Threading bug on the /~user/threads page

[jmiven]

On my threads page, a 3 days old comment on a story is threaded under a more recent comment on another story.

[pushcx]

Well, this is the result of my big clever redesign of the threading code in #1190. I saw this bug and hoped I could override the parent_depth test in app/views/comments/_threads.html.erb by comparing thread_id, but with @lonjil’s report of #1251 it’s clear this is a more general problem. In that one, like here, the comments are in the wrong place because the parent comments have identical votes (~half of comments have one or two upvotes and zero flags) and the low byte of their id is identical. (We get ~200 comments per day, note that in both reports’ screenshots the parents are posted 2 days apart.)

This is going to be some amount of redesign to the confidence_order field and Comment#update_score_and_recalculate! for a reliable fix. I had considered that siblings might swap order a bit (see comment) but I didn’t realize that I was setting up a 1 in 256 chance that they’d swap children because of it.

An obvious fix would be to increase the comment id component of confidence_order to two bytes. It would seem that it reduces the chances from 1 in 256 to 1 in 65,536. However! We get ~200 comments per day and stories accept comments for 90 days (Story::COMMENTABLE_DAYS), so we only see ~18,000 comment IDs in the period available for a collision.

That works in practice but not in theory. I don’t love that it would set up an ever rarer bug if we grow. I have a hunch there’s a better fix available that I’m not immediately seeing, one that doesn’t increase the width of confidence_order_path. Maybe it’ll come to me, or maybe someone else has an idea for a fix?