFromVector<std::vector<char>>>(*query_result_memory);
+        query_result_memory = nullptr;
+        query_result_buf.reset();
         return result;
     }