[manual] Cache results of the Twitter API in the database.

This should substantially improve the repeat-rendering time for pages
with large numbers of tweets since we don't need to go all the way to
twitter.com, which can take like a second, to render tweets properly.

To deploy this commit properly, one needs to run

./manage.py createcachetable third_party_api_results

(imported from commit 01b528e61f9dde2ee718bdec0490088907b6017e)

zephyr/lib/cache.py

@@ -1,6 +1,7 @@
 from functools import wraps
 
 from django.core.cache import cache as djcache
+from django.core.cache import get_cache
 
 def cache_with_key(keyfunc):
     """Decorator which applies Django caching to a function.
@@ -44,3 +45,31 @@ def cache(func):
         return key.replace('-','--').replace(' ','-s')
 
     return cache_with_key(keyfunc)(func)
+
+def db_cache_with_key(keyfunc):
+    """Decorator which applies Django caching to a function.
+
+       Decorator argument is a function which computes a cache key
+       from the original function's arguments.  You are responsible
+       for avoiding collisions with other uses of this decorator or
+       other uses of caching."""
+
+    def decorator(func):
+        @wraps(func)
+        def func_with_caching(*args, **kwargs):
+            key = keyfunc(*args, **kwargs)
+            database_cache = get_cache("database")
+            val = database_cache.get(key)
+
+            # Values are singleton tuples so that we can distinguish
+            # a result of None from a missing key.
+            if val is not None:
+                return val[0]
+
+            val = func(*args, **kwargs)
+            database_cache.set(key, (val,))
+            return val
+
+        return func_with_caching
+
+    return decorator