Add RetryingMemcacheClient class

Add a RetryingMemcacheClient class. Using this will allow
momentary network errors while fetching course configurations
to not stop and fail the whole course build process.

Author: murhum1

gitmanager/settings.py

@@ -169,6 +169,13 @@
 #SESSION_ENGINE = 'django.contrib.sessions.backends.cached_db'
 ##########################################################################
 
+# pymemcache's RetryingClient configuration options
+# https://pymemcache.readthedocs.io/en/latest/apidoc/pymemcache.client.retrying.html
+RETRYING_MEMCACHE_CLIENT_OPTIONS = {
+    'attempts': 10,
+    'retry_delay': 3,
+}
+
 # Internationalization
 # https://docs.djangoproject.com/en/1.6/topics/i18n/
 LANGUAGE_CODE = 'en-us'

util/retrying_memcache_client.py

@@ -0,0 +1,11 @@
+from pymemcache.client import retrying, hash
+from django.core.cache.backends.memcached import PyMemcacheCache
+from django.conf import settings
+
+class RetryingMemcacheClient(PyMemcacheCache):
+    def __init__(self, server, params):
+        super().__init__(server, params)
+        self._cache = retrying.RetryingClient(
+            hash.HashClient(self.client_servers, **self._options),
+            **settings.RETRYING_MEMCACHE_CLIENT_OPTIONS
+        )