MDEV-30646 View created via JSON_ARRAYAGG returns incorrect json object

Backporting add782a13e from 10.6, this fixes the problem.

mysql-test/main/func_json.result

@@ -1640,9 +1640,9 @@ SELECT t1.id, JSON_ARRAYAGG(JSON_OBJECT('id',t2.id) ORDER BY t2.id) as materials
 from t1 LEFT JOIN t2 on t1.id = t2.owner_id
 GROUP BY t1.id ORDER BY id;
 id	materials
-1	["{\"id\": 1}","{\"id\": 2}"]
-2	["{\"id\": 3}"]
-3	["{\"id\": 4}"]
+1	[{"id": 1},{"id": 2}]
+2	[{"id": 3}]
+3	[{"id": 4}]
 DROP TABLE t1;
 DROP TABLE t2;
 #

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

@@ -2880,5 +2880,13 @@ DROP PROCEDURE p1;
 DROP PROCEDURE p2;
 DROP TABLE t1, t1c, t2;
 #
+# MDEV-30646 View created via JSON_ARRAYAGG returns incorrect json object
+#
+CREATE VIEW v1 AS SELECT JSON_OBJECT('plugin','unix_socket') as v1_json;
+SELECT JSON_ARRAYAGG(v1_json) FROM v1;
+JSON_ARRAYAGG(v1_json)
+[{"plugin": "unix_socket"}]
+DROP VIEW v1;
+#
 # End of 10.5 tests
 #

mysql-test/suite/json/t/type_json.test

@@ -138,6 +138,15 @@ DROP PROCEDURE p2;
 
 DROP TABLE t1, t1c, t2;
 
+--echo #
+--echo # MDEV-30646 View created via JSON_ARRAYAGG returns incorrect json object
+--echo #
+
+CREATE VIEW v1 AS SELECT JSON_OBJECT('plugin','unix_socket') as v1_json;
+SELECT JSON_ARRAYAGG(v1_json) FROM v1;
+DROP VIEW v1;
+
+
 --echo #
 --echo # End of 10.5 tests
 --echo #

sql/item_jsonfunc.cc

@@ -1633,7 +1633,7 @@ null_return:
   `CONVERT(arg USING charset)` is actually a general purpose string
   expression, not a JSON expression.
 */
-static bool is_json_type(const Item *item)
+bool is_json_type(const Item *item)
 {
   for ( ; ; )
   {

sql/item_jsonfunc.h

@@ -640,5 +640,6 @@ public:
   { return get_item_copy<Item_func_json_objectagg>(thd, this); }
 };
 
+extern bool is_json_type(const Item *item);
 
 #endif /* ITEM_JSONFUNC_INCLUDED */

sql/sql_select.cc

@@ -18569,6 +18569,25 @@ Field *Item_func_sp::create_tmp_field_ex(MEM_ROOT *root, TABLE *table,
   return result;
 }
 
+
+static bool make_json_valid_expr(TABLE *table, Field *field)
+{
+  THD *thd= table->in_use;
+  Query_arena backup_arena;
+  Item *expr, *item_field;
+
+  if (!table->expr_arena && table->init_expr_arena(thd->mem_root))
+    return 1;
+
+  thd->set_n_backup_active_arena(table->expr_arena, &backup_arena);
+  if ((item_field= new (thd->mem_root) Item_field(thd, field)) &&
+      (expr= new (thd->mem_root) Item_func_json_valid(thd, item_field)))
+    field->check_constraint= add_virtual_expression(thd, expr);
+  thd->restore_active_arena(table->expr_arena, &backup_arena);
+  return field->check_constraint == NULL;
+}
+
+
 /**
   Create field for temporary table.
 
@@ -18614,6 +18633,9 @@ Field *create_tmp_field(TABLE *table, Item *item,
                       make_copy_field);
   Field *result= item->create_tmp_field_ex(table->in_use->mem_root,
                                            table, &src, &prm);
+  if (is_json_type(item) && make_json_valid_expr(table, result))
+    result= NULL;
+
   *from_field= src.field();
   *default_field= src.default_field();
   if (src.item_result_field())

sql/table.cc

@@ -53,6 +53,17 @@
 #define MYSQL57_GENERATED_FIELD 128
 #define MYSQL57_GCOL_HEADER_SIZE 4
 
+bool TABLE::init_expr_arena(MEM_ROOT *mem_root)
+{
+  /*
+    We need to use CONVENTIONAL_EXECUTION here to ensure that
+    any new items created by fix_fields() are not reverted.
+  */
+  expr_arena= new (alloc_root(mem_root, sizeof(Query_arena)))
+                Query_arena(mem_root, Query_arena::STMT_CONVENTIONAL_EXECUTION);
+  return expr_arena == NULL;
+}
+
 struct extra2_fields
 {
   LEX_CUSTRING version;
@@ -1145,14 +1156,8 @@ bool parse_vcol_defs(THD *thd, MEM_ROOT *mem_root, TABLE *table,
            table->s->table_check_constraints * sizeof(Virtual_column_info*));
 
   DBUG_ASSERT(table->expr_arena == NULL);
-  /*
-    We need to use CONVENTIONAL_EXECUTION here to ensure that
-    any new items created by fix_fields() are not reverted.
-  */
-  table->expr_arena= new (alloc_root(mem_root, sizeof(Query_arena)))
-                        Query_arena(mem_root,
-                                    Query_arena::STMT_CONVENTIONAL_EXECUTION);
-  if (!table->expr_arena)
+
+  if (table->init_expr_arena(mem_root))
     DBUG_RETURN(1);
 
   thd->set_n_backup_active_arena(table->expr_arena, &backup_arena);

sql/table.h

@@ -1645,6 +1645,8 @@ public:
     m_needs_reopen= value;
   }
 
+  bool init_expr_arena(MEM_ROOT *mem_root);
+
   bool alloc_keys(uint key_count);
   bool check_tmp_key(uint key, uint key_parts,
                      uint (*next_field_no) (uchar *), uchar *arg);