MDEV-5840: group_concat( column_json(dynamic_column )) return empty result

Fixed max_length of dynamic columns json/create/add functions.

mysql-test/r/dyncol.result

@@ -1755,3 +1755,16 @@ Warning	1265	Data truncated for column 'dyncol' at row 1
 SELECT COLUMN_ADD( COLUMN_ADD( dyncol, 1, REPEAT('b',130) ), 3, 'y' ) FROM t1;
 ERROR HY000: Encountered illegal format of dynamic column string
 DROP TABLE t1;
+#
+#MDEV-5840: group_concat( column_json(dynamic_column )) return empty
+#result
+#
+create table t1 (dyn blob);
+insert into t1 values (column_create('name1','value1','name2','value2'));
+select group_concat(cast(column_json(dyn) as char)) from t1;
+group_concat(cast(column_json(dyn) as char))
+{"name1":"value1","name2":"value2"}
+drop table t1;
+#
+# end of 10.0 tests
+#

mysql-test/t/dyncol.test

@@ -820,3 +820,17 @@ INSERT INTO t1 SET dyncol = COLUMN_CREATE( 3, REPEAT('a',330), 4, 'x' );
 SELECT COLUMN_ADD( COLUMN_ADD( dyncol, 1, REPEAT('b',130) ), 3, 'y' ) FROM t1;
 
 DROP TABLE t1;
+
+--echo #
+--echo #MDEV-5840: group_concat( column_json(dynamic_column )) return empty
+--echo #result
+--echo #
+create table t1 (dyn blob);
+insert into t1 values (column_create('name1','value1','name2','value2'));
+select group_concat(cast(column_json(dyn) as char)) from t1;
+
+drop table t1;
+
+--echo #
+--echo # end of 10.0 tests
+--echo #

sql/item_strfunc.cc

@@ -4279,6 +4279,7 @@ bool Item_func_dyncol_create::fix_fields(THD *thd, Item **ref)
 
 void Item_func_dyncol_create::fix_length_and_dec()
 {
+  max_length= MAX_BLOB_WIDTH;
   maybe_null= TRUE;
   collation.set(&my_charset_bin);
   decimals= 0;

sql/item_strfunc.h

@@ -1174,6 +1174,7 @@ public:
   String *val_str(String *);
   void fix_length_and_dec()
   {
+    max_length= MAX_BLOB_WIDTH;
     maybe_null= 1;
     collation.set(&my_charset_bin);
     decimals= 0;