Bug#12371: executing prepared statement fails (illegal mix of collations)

ctype_utf8.test, ctype_utf8.result:
  Adding test case.
item.h:
item.cc:
  Adding Item_param::safe_charset_converter

mysql-test/r/ctype_utf8.result

@@ -1131,3 +1131,20 @@ a
 i
 い
 drop table t1,t2;
+set names utf8;
+create table t1 (a char(10), b varchar(10));
+insert into t1 values ('bar','kostja');
+insert into t1 values ('kostja','bar');
+prepare my_stmt from "select * from t1 where a=?";
+set @a:='bar';
+execute my_stmt using @a;
+a	b
+bar	kostja
+set @a:='kostja';
+execute my_stmt using @a;
+a	b
+kostja	bar
+set @a:=null;
+execute my_stmt using @a;
+a	b
+drop table if exists t1;

mysql-test/t/ctype_utf8.test

@@ -916,3 +916,20 @@ insert into t1 values('a'),('a'),(0xE38182),(0xE38182);
 insert into t1 values('i'),('i'),(0xE38184),(0xE38184);
 select * from t1 union distinct select * from t2;
 drop table t1,t2;
+
+
+#
+# Bug#12371: executing prepared statement fails (illegal mix of collations)
+#
+set names utf8;
+create table t1 (a char(10), b varchar(10));
+insert into t1 values ('bar','kostja');
+insert into t1 values ('kostja','bar');
+prepare my_stmt from "select * from t1 where a=?";
+set @a:='bar';
+execute my_stmt using @a;
+set @a:='kostja';
+execute my_stmt using @a;
+set @a:=null;
+execute my_stmt using @a;
+drop table if exists t1;

sql/item.cc

@@ -696,6 +696,32 @@ Item *Item_string::safe_charset_converter(CHARSET_INFO *tocs)
 }
 
 
+Item *Item_param::safe_charset_converter(CHARSET_INFO *tocs)
+{
+  if (const_item())
+  {
+    Item_string *conv;
+    uint cnv_errors;
+    char buf[MAX_FIELD_WIDTH];
+    String tmp(buf, sizeof(buf), &my_charset_bin);
+    String cstr, *ostr= val_str(&tmp);
+    /*
+      As safe_charset_converter is not executed for
+      a parameter bound to NULL, ostr should never be 0.
+    */
+    cstr.copy(ostr->ptr(), ostr->length(), ostr->charset(), tocs, &cnv_errors);
+    if (cnv_errors || !(conv= new Item_string(cstr.ptr(), cstr.length(),
+                                              cstr.charset(),
+                                              collation.derivation)))
+      return NULL;
+    conv->str_value.copy();
+    conv->str_value.mark_as_const();
+    return conv;
+  }
+  return NULL;
+}
+
+
 Item *Item_static_string_func::safe_charset_converter(CHARSET_INFO *tocs)
 {
   Item_string *conv;
@@ -1351,7 +1377,7 @@ bool agg_item_charsets(DTCollation &coll, const char *fname,
       been created in prepare. In this case register the change for
       rollback.
     */
-    if (arena)
+    if (arena && arena->is_conventional())
       *arg= conv;
     else
       thd->change_item_tree(arg, conv);

sql/item.h

@@ -1194,6 +1194,7 @@ public:
     constant, assert otherwise. This method is called only if
     basic_const_item returned TRUE.
   */
+  Item *safe_charset_converter(CHARSET_INFO *tocs);
   Item *new_item();
   /*
     Implement by-value equality evaluation if parameter value