Merge pull request xapi-project#3284 from edwintorok/is-valid-ref-no-…

…rpc-null

CA-267946: Avoid Db.is_valid_ref RPC call when Ref is NULL

ocaml/xapi/db.ml

@@ -17,6 +17,8 @@
 
 include Db_actions.DB_Action
 let is_valid_ref __context r =
-  let t = Context.database_of __context in
-  let module DB = (val (Db_cache.get t) : Db_interface.DB_ACCESS) in
-  DB.is_valid_ref t (Ref.string_of r)
+  if r = Ref.null then false
+  else
+    let t = Context.database_of __context in
+    let module DB = (val (Db_cache.get t) : Db_interface.DB_ACCESS) in
+    DB.is_valid_ref t (Ref.string_of r)