Sticky Comments: Move sorting to builder

Using a sort value for sticky comments was leading to a lot of pain
when dealing with rendering edge cases (specifically hiding children).
Using this approach we more directly special case the sticky comment
and explicitly do not render its children when viewing top level
comments, which is much cleaner.

P.S. This is almost entirely @bsimpson63's code and idea, with some
extra docs. Thank you Brian!

Author: umbrae

r2/r2/lib/comment_tree.py

@@ -42,10 +42,6 @@ def parent_comments_key(link_id):
 
 
 def _get_sort_value(comment, sort, link, children=None):
-    # Sticky comments get a very high score for all sorts.
-    if link and comment._id == link.sticky_comment_id:
-        return 2 ** 31 - 1
-
     if sort == "_date":
         return epoch_seconds(comment._date)
     if sort == '_qa':

r2/r2/models/builder.py

@@ -853,6 +853,7 @@ def _get_comments(self):
         dont_collapse = []
         candidates = []
         offset_depth = 0
+        items = []
 
         if self.children:
             # requested specific child comments
@@ -886,14 +887,39 @@ def _get_comments(self):
             offset_depth = depth.get(path[-1], 0)
 
         else:
-            # full tree requested, start with the top level comments
+            # full tree requested, add all top level comments as candidates to
+            # be considered for display
             top_level_comments = cid_tree.get(None, ())
+
+            # If we have a sticky comment and we're viewing top level comments,
+            # we shove the sticky comment in the top and remove it from the
+            # remainder of the top level comments list so it's displayed first.
+            if self.link.sticky_comment_id:
+                try:
+                    # Remove the sticky comment from the candidates list so it
+                    # isn't displayed twice
+                    top_level_comments.remove(self.link.sticky_comment_id)
+                except ValueError:
+                    g.log.warning("Non-top-level sticky comment detected on "
+                                  "link %r.", self.link)
+                    pass
+                else:
+                    # Make the sticky comment the first item, meaning it is the
+                    # first comment displayed regardless of sort score.
+                    items.append(self.link.sticky_comment_id)
+
+                    # Don't add its children as candidates - we don't want to
+                    # show any replies to the sticky comment when viewing a top
+                    # level list. This will still render the "load more
+                    # comments" link for viewing its children. When viewing
+                    # this comment as non-top-level (for example a permalink)
+                    # sticky comment children will render normally.
+
             self.update_candidates(candidates, sorter, top_level_comments)
 
         timer.intermediate("pick_candidates")
 
         # choose which comments to show
-        items = []
         while (self.num is None or len(items) < self.num) and candidates:
             sort_val, comment_id = heapq.heappop(candidates)
             if comment_id not in cids:
@@ -994,11 +1020,6 @@ def _make_wrapped_tree(self):
 
             parent = wrapped_by_id.get(comment.parent_id)
 
-            # Children of sticky comments are hidden by default to avoid top
-            # comment hijacking or conversation derailing.
-            if parent and self.link.sticky_comment_id == parent._id:
-                comment.hidden = True
-
             if qa_sort_hiding:
                 # In the Q&A sort type, we want to collapse all comments other
                 # than those that are:

r2/r2/models/link.py

@@ -1012,10 +1012,6 @@ def set_sticky_comment(self, comment, set_by=None):
         self.sticky_comment_id = comment._id
         self._commit()
 
-        # Update votes on this comment to reset its sort values (as sticky
-        # maxes them out in comment_tree.py)
-        update_comment_votes(comment)
-
         if set_by:
             ModAction.create(
                 self.subreddit_slow,
@@ -1047,8 +1043,11 @@ def remove_sticky_comment(self, comment=None, set_by=None):
         self.sticky_comment_id = None
         self._commit()
 
-        # Update votes on this comment to reset its sort values (as sticky
-        # maxes them out in comment_tree.py)
+        # LEGACY: This is to deal with the old way that sticky comments used
+        # to alter sort values. This is only still here so that when a comment
+        # gets unstickied it will have its sort value corrected. We can remove
+        # this line after 2016-02-01 or so when unstickying comments from
+        # before this update will be rare.
         update_comment_votes(prev_sticky_comment)
 
         if set_by: