MDEV-8253 EXPLAIN SELECT prints unexpected characters

Item_string::clone_item() creates a new Item_string that
points exactly to the same buffer that the original one does.
Later, Item_string::print() uses c_ptr() for the original Item_string,
which reallocs the original buffer, and the clone remain with
the old freed buffer.
Refactoring the code not to use c_ptr() in Item_string::print().

1 files changed, 9 insertions, 1 deletions

diff --git a/sql/sql_string.h b/sql/sql_string.h
index 99fe6b60d37..51a11c7a4ff 100644
--- a/sql/sql_string.h
+++ b/sql/sql_string.h
@@ -568,7 +568,15 @@ public:
     str_length+= arg_length;
     return FALSE;
   }
-  void print(String *print) const;
+  void print(String *to) const;
+  void print_with_conversion(String *to, CHARSET_INFO *cs) const;
+  void print(String *to, CHARSET_INFO *cs) const
+  {
+    if (my_charset_same(charset(), cs))
+      print(to);
+    else
+      print_with_conversion(to, cs);
+  }
 
   bool append_for_single_quote(const char *st, uint len);
   bool append_for_single_quote(const String *s)