Use CACHE_MIDDLEWARE_KEY_PREFIX in template cache.

[bzed@…]

Running two django instances with the same memcached as cache
backend might result in displaying the wrong template from the cache
in case we have template files with the same name but a different
content.

ba95e1ae8fb810bbc4e9a2579cff3620df7300f5
diff --git a/Django-1.1.1/django/templatetags/cache.py b/Django-1.1.1/django/templatetags/cache.py
index 387dd87..6392731 100644
--- a/Django-1.1.1/django/templatetags/cache.py
+++ b/Django-1.1.1/django/templatetags/cache.py
@@ -1,3 +1,4 @@
+from django.conf import settings
 from django.template import Library, Node, TemplateSyntaxError, Variable, VariableDoesNotExist
 from django.template import resolve_variable
 from django.core.cache import cache
@@ -13,6 +14,7 @@ class CacheNode(Node):
         self.expire_time_var = Variable(expire_time_var)
         self.fragment_name = fragment_name
         self.vary_on = vary_on
+        self.key_prefix = settings.CACHE_MIDDLEWARE_KEY_PREFIX
 
     def render(self, context):
         try:
@@ -25,7 +27,7 @@ class CacheNode(Node):
             raise TemplateSyntaxError('"cache" tag got a non-integer timeout value: %r' % expire_time)
         # Build a unicode key for this fragment and all vary-on's.
         args = md5_constructor(u':'.join([urlquote(resolve_variable(var, context)) for var in self.vary_on]))
-        cache_key = 'template.cache.%s.%s' % (self.fragment_name, args.hexdigest())
+        cache_key = 'template.cache.%s.%s.%s' % (self.key_prefix, self.fragment_name, args.hexdigest())
         value = cache.get(cache_key)
         if value is None:
             value = self.nodelist.render(context)

As various other applications which use the caching functions of django might run into the same issue, it could make sense to add the CACHE_MIDDLEWARE_KEY_PREFIX to the key at a different place, next to the backend so all keys of the instance will use it.

[Alex Gaynor]

This isn't in middleware though, so this doesnt seem semantically correct.

[bzed]

As I've described on ​my blog post we're using our own cache backend now, which is based on the original memcached backend. In my opinion this issue really needs a design decision.

[Archatas]

I've ran into the same problem running about six different Django websites on the same server which reuse some templates. In my opinion this change is crucial and maybe later CACHE_MIDDLEWARE_KEY_PREFIX can be changed to CACHE_KEY_PREFIX for logical consistency.

[André Cruz]

I also thought the key prefix would be applied to all operations involving the "cache" backend. It makes sense.

[otherjacob]

This use-case should be addressed in #13795 (pass in a site/instance-specific prefix into CACHES[cache_name]KEY_PREFIX).

[nd.nguyen@…]

You modify the cache file in django directly.
I am wondering if we overwrite file in my source code instead of change file in django. It's too dangerous. Any suggestion in this case? Tks :)