MDEV-11570 JSON_MERGE returns incorrect result.

        JSON merging fixed.

mysql-test/r/func_json.result

@@ -315,6 +315,9 @@ Warning	4038	Syntax error in JSON text in argument 1 to function 'json_merge' at
 select json_merge('{"a":"b"}','{"c":"d"}');
 json_merge('{"a":"b"}','{"c":"d"}')
 {"a":"b", "c":"d"}
+SELECT JSON_MERGE('[1, 2]', '{"id": 47}');
+JSON_MERGE('[1, 2]', '{"id": 47}')
+[1, 2, {"id": 47}]
 select json_type('{"k1":123, "k2":345}');
 json_type('{"k1":123, "k2":345}')
 OBJECT

mysql-test/t/func_json.test

@@ -123,6 +123,7 @@ select json_merge('{"1": 2}', '{"true": false}', '{"3": 4}');
 select json_merge(NULL,json_object('foo', 1));
 select json_merge('a','b');
 select json_merge('{"a":"b"}','{"c":"d"}');
+SELECT JSON_MERGE('[1, 2]', '{"id": 47}');
 
 select json_type('{"k1":123, "k2":345}');
 select json_type('[123, "k2", 345]');

sql/item_jsonfunc.cc

@@ -1522,12 +1522,10 @@ String *Item_func_json_merge::val_str(String *str)
       goto error_return;
 
     str->length(0);
-    if ((je1.value_type == JSON_VALUE_ARRAY &&
-         je2.value_type == JSON_VALUE_ARRAY)  ||
-        (je1.value_type == JSON_VALUE_OBJECT &&
-         je2.value_type == JSON_VALUE_OBJECT))
+    if (je1.value_type == JSON_VALUE_OBJECT &&
+        je2.value_type == JSON_VALUE_OBJECT)
     {
-      /* Merge the adjancent arrays or objects. */
+      /* Wrap as a single objects. */
       if (json_skip_level(&je1))
         goto error_return;
       if (str->append(js1->ptr(),
@@ -1539,11 +1537,35 @@ String *Item_func_json_merge::val_str(String *str)
     }
     else
     {
-      /* Wrap as an array. */
-      if (str->append("[", 1) ||
-          str->append(js1->ptr(), js1->length()) ||
-          str->append(", ", 2) ||
-          str->append(js2->ptr(), js2->length()) ||
+      const char *end1, *beg2;
+
+      /* Merge as a single array. */
+      if (je1.value_type == JSON_VALUE_ARRAY)
+      {
+        if (json_skip_level(&je1))
+          goto error_return;
+        end1= (const char *) (je1.s.c_str - je1.sav_c_len);
+      }
+      else
+      {
+        if (str->append("[", 1))
+          goto error_return;
+        end1= js1->end();
+      }
+
+      if (str->append(js1->ptr(), end1 - js1->ptr()),
+          str->append(", ", 2))
+        goto error_return;
+
+      if (je2.value_type == JSON_VALUE_ARRAY)
+        beg2= (const char *) je2.s.c_str;
+      else
+        beg2= js2->ptr();
+
+      if (str->append(beg2, js2->end() - beg2))
+        goto error_return;
+
+      if (je2.value_type != JSON_VALUE_ARRAY &&
           str->append("]", 1))
         goto error_return;
     }