'RegexURLResolver._populate() can be called concurrently, but not more
than once per thread (#26888).'

def test_populate_concurrency(self):

    resolver = RegexURLResolver('^/', 'urlpatterns_reverse.urls')
    resolver._local.populating = True
    thread = threading.Thread(target=resolver._populate)
    thread.start()
    thread.join()
    self.assertNotEqual(resolver._reverse_dict, {})
