CommentTreeStorageV1: Don't trigger a rebuild on miss

Instead just assume that the comment tree is empty. One downside
of this approach is that until a comment tree is created (when
a new comment is added), each attempt to retrieve the comment tree
will result in a read hitting Cassandra (permacache).

Doing a rebuild was bad because it caused lock contention if there
were simultaneous attempts to:

1) View the link's comments page
2) Add a new comment
3) Vote on a comment (update_comment_votes retrieves the tree for qa sort)

Author: bsimpson63

r2/r2/lib/comment_tree.py

@@ -281,9 +281,7 @@ def get_comment_tree(link, _update=False, timer=None):
     if not _update:
         cache = CommentTree.by_link(link)
         timer.intermediate('load')
-
-        if cache:
-            return cache
+        return cache
 
     with CommentTree.mutation_context(link, timeout=180):
         timer.intermediate('lock')

r2/r2/models/comment_tree.py

@@ -413,7 +413,11 @@ def by_link(cls, link):
 
         r = g.permacache.get(key)
         if not r:
-            return None
+            # this link has not had an empty tree written for it. make an empty
+            # tree here and return it. the downside is that until a comment is
+            # added every time the storage for this link is requested it will
+            # have a cache miss and fall through to cassandra
+            return dict(cids=[], tree={}, depth={}, parents={})
 
         try:
             cids, cid_tree, depth = r
@@ -479,10 +483,7 @@ def mutation_context(cls, link, timeout=None):
     def by_link(cls, link):
         impl = cls.IMPLEMENTATIONS[link.comment_tree_version]
         data = impl.by_link(link)
-        if data is None:
-            return None
-        else:
-            return cls(link, **data)
+        return cls(link, **data)
 
     def add_comments(self, comments):
         impl = self.IMPLEMENTATIONS[self.link.comment_tree_version]