CA-413412: Fail to designate new master (#6582)

The user attempted to designate a new master, but the operation failed.

The root cause is as follows:
After the new proposed master successfully sent the `commit_new_master`
API call to the old master, it attempted to send a `logout` request.
However, at this point, the old master was already rebooting its xapi
service, causing the `logout` to fail. As a result, the process of
designating the new master was marked as failed, and the status changed
to `broken`. In high-load environments, there can be a delay in sending
the logout request, increasing the likelihood that it is sent after the
old master has already started rebooting.

If `commit_new_master` has already been successful, the success of the
subsequent `logout` operation should not be considered critical.
Therefore, the solution is to ignore the result of the `logout` request
if `commit_new_master` was successful.

Author: BengangY

ocaml/xapi/helpers.ml

@@ -591,6 +591,7 @@ let call_api_functions ~__context f =
       call_api_functions_internal ~__context f
 
 let call_emergency_mode_functions hostname f =
+  let __FUN = __FUNCTION__ in
   let open Xmlrpc_client in
   let transport =
     SSL
@@ -609,7 +610,13 @@ let call_emergency_mode_functions hostname f =
   in
   finally
     (fun () -> f rpc session_id)
-    (fun () -> Client.Client.Session.local_logout ~rpc ~session_id)
+    (fun () ->
+      try Client.Client.Session.local_logout ~rpc ~session_id
+      with _ ->
+        (* This is an emergency mode function, so we don't care about the error
+           in logout *)
+        debug "%s: The logout failed in emergency mode function" __FUN
+    )
 
 let is_domain_zero_with_record ~__context vm_ref vm_rec =
   let host_ref = vm_rec.API.vM_resident_on in