Bug#16712: group_concat returns odd srting insead of intended result

  when calculating GROUP_CONCAT all blob fields are transformed
  to varchar when making the temp table.
  However a varchar has at max 2 bytes for length. 
  This fix makes the conversion only for blobs whose max length 
  is below that limit. 
  Otherwise blob field is created by make_string_field() call.

mysql-test/r/func_gconcat.result

@@ -641,3 +641,16 @@ select charset(group_concat(c1 order by c2)) from t1;
 charset(group_concat(c1 order by c2))
 latin1
 drop table t1;
+CREATE TABLE t1 (a INT(10), b LONGTEXT, PRIMARY KEY (a));
+SET GROUP_CONCAT_MAX_LEN = 20000000;
+INSERT INTO t1 VALUES (1,REPEAT(CONCAT('A',CAST(CHAR(0) AS BINARY),'B'), 40000));
+INSERT INTO t1 SELECT a + 1, b FROM t1;
+SELECT a, CHAR_LENGTH(b) FROM t1;
+a	CHAR_LENGTH(b)
+1	120000
+2	120000
+SELECT CHAR_LENGTH( GROUP_CONCAT(b) ) FROM t1;
+CHAR_LENGTH( GROUP_CONCAT(b) )
+240001
+SET GROUP_CONCAT_MAX_LEN = 1024;
+DROP TABLE t1;

mysql-test/t/func_gconcat.test

@@ -433,3 +433,17 @@ create table t1 (c1 varchar(10), c2 int);
 select charset(group_concat(c1 order by c2)) from t1;
 drop table t1;
 
+#
+# Bug #16712: group_concat returns odd string instead of intended result
+#
+CREATE TABLE t1 (a INT(10), b LONGTEXT, PRIMARY KEY (a));
+
+SET GROUP_CONCAT_MAX_LEN = 20000000;
+
+INSERT INTO t1 VALUES (1,REPEAT(CONCAT('A',CAST(CHAR(0) AS BINARY),'B'), 40000));
+INSERT INTO t1 SELECT a + 1, b FROM t1;
+
+SELECT a, CHAR_LENGTH(b) FROM t1;
+SELECT CHAR_LENGTH( GROUP_CONCAT(b) ) FROM t1;
+SET GROUP_CONCAT_MAX_LEN = 1024;
+DROP TABLE t1;

sql/item_sum.cc

@@ -377,7 +377,13 @@ Field *Item_sum::create_tmp_field(bool group, TABLE *table,
   case INT_RESULT:
     return new Field_longlong(max_length,maybe_null,name,table,unsigned_flag);
   case STRING_RESULT:
-    if (max_length/collation.collation->mbmaxlen > 255 && convert_blob_length)
+    /* 
+      Make sure that the blob fits into a Field_varstring which has 
+      2-byte lenght. 
+    */
+    if (max_length/collation.collation->mbmaxlen > 255 && 
+        max_length/collation.collation->mbmaxlen < UINT_MAX16 &&
+        convert_blob_length)
       return new Field_varstring(convert_blob_length, maybe_null,
                                  name, table,
                                  collation.collation);

sql/sql_select.cc

@@ -8011,7 +8011,12 @@ Field* create_tmp_field_from_field(THD *thd, Field* org_field,
 {
   Field *new_field;
 
-  if (convert_blob_length && (org_field->flags & BLOB_FLAG))
+  /* 
+    Make sure that the blob fits into a Field_varstring which has 
+    2-byte lenght. 
+  */
+  if (convert_blob_length && convert_blob_length < UINT_MAX16 &&
+      (org_field->flags & BLOB_FLAG))
     new_field= new Field_varstring(convert_blob_length,
                                    org_field->maybe_null(),
                                    org_field->field_name, table,
@@ -8087,8 +8092,13 @@ static Field *create_tmp_field_from_item(THD *thd, Item *item, TABLE *table,
     if ((type= item->field_type()) == MYSQL_TYPE_DATETIME ||
         type == MYSQL_TYPE_TIME || type == MYSQL_TYPE_DATE)
       new_field= item->tmp_table_field_from_field_type(table);
+    /* 
+      Make sure that the blob fits into a Field_varstring which has 
+      2-byte lenght. 
+    */
     else if (item->max_length/item->collation.collation->mbmaxlen > 255 &&
-             convert_blob_length)
+             item->max_length/item->collation.collation->mbmaxlen < UINT_MAX16
+             && convert_blob_length)
       new_field= new Field_varstring(convert_blob_length, maybe_null,
                                      item->name, table,
                                      item->collation.collation);