invoker name
The bug was fixed similar to how context switch is handled in
Item_func_sp::execute_impl(): we store pointer to current
Name_resolution_context in Item_func_current_user class, and use
its Security_context in Item_func_current_user::fix_fields().
When reading a view definition from a .frm file it was
throwing a SQL error if the DEFINER user is not defined.
Changed it to a warning to match the (documented) case
when a view with undefined DEFINER user is created.
The check for view security was lacking several points :
1. Check with the right set of permissions : for each table ref that
participates in a view there were the right credentials to use in it's
security_ctx member, but these weren't used for checking the credentials.
This makes hard enforcing the SQL SECURITY DEFINER|INVOKER property
consistently.
2. Because of the above the security checking for views was just ruled out
in explicit ways in several places.
3. The security was checked only for the columns of the tables that are
brought into the query from a view. So if there is no column reference
outside of the view definition it was not detecting the lack of access to
the tables in the view in SQL SECURITY INVOKER mode.
The fix below tries to fix the above 3 points.
Changed the parser test for wildcards in hostname to checking for empty
strings instead (analogous with the test in default_view_definer()),
since wildcards do appear in the definer's host-part sometimes.
