Override the RedisCache set method to not return a value in line with…

… Django's expectations of a cache backend.

kolibri/core/utils/cache.py

@@ -3,6 +3,7 @@
 from django.core.cache import caches
 from django.core.cache import InvalidCacheBackendError
 from django.utils.functional import SimpleLazyObject
+from redis_cache import RedisCache as BaseRedisCache
 
 
 logger = logging.getLogger(__name__)
@@ -61,3 +62,15 @@ def save(self):
             logger.info("Overwriting Redis config")
             self.client.config_rewrite()
             self.changed = False
+
+
+class RedisCache(BaseRedisCache):
+    def set(self, *args, **kwargs):
+        """
+        Overwrite the set method to not return a value, in line with the Django cache interface
+        This causes particular issues for Django's caching middleware, which expects the set method to return None
+        as it invokes it directly in a lambda in the response.add_post_render_callback method
+        We use a similar pattern in our own caching decorator in kolibri/core/content/api.py and saw errors
+        due to the fact if the lambda returns a value, it is interpreted as a replacement for the response object.
+        """
+        super(RedisCache, self).set(*args, **kwargs)

kolibri/deployment/default/cache.py

@@ -38,7 +38,7 @@
 
 if cache_options["CACHE_BACKEND"] == "redis":
     base_cache = {
-        "BACKEND": "redis_cache.RedisCache",
+        "BACKEND": "kolibri.core.utils.cache.RedisCache",
         "LOCATION": cache_options["CACHE_LOCATION"],
         # Default time out of each cache key
         "TIMEOUT": cache_options["CACHE_TIMEOUT"],