update the choosing of the reddits to fix the empty-next-page bug

r2/r2/controllers/listingcontroller.py

@@ -224,8 +224,7 @@ class HotController(FixListing, ListingController):
             self.fix_listing = False
 
         if c.site == Default:
-            sr_ids = Subreddit.user_subreddits(c.user,
-                                               limit = g.num_default_reddits)
+            sr_ids = Subreddit.user_subreddits(c.user)
             return normalized_hot(sr_ids)
         #if not using the query_cache we still want cached front pages
         elif (not g.use_query_cache

r2/r2/models/subreddit.py

@@ -323,6 +323,14 @@ class Subreddit(Thing, Printable):
             srs = cls.top_lang_srs(c.content_langs, limit)
         return [s._id for s in srs] if ids else srs
 
+    @classmethod
+    @memoize('random_reddits', time = 1800)
+    def random_reddits(cls, user_name, sr_ids, limit):
+        """This gets called when a user is subscribed to more than 50
+        reddits. Randomly choose 50 of those reddits and cache it for
+        a while so their front page doesn't jump around."""
+        return random.sample(sr_ids, limit)
+
     @classmethod
     def user_subreddits(cls, user, ids = True, limit = sr_limit):
         """
@@ -336,7 +344,8 @@ class Subreddit(Thing, Printable):
         if user and user.has_subscribed:
             sr_ids = Subreddit.reverse_subscriber_ids(user)
             if limit and len(sr_ids) > limit:
-                sr_ids = random.sample(sr_ids, limit)
+                sr_ids.sort()
+                sr_ids = cls.random_reddits(user.name, sr_ids, limit)
             return sr_ids if ids else Subreddit._byID(sr_ids, True, False)
         else:
             # if there is a limit, we want *at most* limit subreddits.