def METHOD_NAME(self, spawn, find_binary):
    # fake external syncer
    class FooSyncer(base.ExternalSyncer):
        binary = "foo"
    # fake that the external binary exists
    find_binary.side_effect = lambda x: x
    o = FooSyncer(self.repo_path, "http://dar")
    o.uid = 1234
    o.gid = 2345
    o._spawn("cmd", pipes={})
    assert spawn.call_args[1]["uid"] == o.uid
    assert spawn.call_args[1]["gid"] == o.gid