Cache edxmako request context computation.

When capa problem rendering was moved to happen inline on courseware
page loads, we started executing many more Mako templates on sequences
with large numbers of thse problems. To help offset this, we're caching
the context generation (it showed up as the easiest piece of low
hanging fruit on profiles of the courseware index page).

[PERF-261]

common/djangoapps/edxmako/middleware.py

@@ -19,6 +19,8 @@ from django.template.context import _builtin_context_processors
 from django.utils.module_loading import import_string
 from util.request import safe_get_host
 
+from request_cache.middleware import RequestCache
+
 REQUEST_CONTEXT = threading.local()
 
 
@@ -51,6 +53,12 @@ def get_template_request_context():
     request = getattr(REQUEST_CONTEXT, "request", None)
     if not request:
         return None
+
+    request_cache_dict = RequestCache.get_request_cache().data
+    cache_key = "edxmako_request_context"
+    if cache_key in request_cache_dict:
+        return request_cache_dict[cache_key]
+
     context = RequestContext(request)
     context['is_secure'] = request.is_secure()
     context['site'] = safe_get_host(request)
@@ -62,4 +70,6 @@ def get_template_request_context():
     for processor in get_template_context_processors():
         context.update(processor(request))
 
+    request_cache_dict[cache_key] = context
+
     return context

common/djangoapps/edxmako/shortcuts.py

@@ -33,7 +33,6 @@ def marketing_link(name):
     possible URLs for certain links. This function is to decides
     which URL should be provided.
     """
-
     # link_map maps URLs from the marketing site to the old equivalent on
     # the Django site
     link_map = settings.MKTG_URL_LINK_MAP