A clean-up patch for Item_func_conv_charset (needed for MDEV-9181)

Removing the "conv_charset" member and using collation.collation
instead, as they duplicated each other.

sql/item_strfunc.cc

@@ -3404,7 +3404,7 @@ String *Item_func_conv_charset::val_str(String *str)
   String *arg= args[0]->val_str(str);
   String_copier_for_item copier(current_thd);
   return ((null_value= args[0]->null_value ||
-                       copier.copy_with_warn(conv_charset, &tmp_value,
+                       copier.copy_with_warn(collation.collation, &tmp_value,
                                              arg->charset(), arg->ptr(),
                                              arg->length(), arg->length()))) ?
     0 : &tmp_value;
@@ -3412,7 +3412,7 @@ String *Item_func_conv_charset::val_str(String *str)
 
 void Item_func_conv_charset::fix_length_and_dec()
 {
-  collation.set(conv_charset, DERIVATION_IMPLICIT);
+  DBUG_ASSERT(collation.derivation == DERIVATION_IMPLICIT);
   fix_char_length(args[0]->max_char_length());
 }
 
@@ -3421,7 +3421,7 @@ void Item_func_conv_charset::print(String *str, enum_query_type query_type)
   str->append(STRING_WITH_LEN("convert("));
   args[0]->print(str, query_type);
   str->append(STRING_WITH_LEN(" using "));
-  str->append(conv_charset->csname);
+  str->append(collation.collation->csname);
   str->append(')');
 }
 

sql/item_strfunc.h

@@ -959,20 +959,22 @@ class Item_func_conv_charset :public Item_str_func
   String tmp_value;
 public:
   bool safe;
-  CHARSET_INFO *conv_charset; // keep it public
   Item_func_conv_charset(THD *thd, Item *a, CHARSET_INFO *cs):
     Item_str_func(thd, a)
-  { conv_charset= cs; use_cached_value= 0; safe= 0; }
+  {
+    collation.set(cs, DERIVATION_IMPLICIT);
+    use_cached_value= 0; safe= 0;
+  }
   Item_func_conv_charset(THD *thd, Item *a, CHARSET_INFO *cs, bool cache_if_const):
     Item_str_func(thd, a)
   {
-    conv_charset= cs;
+    collation.set(cs, DERIVATION_IMPLICIT);
     if (cache_if_const && args[0]->const_item() && !args[0]->is_expensive())
     {
       uint errors= 0;
       String tmp, *str= args[0]->val_str(&tmp);
       if (!str || str_value.copy(str->ptr(), str->length(),
-                                 str->charset(), conv_charset, &errors))
+                                 str->charset(), cs, &errors))
         null_value= 1;
       use_cached_value= 1;
       str_value.mark_as_const();

storage/spider/spd_db_mysql.cc

@@ -4041,7 +4041,7 @@ int spider_db_mysql_util::open_item_func(
       {
         Item_func_conv_charset *item_func_conv_charset =
           (Item_func_conv_charset *)item_func;
-        CHARSET_INFO *conv_charset = item_func_conv_charset->conv_charset;
+        CHARSET_INFO *conv_charset = item_func_conv_charset->collation.collation;
         uint cset_length = strlen(conv_charset->csname);
         if (str->reserve(SPIDER_SQL_USING_LEN + cset_length))
           DBUG_RETURN(HA_ERR_OUT_OF_MEM);