Bug #50087 Interval arithmetic for Event_queue_element is not portable.

Subtraction of two unsigned months yielded a (very large) positive value.
Conversion of this to a signed value was not necessarily well defined.

Solution: do the subtraction on signed values.

mysql-test/r/events_scheduling.result

@@ -82,5 +82,24 @@ DROP TABLE table_1;
 DROP TABLE table_2;
 DROP TABLE table_3;
 DROP TABLE table_4;
+
+Bug #50087 Interval arithmetic for Event_queue_element is not portable.
+
+CREATE TABLE t1(a int);
+CREATE EVENT e1 ON SCHEDULE EVERY 1 MONTH
+STARTS NOW() - INTERVAL 1 MONTH
+ENDS NOW() + INTERVAL 2 MONTH
+ON COMPLETION PRESERVE
+DO
+INSERT INTO t1 VALUES (1);
+CREATE EVENT e2 ON SCHEDULE EVERY 1 MONTH
+STARTS NOW()
+ENDS NOW() + INTERVAL 11 MONTH
+ON COMPLETION PRESERVE
+DO
+INSERT INTO t1 VALUES (1);
+DROP TABLE t1;
+DROP EVENT e1;
+DROP EVENT e2;
 DROP DATABASE events_test;
 SET GLOBAL event_scheduler=@event_scheduler;

mysql-test/t/events_scheduling.test

@@ -108,6 +108,32 @@ DROP TABLE table_1;
 DROP TABLE table_2;
 DROP TABLE table_3;
 DROP TABLE table_4;
+
+-- echo
+-- echo Bug #50087 Interval arithmetic for Event_queue_element is not portable.
+-- echo
+
+CREATE TABLE t1(a int);
+
+CREATE EVENT e1 ON SCHEDULE EVERY 1 MONTH
+STARTS NOW() - INTERVAL 1 MONTH
+ENDS NOW() + INTERVAL 2 MONTH
+ON COMPLETION PRESERVE
+DO
+  INSERT INTO t1 VALUES (1);
+
+CREATE EVENT e2 ON SCHEDULE EVERY 1 MONTH
+STARTS NOW()
+ENDS NOW() + INTERVAL 11 MONTH
+ON COMPLETION PRESERVE
+DO
+  INSERT INTO t1 VALUES (1);
+
+DROP TABLE t1;
+DROP EVENT e1;
+DROP EVENT e2;
+
+
 DROP DATABASE events_test;
 SET GLOBAL event_scheduler=@event_scheduler;
 

sql/event_data_objects.cc

@@ -834,8 +834,9 @@ bool get_next_time(const Time_zone *time_zone, my_time_t *next,
   }
   else
   {
-    long diff_months= (long) (local_now.year - local_start.year)*12 +
-                      (local_now.month - local_start.month);
+    long diff_months= ((long) local_now.year - (long) local_start.year)*12 +
+                      ((long) local_now.month - (long) local_start.month);
+
     /*
       Unlike for seconds above, the formula below returns the interval
       that, when added to the local_start, will give the time in the