Add test for handling event if queue was empty in periodic scheduler

Previously if the queue was empty and the loop thread was active
the scheduler took quite some time to pick up the new event.
Check that this is done in a timely fashion to avoid regressions in code.

Signed-off-by: Frediano Ziglio <[email protected]>

ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/scheduler_test.ml

@@ -56,10 +56,28 @@ let test_remove_self () =
   let elapsed_ms = elapsed_ms cnt in
   Alcotest.check is_less "small time" 300 elapsed_ms
 
+let test_empty () =
+  let finished = Event.new_channel () in
+  Scheduler.add_to_queue "one" Scheduler.OneShot 0.001 (fun () ->
+      send finished true
+  ) ;
+  start_schedule () ;
+  Alcotest.(check bool) "finished" true (receive finished) ;
+  (* wait loop to go to wait with no work to do *)
+  Thread.delay 0.1 ;
+  Scheduler.add_to_queue "two" Scheduler.OneShot 0.001 (fun () ->
+      send finished true
+  ) ;
+  let cnt = Mtime_clock.counter () in
+  Alcotest.(check bool) "finished" true (receive finished) ;
+  let elapsed_ms = elapsed_ms cnt in
+  Alcotest.check is_less "small time" 100 elapsed_ms
+
 let tests =
   [
     ("test_single", `Quick, test_single)
   ; ("test_remove_self", `Quick, test_remove_self)
+  ; ("test_empty", `Quick, test_empty)
   ]
 
 let () = Alcotest.run "Scheduler" [("generic", tests)]