CP-51692: feat(use-event-next): cli_util: use Event.from instead of Event.next

This is more efficient: we can watch a single task, instead of everything in
the DB.

Feature-flag: use-event-next

Signed-off-by: Edwin Török <edwin.torok@cloud.com>

Author: edwintorok

ocaml/xapi-cli-server/cli_util.ml

@@ -42,21 +42,41 @@ exception Cli_failure of string
 
 (** call [callback task_record] on every update to the task, until it completes or fails *)
 let track callback rpc (session_id : API.ref_session) task =
-  let classes = ["task"] in
+  let use_event_next = !Constants.use_event_next in
+  let classes =
+    if use_event_next then
+      ["task"]
+    else
+      [Printf.sprintf "task/%s" (Ref.string_of task)]
+  in
   finally
     (fun () ->
       let finished = ref false in
       while not !finished do
-        Client.Event.register ~rpc ~session_id ~classes ;
+        if use_event_next then
+          Client.Event.register ~rpc ~session_id ~classes ;
         try
           (* Need to check once after registering to avoid a race *)
           finished :=
             Client.Task.get_status ~rpc ~session_id ~self:task <> `pending ;
+          let token = ref "" in
           while not !finished do
             let events =
-              Event_types.events_of_rpc (Client.Event.next ~rpc ~session_id)
+              if use_event_next then
+                let events =
+                  Event_types.events_of_rpc (Client.Event.next ~rpc ~session_id)
+                in
+                List.map Event_helper.record_of_event events
+              else
+                let event_from =
+                  Event_types.event_from_of_rpc
+                    (Client.Event.from ~rpc ~session_id ~classes ~token:!token
+                       ~timeout:30.
+                    )
+                in
+                token := event_from.token ;
+                List.map Event_helper.record_of_event event_from.events
             in
-            let events = List.map Event_helper.record_of_event events in
             List.iter
               (function
                 | Event_helper.Task (t, Some t_rec) when t = task ->