MDEV-12666: CURRENT_ROLE() and DATABASE() does not work in a view

The problem lies in how CURRENT_ROLE is defined. The
Item_func_current_role inherits from Item_func_sysconst, which defines
a safe_charset_converter to be a const_charset_converter.

During view creation, if there is no role previously set, the current_role()
function returns NULL.

This is captured on item instantiation and the
const_charset_converter call subsequently returns an Item_null.
In turn, the function is replaced with Item_null and the view is
then created with an Item_null instead of Item_func_current_role.

Without this patch, the first SHOW CREATE VIEW from the testcase would
have a where clause of WHERE role_name = NULL, while the second SHOW
CREATE VIEW would show a correctly created view.

The same applies for the DATABASE function, as it can change as well.

There is an additional problem with CURRENT_ROLE() when used in a
prepared statement. During prepared statement creation we used to set
the string_value of the function to the current role as well as the
null_value flag. During execution, if CURRENT_ROLE was not null, the
null_value flag was never set to not-null during fix_fields.

Item_func_current_user however can never be NULL so it did not show this
problem in a view before. At the same time, the CURRENT_USER() can not
be changed between prepared statement execution and creation so the
implementation where the value is stored during fix_fields is
sufficient.

Note also that DATABASE() function behaves differently during prepared
statements. See bug 25843 for details or commit
7e0ad09edff587dadc3e9855fc81e1b7de8f2199

1 files changed, 3 insertions, 5 deletions

diff --git a/sql/item_strfunc.h b/sql/item_strfunc.h
index 2886cb68f9b..78989e9f517 100644
--- a/sql/item_strfunc.h
+++ b/sql/item_strfunc.h
@@ -542,10 +542,7 @@ class Item_func_sysconst :public Item_str_func
 public:
   Item_func_sysconst()
   { collation.set(system_charset_info,DERIVATION_SYSCONST); }
-  Item *safe_charset_converter(CHARSET_INFO *tocs)
-  {
-    return const_charset_converter(tocs, true, fully_qualified_func_name());
-  }
+  Item *safe_charset_converter(CHARSET_INFO *tocs);
   /*
     Used to create correct Item name in new converted item in
     safe_charset_converter, return string representation of this function
@@ -557,6 +554,7 @@ public:
     return trace_unsupported_by_check_vcol_func_processor(
                                            fully_qualified_func_name());
   }
+  bool const_item() const;
 };
 
 
@@ -635,7 +633,7 @@ public:
   String *val_str(String *)
   {
     DBUG_ASSERT(fixed == 1);
-    return (null_value ? 0 : &str_value);
+    return null_value ? NULL : &str_value;
   }
 };