CA-411319: Concurrent VM.assert_can_migrate failure (#6476)

When the customers open "Migrate VM Wizard" on XenCenter, XenCenter will
call
`VM.assert_can_migrate` to check each host in each pool connected to
XenCenter
if the VM can be migrated to it. The API `VM.assert_can_migrate` then
calls
`VM.export_metadata`. `VM.export_metadata` will lock VM. During this
time, other
`VM.export_metadata` requests will fail as they can't get VM lock.

The solution is to add retry when failing to lock VM.

Author: psafont

ocaml/xapi/export.ml

@@ -713,11 +713,15 @@ open Http
 open Client
 
 let lock_vm ~__context ~vm ~task_id op =
-  (* Note slight race here because we haven't got the master lock *)
-  Xapi_vm_lifecycle.assert_operation_valid ~__context ~self:vm ~op ~strict:true ;
-  (* ... small race lives here ... *)
-  Db.VM.add_to_current_operations ~__context ~self:vm ~key:task_id ~value:op ;
-  Xapi_vm_lifecycle.update_allowed_operations ~__context ~self:vm
+  Helpers.retry ~__context ~doc:task_id ~policy:Helpers.Policy.fail_quickly
+    (fun () ->
+      (* Note slight race here because we haven't got the master lock *)
+      Xapi_vm_lifecycle.assert_operation_valid ~__context ~self:vm ~op
+        ~strict:true ;
+      (* ... small race lives here ... *)
+      Db.VM.add_to_current_operations ~__context ~self:vm ~key:task_id ~value:op ;
+      Xapi_vm_lifecycle.update_allowed_operations ~__context ~self:vm
+  )
 
 let unlock_vm ~__context ~vm ~task_id =
   Db.VM.remove_from_current_operations ~__context ~self:vm ~key:task_id ;