Fixed bug #26738: incomplete string values in a result set column

when the column is to be read from a derived table column which 
was specified as a concatenation of string literals.
The bug happened because the Item_string::append did not adjust the
value of Item_string::max_length. As a result of it the temporary 
table column  defined to store the concatenation of literals was 
not wide enough to hold the whole value.

mysql-test/r/subselect.result

@@ -3867,3 +3867,16 @@ id_1
 DROP TABLE t1;
 DROP TABLE t2;
 DROP TABLE t1xt2;
+CREATE TABLE t1 (a int);
+INSERT INTO t1 VALUES (3), (1), (2);
+SELECT 'this is ' 'a test.' AS col1, a AS col2 FROM t1;
+col1	col2
+this is a test.	3
+this is a test.	1
+this is a test.	2
+SELECT * FROM (SELECT 'this is ' 'a test.' AS col1, a AS t2 FROM t1) t;
+col1	t2
+this is a test.	3
+this is a test.	1
+this is a test.	2
+DROP table t1;

mysql-test/t/subselect.test

@@ -2729,3 +2729,16 @@ DROP TABLE t1;
 DROP TABLE t2;
 DROP TABLE t1xt2;
 
+#
+# Bug #26728: derived table with concatanation of literals in select list
+#  
+
+CREATE TABLE t1 (a int);
+INSERT INTO t1 VALUES (3), (1), (2);           
+
+SELECT 'this is ' 'a test.' AS col1, a AS col2 FROM t1;
+SELECT * FROM (SELECT 'this is ' 'a test.' AS col1, a AS t2 FROM t1) t;
+
+DROP table t1;
+
+

sql/item.h

@@ -1713,7 +1713,11 @@ public:
     			   str_value.length(), collation.collation);
   }
   Item *safe_charset_converter(CHARSET_INFO *tocs);
-  inline void append(char *str, uint length) { str_value.append(str, length); }
+  inline void append(char *str, uint length)
+  {
+    str_value.append(str, length);
+    max_length= str_value.numchars() * collation.collation->mbmaxlen;
+  }
   void print(String *str);
   // to prevent drop fixed flag (no need parent cleanup call)
   void cleanup() {}