CA-85940: fix a race between deleting the guest metrics and updating it

If the VM exposes guest agent data very early then we will delete
the old guest metrics (because a reboot has happened) but fail to
notice the new values are present and fail to create the new guest metrics.

We now check the data from the guest agent immediately after we
spot a reboot and delete the old guest agent data.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>

ocaml/xapi/xapi_xenops.ml

@@ -883,33 +883,40 @@ let update_vm ~__context id =
 						with e ->
 							error "Caught %s: while updating VM %s rtc/timeoffset" (Printexc.to_string e) id
 					end;
+					let check_guest_agent () =
+						Opt.iter
+							(fun (_, state) ->
+								List.iter
+									(fun domid ->
+										let lookup key =
+											if List.mem_assoc key state.guest_agent then Some (List.assoc key state.guest_agent) else None in
+										let list dir =
+											let dir = if dir.[0] = '/' then String.sub dir 1 (String.length dir - 1) else dir in
+											let results = Listext.List.filter_map (fun (path, value) ->
+												if String.startswith dir path then begin
+													let rest = String.sub path (String.length dir) (String.length path - (String.length dir)) in
+													match List.filter (fun x -> x <> "") (String.split '/' rest) with
+														| x :: _ -> Some x
+														| _ -> None
+												end else None
+											) state.guest_agent |> Listext.List.setify in
+											results in
+										try
+											debug "xenopsd event: Updating VM %s domid %d guest_agent" id domid;
+											Xapi_guest_agent.all lookup list ~__context ~domid ~uuid:id
+										with e ->
+											error "Caught %s: while updating VM %s guest_agent" (Printexc.to_string e) id
+									) state.domids
+							) info in
 					Opt.iter
 						(fun (_, state) ->
 							List.iter
 								(fun domid ->
 									if different (fun x -> x.uncooperative_balloon_driver) then begin
 										debug "xenopsd event: VM %s domid %d uncooperative_balloon_driver = %b" id domid state.uncooperative_balloon_driver;
 									end;
-									let lookup key =
-										if List.mem_assoc key state.guest_agent then Some (List.assoc key state.guest_agent) else None in
-									let list dir =
-										let dir = if dir.[0] = '/' then String.sub dir 1 (String.length dir - 1) else dir in
-										let results = Listext.List.filter_map (fun (path, value) ->
-											if String.startswith dir path then begin
-												let rest = String.sub path (String.length dir) (String.length path - (String.length dir)) in
-												match List.filter (fun x -> x <> "") (String.split '/' rest) with
-													| x :: _ -> Some x
-													| _ -> None
-											end else None
-										) state.guest_agent |> Listext.List.setify in
-										results in
-									if different (fun x -> x.guest_agent) then begin
-										try
-											debug "xenopsd event: Updating VM %s domid %d guest_agent" id domid;
-											Xapi_guest_agent.all lookup list ~__context ~domid ~uuid:id
-										with e ->
-											error "Caught %s: while updating VM %s guest_agent" (Printexc.to_string e) id
-									end;
+									if different (fun x -> x.guest_agent) then check_guest_agent ();
+
 									if different (fun x -> x.xsdata_state) then begin
 										try
 											debug "xenopsd event: Updating VM %s domid %d xsdata" id domid;
@@ -952,7 +959,8 @@ let update_vm ~__context id =
 											if update_time < start_time then begin
 												debug "VM %s guest metrics update time (%s) < VM start time (%s): deleting"
 													id (Date.to_string update_time) (Date.to_string start_time);
-												Xapi_vm_helpers.delete_guest_metrics ~__context ~self
+												Xapi_vm_helpers.delete_guest_metrics ~__context ~self;
+												check_guest_agent ();
 											end
 										with _ -> () (* The guest metrics didn't exist *)
 									end