Added a cache_if_anonymous decorator.

common/djangoapps/util/cache.py

@@ -5,6 +5,8 @@ invalidation. Import these instead of django.core.cache.
 Note that 'default' is being preserved for user session caching, which we're 
 not migrating so as not to inconvenience users by logging them all out.
 """
+from functools import wraps
+
 from django.core import cache
 
 # If we can't find a 'general' CACHE defined in settings.py, we simply fall back
@@ -14,3 +16,41 @@ try:
 except Exception:
     cache = cache.cache
 
+def cache_if_anonymous(view_func):
+    """
+    Many of the pages in edX are identical when the user is not logged
+    in, but should not be cached when the user is logged in (because 
+    of the navigation bar at the top with the username). 
+    
+    The django middleware cache does not handle this correctly, because
+    we access the session to put the csrf token in the header. This adds
+    the cookie to the vary header, and so every page is cached seperately
+    for each user (because each user has a different csrf token).
+    
+    Note that this decorator should only be used on views that do not
+    contain the csrftoken within the html. The csrf token can be included
+    in the header by ordering the decorators as such:
+    
+    @ensure_csrftoken
+    @cache_if_anonymous
+    def myView(request):
+    """
+    
+    @wraps(view_func)
+    def _decorated(request, *args, **kwargs):
+        if not request.user.is_authenticated():
+            #Use the cache
+            cache_key = "cache_if_anonymous." + request.path
+            response = cache.get(cache_key)
+            if not response:
+                response = view_func(request, *args, **kwargs)
+                cache.set(cache_key, response, 60 * 3)
+            
+            return response
+            
+        else:
+            #Don't use the cache
+            return view_func(request, *args, **kwargs)
+    
+    return _decorated
+