# Bug#8785 Problem with nested concats and

character set conversion of a string constant.

mysql-test/r/ctype_utf8.result

@@ -861,6 +861,13 @@ user	c
 one	<one>
 two	<two>
 DROP TABLE t1;
+create table t1 (f1 varchar(1) not null) default charset utf8;
+insert into t1 values (''), ('');
+select concat(concat(_latin1'->',f1),_latin1'<-') from t1;
+concat(concat(_latin1'->',f1),_latin1'<-')
+-><-
+-><-
+drop table t1;
 select convert(_koi8r'' using utf8) < convert(_koi8r'' using utf8);
 convert(_koi8r'' using utf8) < convert(_koi8r'' using utf8)
 1

mysql-test/t/ctype_utf8.test

@@ -694,6 +694,15 @@ SELECT CHARSET('a');
 SELECT user, CONCAT('<', user, '>') AS c FROM t1;
 DROP TABLE t1;
 
+#
+# Bug#8785
+# the same problem with the above, but with nested CONCATs
+#
+create table t1 (f1 varchar(1) not null) default charset utf8;
+insert into t1 values (''), ('');
+select concat(concat(_latin1'->',f1),_latin1'<-') from t1;
+drop table t1;
+
 #
 # Bug#8385: utf8_general_ci treats Cyrillic letters I and SHORT I as the same
 #

sql/item.cc

@@ -236,6 +236,18 @@ Item *Item_string::safe_charset_converter(CHARSET_INFO *tocs)
     return NULL;
   }
   conv->str_value.copy();
+  /* 
+    The above line executes str_value.realloc() internally,
+    which alligns Alloced_length using ALLIGN_SIZE.
+    In the case of Item_string::str_value we don't want
+    Alloced_length to be longer than str_length.
+    Otherwise, some functions like Item_func_concat::val_str()
+    try to reuse str_value as a buffer for concatenation result
+    for optimization purposes, so our string constant become
+    corrupted. See bug#8785 for more details.
+    Let's shrink Alloced_length to str_length to avoid this problem.
+  */
+  conv->str_value.shrink_to_length();
   return conv;
 }
 

sql/item_strfunc.cc

@@ -277,8 +277,7 @@ String *Item_func_concat::val_str(String *str)
 			    current_thd->variables.max_allowed_packet);
 	goto null;
       }
-      if (!args[0]->const_item() && 
-          res->alloced_length() >= res->length()+res2->length())
+      if (res->alloced_length() >= res->length()+res2->length())
       {						// Use old buffer
 	res->append(*res2);
       }

sql/sql_string.h

@@ -177,6 +177,10 @@ public:
       }
     }
   }
+  inline void shrink_to_length()
+  {
+    Alloced_length= str_length;
+  }
   bool is_alloced() { return alloced; }
   inline String& operator = (const String &s)
   {