MDEV-27151: JSON_VALUE() does not parse NULL properties properly

Analysis: JSON_VALUE() returns "null" string instead of NULL pointer.
Fix: When the type is JSON_VALUE_NULL (which is also a scalar) set
null_value to true and return 0 instead of returning string.

mysql-test/main/func_json.result

@@ -1016,5 +1016,32 @@ j
 {"ID": "4", "Name": "Betty", "Age": 19}
 drop table t1;
 #
+# MDEV-27151: JSON_VALUE() does not parse NULL properties properly
+#
+#
+# It is correct for JSON_EXTRACT() to give null instead of "NULL" because
+# it returns the json literal that is put inside json.
+# Hence it should return null as in 'null' string and not SQL NULL.
+# JSON_VALUE() returns the "VALUE" so it is correct for it to return SQl NULL
+#
+SELECT NULL;
+NULL
+NULL
+SELECT JSON_VALUE('{"nulltest": null}', '$.nulltest');
+JSON_VALUE('{"nulltest": null}', '$.nulltest')
+NULL
+SELECT 1 + NULL;
+1 + NULL
+NULL
+SELECT 1 + JSON_VALUE('{"nulltest": null}', '$.nulltest');
+1 + JSON_VALUE('{"nulltest": null}', '$.nulltest')
+NULL
+SELECT NULL;
+NULL
+NULL
+SELECT JSON_EXTRACT('{"a":null, "b":10, "c":"null"}', '$.a');
+JSON_EXTRACT('{"a":null, "b":10, "c":"null"}', '$.a')
+null
+#
 # End of 10.3 tests
 #

mysql-test/main/func_json.test

@@ -627,6 +627,25 @@ SELECT * FROM t1 WHERE JSON_EXTRACT(j, '$.Age')=19;
 
 drop table t1;
 
+--echo #
+--echo # MDEV-27151: JSON_VALUE() does not parse NULL properties properly
+--echo #
+--echo #
+--echo # It is correct for JSON_EXTRACT() to give null instead of "NULL" because
+--echo # it returns the json literal that is put inside json.
+--echo # Hence it should return null as in 'null' string and not SQL NULL.
+--echo # JSON_VALUE() returns the "VALUE" so it is correct for it to return SQl NULL
+--echo #
+
+SELECT NULL;
+SELECT JSON_VALUE('{"nulltest": null}', '$.nulltest');
+SELECT 1 + NULL;
+SELECT 1 + JSON_VALUE('{"nulltest": null}', '$.nulltest');
+
+
+SELECT NULL;
+SELECT JSON_EXTRACT('{"a":null, "b":10, "c":"null"}', '$.a');
+
 --echo #
 --echo # End of 10.3 tests
 --echo #

sql/item_jsonfunc.cc

@@ -619,6 +619,12 @@ continue_search:
   if (json_read_value(&je))
     goto err_return;
 
+  if (je.value_type == JSON_VALUE_NULL)
+  {
+    null_value= 1;
+    return NULL;
+  }
+
   if (unlikely(check_and_get_value(&je, str, &error)))
   {
     if (error)
@@ -1111,7 +1117,6 @@ my_decimal *Item_func_json_extract::val_decimal(my_decimal *to)
       case JSON_VALUE_OBJECT:
       case JSON_VALUE_ARRAY:
       case JSON_VALUE_FALSE:
-      // TODO: fix: NULL should be NULL
       case JSON_VALUE_NULL:
         int2my_decimal(E_DEC_FATAL_ERROR, 0, false/*unsigned_flag*/, to);
         return to;