routers: invalidate _urls cache on register (#6407)

see https://github.com/encode/django-rest-framework/issues/5660

Trying to register new routes on a router after having accessed the
router `urls` attribute leads to surprising results.
The route is added without error to the router's `registry` but the urls
are not updated, because they are cached in `_urls`.
This commit invalidates the cache after each new registration.

rest_framework/routers.py

@@ -100,6 +100,10 @@ class BaseRouter(six.with_metaclass(RenameRouterMethods)):
             basename = self.get_default_basename(viewset)
         self.registry.append((prefix, viewset, basename))
 
+        # invalidate the urls cache
+        if hasattr(self, '_urls'):
+            del self._urls
+
     def get_default_basename(self, viewset):
         """
         If `basename` is not specified, attempt to automatically determine

tests/test_routers.py

@@ -158,6 +158,12 @@ class TestSimpleRouter(URLPatternsTestCase, TestCase):
         response = self.client.delete(reverse('basic-action3', args=[1]))
         assert response.data == {'delete': '1'}
 
+    def test_register_after_accessing_urls(self):
+        self.router.register(r'notes', NoteViewSet)
+        assert len(self.router.urls) == 2  # list and detail
+        self.router.register(r'notes_bis', NoteViewSet)
+        assert len(self.router.urls) == 4
+
 
 class TestRootView(URLPatternsTestCase, TestCase):
     urlpatterns = [