models: Fix performance of supported_auth_backends with caching.

See the comment, but this is a significant performance optimization
for all of our pages using common_context, because this code path is
called more than a dozen times (recursively) by common_context.

zerver/lib/test_classes.py

@@ -38,6 +38,7 @@ from zerver.lib.test_helpers import (
 )
 
 from zerver.models import (
+    clear_supported_auth_backends_cache,
     get_stream,
     get_client,
     get_display_recipient,
@@ -94,6 +95,7 @@ class ZulipTestCase(TestCase):
         super().tearDown()
         # Important: we need to clear event queues to avoid leaking data to future tests.
         clear_client_event_queues_for_testing()
+        clear_supported_auth_backends_cache()
 
     '''
     WRAPPER_COMMENT:

zerver/models.py

@@ -133,6 +133,22 @@ def get_realm_emoji_cache_key(realm: 'Realm') -> str:
 def get_active_realm_emoji_cache_key(realm: 'Realm') -> str:
     return u'active_realm_emoji:%s' % (realm.id,)
 
+# This simple call-once caching saves ~500us in auth_enabled_helper,
+# which is a significant optimization for common_context.  Note that
+# these values cannot change in a running production system, but do
+# regularly change within unit tests; we address the latter by calling
+# clear_supported_auth_backends_cache in our standard tearDown code.
+supported_backends = None  # type: Optional[Set[type]]
+def supported_auth_backends() -> Set[type]:
+    global supported_backends
+    if supported_backends is None:
+        supported_backends = django.contrib.auth.get_backends()
+    return supported_backends
+
+def clear_supported_auth_backends_cache() -> None:
+    global supported_backends
+    supported_backends = None
+
 class Realm(models.Model):
     MAX_REALM_NAME_LENGTH = 40
     MAX_REALM_SUBDOMAIN_LENGTH = 40
@@ -339,7 +355,7 @@ class Realm(models.Model):
         from zproject.backends import AUTH_BACKEND_NAME_MAP
 
         ret = {}  # type: Dict[str, bool]
-        supported_backends = {backend.__class__ for backend in django.contrib.auth.get_backends()}
+        supported_backends = [backend.__class__ for backend in supported_auth_backends()]
         for k, v in self.authentication_methods.iteritems():
             backend = AUTH_BACKEND_NAME_MAP[k]
             if backend in supported_backends:

zerver/tests/test_auth_backends.py

@@ -43,7 +43,8 @@ from zerver.lib.test_classes import (
 )
 from zerver.models import \
     get_realm, email_to_username, CustomProfileField, CustomProfileFieldValue, \
-    UserProfile, PreregistrationUser, Realm, RealmDomain, get_user, MultiuseInvite
+    UserProfile, PreregistrationUser, Realm, RealmDomain, get_user, MultiuseInvite, \
+    clear_supported_auth_backends_cache
 from zerver.signals import JUST_CREATED_THRESHOLD
 
 from confirmation.models import Confirmation, create_confirmation_link
@@ -78,7 +79,7 @@ class AuthBackendTest(ZulipTestCase):
         return username
 
     def verify_backend(self, backend: Any, good_kwargs: Optional[Dict[str, Any]]=None, bad_kwargs: Optional[Dict[str, Any]]=None) -> None:
-
+        clear_supported_auth_backends_cache()
         user_profile = self.example_user('hamlet')
 
         assert good_kwargs is not None
@@ -116,7 +117,9 @@ class AuthBackendTest(ZulipTestCase):
 
         # Verify auth fails if the auth backend is disabled on server
         with self.settings(AUTHENTICATION_BACKENDS=('zproject.backends.ZulipDummyBackend',)):
+            clear_supported_auth_backends_cache()
             self.assertIsNone(backend.authenticate(**good_kwargs))
+        clear_supported_auth_backends_cache()
 
         # Verify auth fails if the auth backend is disabled for the realm
         for backend_name in AUTH_BACKEND_NAME_MAP.keys():