fix: Make Tiered cache compatible with both cache backends (python-me…

…mcache and pymemcache)

CHANGELOG.rst

@@ -15,8 +15,9 @@ Unreleased
 ----------
 
 Added
-
+~~~~~
 * Support added for Django 4.2
+* A new django setting ``ENABLE_FORCE_CACHE_MISS_FOR_NONE`` has been added to make ``TieredCache`` backward compatible
 
 [5.6.0] - 2023-07-24
 --------------------

edx_django_utils/cache/tests/test_utils.py

@@ -168,6 +168,35 @@ def test_get_cached_response_django_cache_hit(self, mock_cache_get):
         cached_response = self.request_cache.get_cached_response(TEST_KEY)
         self.assertTrue(cached_response.is_found, 'Django cache hit should cache value in request cache.')
 
+    @mock.patch('edx_django_utils.cache.utils.waffle.switch_is_active')
+    def test_get_cached_response_hit_with_cached_none(self, mock_switch_is_active):
+        mock_switch_is_active.return_value = True
+        TieredCache.set_all_tiers(TEST_KEY, None)
+        # Test retrieval from tier 1: RequestCache
+        cached_response = TieredCache.get_cached_response(TEST_KEY)
+        self.assertTrue(cached_response.is_found)
+        self.assertEqual(cached_response.value, None)
+
+        self.request_cache.clear()
+        # Test retrieval from tier 2: Django Cache
+        cached_response = TieredCache.get_cached_response(TEST_KEY)
+        self.assertTrue(cached_response.is_found)
+        self.assertEqual(cached_response.value, None)
+
+    @mock.patch('edx_django_utils.cache.utils.waffle.switch_is_active')
+    def test_get_cached_response_miss_with_cached_none(self, mock_switch_is_active):
+        mock_switch_is_active.return_value = False
+        TieredCache.set_all_tiers(TEST_KEY, None)
+        # Test retrieval from tier 1: RequestCache
+        cached_response = TieredCache.get_cached_response(TEST_KEY)
+        self.assertTrue(cached_response.is_found)
+        self.assertEqual(cached_response.value, None)
+
+        self.request_cache.clear()
+        # Test retrieval from tier 2: Django Cache
+        cached_response = TieredCache.get_cached_response(TEST_KEY)
+        self.assertFalse(cached_response.is_found)
+
     @mock.patch('django.core.cache.cache.get')
     def test_get_cached_response_force_cache_miss(self, mock_cache_get):
         self.request_cache.set(SHOULD_FORCE_CACHE_MISS_KEY, True)

edx_django_utils/cache/utils.py

@@ -4,6 +4,7 @@
 import hashlib
 import threading
 
+import waffle  # pylint: disable=invalid-django-waffle-import
 from django.core.cache import cache as django_cache
 from django.core.cache.backends.base import DEFAULT_TIMEOUT
 from django.utils.encoding import force_str
@@ -259,6 +260,10 @@ def _get_cached_response_from_django_cache(key):
             return CachedResponse(is_found=False, key=key, value=None)
 
         cached_value = django_cache.get(key, _CACHE_MISS)
+
+        if cached_value is None and not TieredCache._is_forced_cache_miss_for_none_disabled():
+            cached_value = _CACHE_MISS
+
         is_found = cached_value is not _CACHE_MISS
         return CachedResponse(is_found, key, cached_value)
 
@@ -303,6 +308,12 @@ def _should_force_django_cache_miss(cls):
         cached_response = DEFAULT_REQUEST_CACHE.get_cached_response(SHOULD_FORCE_CACHE_MISS_KEY)
         return False if not cached_response.is_found else cached_response.value
 
+    @staticmethod
+    def _is_forced_cache_miss_for_none_disabled():
+        # This switch is to provide backward compatibility.
+        # As we have been treating None as CACHE_MISS.
+        return waffle.switch_is_active('edx_django_utils.cache.utils.disable_forced_cache_miss_for_none')
+
 
 class CachedResponseError(Exception):
     """