lp:910817: Race condition in kill_threads_for_user()

The code was accessing a pointer in a mem_root that might be freed by
another concurrent thread. Fix by moving the access to be done while the
LOCK_thd_data is held, preventing the memory from being freed too early.

sql/sql_parse.cc

@@ -7363,13 +7363,23 @@ static uint kill_threads_for_user(THD *thd, LEX_USER *user,
   if (!threads_to_kill.is_empty())
   {
     List_iterator_fast<THD> it(threads_to_kill);
-    THD *ptr;
-    while ((ptr= it++))
+    THD *next_ptr;
+    THD *ptr= it++;
+    do
     {
       ptr->awake(kill_signal);
+      /*
+        Careful here: The list nodes are allocated on the memroots of the
+        THDs to be awakened.
+        But those THDs may be terminated and deleted as soon as we release
+        LOCK_thd_data, which will make the list nodes invalid.
+        Since the operation "it++" dereferences the "next" pointer of the
+        previous list node, we need to do this while holding LOCK_thd_data.
+      */
+      next_ptr= it++;
       pthread_mutex_unlock(&ptr->LOCK_thd_data);
       (*rows)++;
-    }
+    } while ((ptr= next_ptr));
   }
   DBUG_RETURN(0);
 }