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

@@ -6,6 +6,7 @@
 from unittest import TestCase, mock
 
 import ddt
+from django.test.utils import override_settings
 
 from edx_django_utils.cache.utils import (
     DEFAULT_REQUEST_CACHE_NAMESPACE,
@@ -168,6 +169,32 @@ 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.')
 
+    @override_settings(ENABLE_FORCE_CACHE_MISS_FOR_NONE=False)
+    def test_get_cached_response_hit_with_cached_none(self):
+        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)
+
+    def test_get_cached_response_miss_with_cached_none(self):
+        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

@@ -4,10 +4,12 @@
 import hashlib
 import threading
 
+from django.conf import settings
 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
 
+
 FORCE_CACHE_MISS_PARAM = 'force_cache_miss'
 DEFAULT_NAMESPACE = 'edx_django_utils.cache'
 DEFAULT_REQUEST_CACHE_NAMESPACE = f'{DEFAULT_NAMESPACE}.default'
@@ -259,6 +261,19 @@ 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 getattr(settings, 'ENABLE_FORCE_CACHE_MISS_FOR_NONE', True):
+            # `ENABLE_FORCE_CACHE_MISS_FOR_NONE` is to provide backward compatibility.
+            # As we have been treating None as CACHE_MISS.
+            cached_value = _CACHE_MISS
+
+            # Avoiding circular import
+            from edx_django_utils.monitoring import set_custom_attribute
+            # .. custom_attribute_name: forced_cache_miss_occurred_for_none
+            # .. custom_attribute_description: True if we explicitly do CACHE MISS when None is retrieved
+            #   from cache.
+            set_custom_attribute('forced_cache_miss_occurred_for_none', True)
+
         is_found = cached_value is not _CACHE_MISS
         return CachedResponse(is_found, key, cached_value)