Do not log out session in xapi events loop to re-register VMs

The session.logout was used to interrupt the call to `event.from`, so
that is can be called again with a new list of VMs to watch, in case a
VM started, stopped etc.

The trouble is, that each such re-register event led to SESSION_INVALID
being logged, with stack trace, three times. This caused many people
looking at the logs unnecessary worries, while in reality the behaviour
is completely normal.

This replaces the register mechanism by adding the event loop's task
into the watch list for `event.from`, and injecting an event on the task
to interrupt the call and recompute the VM list.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>

ocaml/xapi/xapi_xenops.ml

@@ -3207,7 +3207,9 @@ end
 (* XXX: PR-1255: we also want to only listen for events on VMs and fields we care about *)
 let events_from_xapi () =
   let open Event_types in
-  Server_helpers.exec_with_new_task "xapi events" (fun __context ->
+  Server_helpers.exec_with_new_task ~task_in_database:true "xapi events"
+    (fun __context ->
+      let task = Context.get_task_id __context |> Ref.string_of in
       let localhost = Helpers.get_localhost ~__context in
       let token = ref "" in
       let stop = ref false in
@@ -3216,19 +3218,18 @@ let events_from_xapi () =
           Helpers.call_api_functions ~__context (fun rpc session_id ->
               (trigger_xenapi_reregister :=
                  fun () ->
-                   (* This causes Event.next () and Event.from () to return SESSION_INVALID *)
+                   debug
+                     "triggering xapi event thread to re-register via event \
+                      injection" ;
                    try
-                     debug
-                       "triggering xapi event thread to re-register via \
-                        session.logout" ;
-                     XenAPI.Session.logout ~rpc ~session_id
-                   with
-                   | Api_errors.Server_error (code, _)
-                     when code = Api_errors.session_invalid ->
-                       debug "Event thread has already woken up"
-                   | e ->
-                       error "Waking up the xapi event thread: %s"
-                         (string_of_exn e)
+                     let _ =
+                       XenAPI.Event.inject ~rpc ~session_id ~_class:"task"
+                         ~_ref:task
+                     in
+                     ()
+                   with e ->
+                     error "Waking up the xapi event thread: %s"
+                       (string_of_exn e)
               ) ;
               (* We register for events on resident_VMs only *)
               let resident_VMs =
@@ -3251,13 +3252,15 @@ let events_from_xapi () =
                 error "events_from_xapi: extra items in the cache: [ %s ]"
                   (String.concat "; " (StringSet.elements extra_in_cache)) ;
               let classes =
-                List.map
-                  (fun x -> Printf.sprintf "VM/%s" (Ref.string_of x))
-                  resident_VMs
+                Printf.sprintf "task/%s" task
+                :: List.map
+                     (fun x -> Printf.sprintf "VM/%s" (Ref.string_of x))
+                     resident_VMs
               in
               (* NB we re-use the old token so we don't get events we've already
-                 							   received BUT we will not necessarily receive events for the new VMs *)
-              while true do
+                 received BUT we will not necessarily receive events for the new VMs *)
+              let reregister = ref false in
+              while not !reregister do
                 let api_timeout = 60. in
                 let timeout =
                   30.
@@ -3307,6 +3310,11 @@ let events_from_xapi () =
                             raise e
                           )
                       )
+                    | {ty= "task"; reference= t} when t = task ->
+                        debug
+                          "Woken event thread: updating list of event \
+                           subscriptions" ;
+                        reregister := true
                     | _ ->
                         warn
                           "Received event for something we didn't register for!"
@@ -3319,15 +3327,20 @@ let events_from_xapi () =
         with
         | Api_errors.Server_error (code, _)
           when code = Api_errors.session_invalid ->
-            debug "Woken event thread: updating list of event subscriptions"
+            debug
+              "Caught SESSION_INVALID listening to events from xapi. \
+               Restarting thread immediately."
         | Api_errors.Server_error (code, _)
           when code = Api_errors.xen_incompatible ->
             warn
               "Stopping events-from-xapi thread due to Xen/libxenctrl \
                incompatibility" ;
             stop := true
         | e ->
-            debug "Caught %s listening to events from xapi" (string_of_exn e) ;
+            debug
+              "Caught %s listening to events from xapi. Restarting thread \
+               after 15 seconds."
+              (string_of_exn e) ;
             (* Start from scratch *)
             token := "" ;
             Thread.delay 15.