CommentTree.add_comments: Add orphaned comments to the tree

Adding them to the tree doesn't hurt anything--they still won't be
displayed until their parent comment is in the tree, but it does allow
for easier cleanup in case comments get processed out of order. Once their
parent is added to the tree everything will be consistent and fine.

r2/r2/models/comment_tree.py

@@ -129,23 +129,22 @@ def add_comments(cls, link, comments):
                 if comment._id not in cids
             }
 
-            # skip adding any comments whose parents are missing from the tree
-            # because they will never be displayed unless the tree is rebuilt.
-            # check for the parent in the existing tree and in the current
-            # batch of comments to be added.
+            if not comments:
+                return
+
+            # warn on any comments whose parents are missing from the tree
+            # because they will never be displayed unless their parent is
+            # added. this can happen in normal operation if there are multiple
+            # queue consumers and a child is processed before its parent.
             parent_ids = set(cids) | {comment._id for comment in comments}
-            orphan_comments = {
+            possible_orphan_comments = {
                 comment for comment in comments
                 if (comment.parent_id and comment.parent_id not in parent_ids)
             }
-            if orphan_comments:
+            if possible_orphan_comments:
                 g.log.error("comment_tree_inconsistent: %s %s", link,
-                    orphan_comments)
+                    possible_orphan_comments)
                 g.stats.simple_event('comment_tree_inconsistent')
-            comments -= orphan_comments
-
-            if not comments:
-                return
 
             for comment in comments:
                 tree.setdefault(comment.parent_id, []).append(comment._id)