CP-51690: [bugfix] Xapi_periodic_scheduler: avoid 10s sleep on empty …

…queue

Wake up the scheduler immediately when there are more tasks.
Otherwise timeouts <10s may not work correctly, and it is difficult to test the
periodic scheduler if you need to wait 10s for it to start working.

If there are no tasks, then it will still sleep efficiently, but as soon as
more tasks are added (with [~signal:true], which is the default) it will
immediately wake up and calculate the next sleep time.

In practice it is probably quite rare for XAPI's queue to be empty (there are usually periodic tasks),
but we cannot rely on this.

Signed-off-by: Edwin Török <[email protected]>

ocaml/tests/test_event.ml

@@ -277,6 +277,37 @@ let object_level_event_test _session_id =
   if !failure then
     Alcotest.fail "failed to see object-level event change"
 
+let test_short_oneshot () =
+  (* don't call event_setup_common here, it'll register a dummy event and hide the bug *)
+  let started = ref false in
+  let m = Mutex.create () in
+  let cond = Condition.create () in
+  let scheduler () =
+    Mutex.lock m ;
+    started := true ;
+    Condition.broadcast cond ;
+    Mutex.unlock m ;
+    Xapi_periodic_scheduler.loop ()
+  in
+  ignore (Thread.create scheduler ()) ;
+  (* ensure scheduler sees an empty queue , by waiting for it to start *)
+  Mutex.lock m ;
+  while not !started do
+    Condition.wait cond m
+  done ;
+  Mutex.unlock m ;
+  (* run the scheduler, let it realize its queue is empty,
+     a Thread.yield is not enough due to the use of debug which would yield back almost immediately.
+  *)
+  Thread.delay 1. ;
+  let fired = Atomic.make false in
+  let fire () = Atomic.set fired true in
+  let task = "test_oneshot" in
+  Xapi_periodic_scheduler.add_to_queue task Xapi_periodic_scheduler.OneShot 1.
+    fire ;
+  Thread.delay 2. ;
+  assert (Atomic.get fired)
+
 let test =
   [
     ("test_event_from_timeout", `Slow, test_event_from_timeout)
@@ -287,4 +318,5 @@ let test =
   ; ("test_event_from", `Quick, event_from_test)
   ; ("test_event_from_parallel", `Slow, event_from_parallel_test)
   ; ("test_event_object_level_event", `Slow, object_level_event_test)
+  ; ("test_short_oneshot", `Slow, test_short_oneshot)
   ]

ocaml/xapi/xapi_periodic_scheduler.ml

@@ -80,7 +80,7 @@ let loop () =
     while true do
       let empty = with_lock lock (fun () -> Ipq.is_empty queue) in
       if empty then
-        Thread.delay 10.0
+        wait_next 10.0
       (* Doesn't happen often - the queue isn't usually empty *)
       else
         let next = with_lock lock (fun () -> Ipq.maximum queue) in