Bug#26867509: JSON_OBJECT CREATES INVALID JSON CODE

When inserting JSON values from a grouped query into a string column,
the inserted values could sometimes include the concatenation of all
the values previously inserted into that column.

The fix is to make Item_copy_json::save_in_field_inner() reset the
buffer before converting the JSON value to text.

(cherry picked from commit 0178fbc193bfaa0af49136e444ad5b73586d6f83)

Author: kahatlen

mysql-test/suite/json/inc/json_insert.inc

@@ -145,3 +145,14 @@ disconnect con1;
 connection default;
 
 SET global max_allowed_packet=default;
+
+--echo #
+--echo # Bug#26867509: JSON_OBJECT CREATES INVALID JSON CODE
+--echo #
+
+CREATE TABLE t1(i INT, KEY(i));
+INSERT INTO t1 VALUES (1), (2), (3);
+CREATE TABLE t2(i INT, vc VARCHAR(100));
+INSERT INTO t2 SELECT i, JSON_ARRAY(i) FROM t1 GROUP BY i;
+SELECT * FROM t2 ORDER BY i;
+DROP TABLE t1, t2;

mysql-test/suite/json/r/json_insert_innodb.result

@@ -174,3 +174,16 @@ ARRAY	6000005	18000015
 INSERT INTO t1(a) VALUES(@a);
 DROP TABLE t1;
 SET global max_allowed_packet=default;
+#
+# Bug#26867509: JSON_OBJECT CREATES INVALID JSON CODE
+#
+CREATE TABLE t1(i INT, KEY(i));
+INSERT INTO t1 VALUES (1), (2), (3);
+CREATE TABLE t2(i INT, vc VARCHAR(100));
+INSERT INTO t2 SELECT i, JSON_ARRAY(i) FROM t1 GROUP BY i;
+SELECT * FROM t2 ORDER BY i;
+i	vc
+1	[1]
+2	[2]
+3	[3]
+DROP TABLE t1, t2;

mysql-test/suite/json/r/json_insert_ndb.result

@@ -180,3 +180,16 @@ ARRAY	6000005	18000015
 INSERT INTO t1(a) VALUES(@a);
 DROP TABLE t1;
 SET global max_allowed_packet=default;
+#
+# Bug#26867509: JSON_OBJECT CREATES INVALID JSON CODE
+#
+CREATE TABLE t1(i INT, KEY(i));
+INSERT INTO t1 VALUES (1), (2), (3);
+CREATE TABLE t2(i INT, vc VARCHAR(100));
+INSERT INTO t2 SELECT i, JSON_ARRAY(i) FROM t1 GROUP BY i;
+SELECT * FROM t2 ORDER BY i;
+i	vc
+1	[1]
+2	[2]
+3	[3]
+DROP TABLE t1, t2;

sql/item.cc

@@ -4838,6 +4838,7 @@ Item_copy_json::save_in_field_inner(Field *field, bool no_conversions)
   }
   else
   {
+    str_value.length(0);
     if (m_value->to_string(&str_value, true, item_name.ptr()))
       return set_field_to_null(field);        /* purecov: inspected */
     return save_str_value_in_field(field, &str_value);