Fixed #29744 -- Fixed caching of URLResolver for a default URLconf.

get_resolver() for a default URLconf (passing no argument) and for
settings.ROOT_URLCONF should return the same cached object.

3 files changed, 18 insertions, 4 deletions

diff --git a/django/urls/base.py b/django/urls/base.py
index 0e1c3d909c..dd9b2e4c22 100644
--- a/django/urls/base.py
+++ b/django/urls/base.py
@@ -7,7 +7,7 @@ from django.utils.functional import lazy
 from django.utils.translation import override
 
 from .exceptions import NoReverseMatch, Resolver404
-from .resolvers import get_ns_resolver, get_resolver
+from .resolvers import _get_cached_resolver, get_ns_resolver, get_resolver
 from .utils import get_callable
 
 # SCRIPT_NAME prefixes for each thread are stored here. If there's no entry for
@@ -92,7 +92,7 @@ reverse_lazy = lazy(reverse, str)
 
 def clear_url_caches():
     get_callable.cache_clear()
-    get_resolver.cache_clear()
+    _get_cached_resolver.cache_clear()
     get_ns_resolver.cache_clear()
 
 
diff --git a/django/urls/resolvers.py b/django/urls/resolvers.py
index 247e3680c0..2154a46320 100644
--- a/django/urls/resolvers.py
+++ b/django/urls/resolvers.py
@@ -63,10 +63,14 @@ class ResolverMatch:
         )
 
 
-@functools.lru_cache(maxsize=None)
 def get_resolver(urlconf=None):
     if urlconf is None:
         urlconf = settings.ROOT_URLCONF
+    return _get_cached_resolver(urlconf)
+
+
+@functools.lru_cache(maxsize=None)
+def _get_cached_resolver(urlconf=None):
     return URLResolver(RegexPattern(r'^/'), urlconf)
 
 
diff --git a/tests/urlpatterns/test_resolvers.py b/tests/urlpatterns/test_resolvers.py
index a32e8edaf7..eaca6096d9 100644
--- a/tests/urlpatterns/test_resolvers.py
+++ b/tests/urlpatterns/test_resolvers.py
@@ -1,5 +1,6 @@
 from django.test import SimpleTestCase
-from django.urls.resolvers import RegexPattern, RoutePattern
+from django.test.utils import override_settings
+from django.urls.resolvers import RegexPattern, RoutePattern, get_resolver
 from django.utils.translation import gettext_lazy as _
 
 
@@ -13,3 +14,12 @@ class RoutePatternTests(SimpleTestCase):
 
     def test_str(self):
         self.assertEqual(str(RoutePattern(_('translated/'))), 'translated/')
+
+
+class ResolverCacheTests(SimpleTestCase):
+    @override_settings(ROOT_URLCONF='urlpatterns.path_urls')
+    def test_resolver_cache_default__root_urlconf(self):
+        # resolver for a default URLconf (passing no argument) and for the
+        # settings.ROOT_URLCONF is the same cached object.
+        self.assertIs(get_resolver(), get_resolver('urlpatterns.path_urls'))
+        self.assertIsNot(get_resolver(), get_resolver('urlpatterns.path_dynamic_urls'))