Merge pull request #23031 from edx/revert-22779-ormsbee/revert_context_processor_cache

Re-enable caching of context processor output

common/djangoapps/edxmako/template.py

@@ -15,6 +15,7 @@
 
 from django.conf import settings
 from django.template import Context, engines
+from edx_django_utils.cache import RequestCache
 from mako.template import Template as MakoTemplate
 from six import text_type
 
@@ -45,9 +46,27 @@ class Template(object):
         """
         This takes a render call with a context (from Django) and translates
         it to a render call on the mako template.
+
+        When rendering a large sequence of XBlocks, we may end up rendering
+        hundreds of small templates. Even if context processors aren't very
+        expensive individually, they will quickly add up in that situation. To
+        help guard against this, we do context processing once for a given
+        request and then cache it.
         """
         context_object = self._get_context_object(request)
-        context_dictionary = self._get_context_processors_output_dict(context_object)
+
+        request_cache = RequestCache('context_processors')
+        cache_response = request_cache.get_cached_response('cp_output')
+        if cache_response.is_found:
+            context_dictionary = dict(cache_response.value)
+        else:
+            context_dictionary = self._get_context_processors_output_dict(context_object)
+            # The context_dictionary is later updated with template specific
+            # variables. There are potentially hundreds of calls to templates
+            # rendering and we don't want them to interfere with each other, so
+            # we make a copy from the output of the context processors and then
+            # recreate a new dict every time we pull from the cache.
+            request_cache.set('cp_output', dict(context_dictionary))
 
         if isinstance(context, Context):
             context_dictionary.update(context.flatten())