Simplify caseup() and casedn() in charsets

After the MDEV-13118 fix there's no code in the server that
wants caseup/casedn to change the argument in place for simple
charsets.  Let's remove this logic and always return the result in a
new string for all charsets, both simple and complex.

1. Removing the optimization that *some* character sets used in casedn()
  and caseup(), which allowed (and required) to change the case in-place,
  overwriting the string passed as the "src" argument.
  Now all CHARSET_INFO's work in the same way:
  non of them change the source string in-place, all of them now convert
  case from the source string to the destination string, leaving
  the source string untouched.

2. Adding "const" qualifier to the "char *src" parameter
   to caseup() and casedn().

3. Removing duplicate implementations in ctype-mb.c.
  Now both caseup() and casedn() implementations for all CJK character sets
  use internally the same function my_casefold_mb()
  (the former my_casefold_mb_varlen()).

4. Removing the "unused" attribute from parameters of some my_case{up|dn}_xxx()
   implementations, as the affected parameters are now *used* in the code.
   Previously these parameters were used only in DBUG_ASSERT().

1 files changed, 3 insertions, 12 deletions

diff --git a/sql/item_strfunc.cc b/sql/item_strfunc.cc
index 160e740a9f7..ba9f14fb63f 100644
--- a/sql/item_strfunc.cc
+++ b/sql/item_strfunc.cc
@@ -1572,19 +1572,10 @@ String *Item_str_conv::val_str(String *str)
                     str->alloc((alloced_length= res->length() * multiply)))))
     return 0;
 
-  if (multiply == 1)
-  {
-    str->copy(*res); // Should not fail (it was alloced above)
-    len= converter(collation.collation, (char*) str->ptr(), str->length(),
-                                        (char*) str->ptr(), alloced_length);
-  }
-  else
-  {
-    len= converter(collation.collation, (char*) res->ptr(), res->length(),
-                                        (char*) str->ptr(), alloced_length);
-    str->set_charset(collation.collation);
-  }
+  len= converter(collation.collation, (char*) res->ptr(), res->length(),
+                                      (char*) str->ptr(), alloced_length);
   DBUG_ASSERT(len <= alloced_length);
+  str->set_charset(collation.collation);
   str->length(len);
   return str;
 }