catstodon/db/migrate/20170920032311_fix_reblogs_in_feeds.rb

class FixReblogsInFeeds < ActiveRecord::Migration[5.1]
  def up
    redis = Redis.current
    fm = FeedManager.instance

    # find_each is batched on the database side.
    User.includes(:account).find_each do |user|
      account = user.account

      # Old scheme:
      # Each user's feed zset had a series of score:value entries,
      # where "regular" statuses had the same score and value (their
      # ID). Reblogs had a score of the reblogging status' ID, and a
      # value of the reblogged status' ID.

      # New scheme:
      # The feed contains only entries with the same score and value.
      # Reblogs result in the reblogging status being added to the
      # feed, with an entry in a reblog tracking zset (where the score
      # is once again set to the reblogging status' ID, and the value
      # is set to the reblogged status' ID). This is safe for Redis'
      # float coersion because in this reblog tracking zset, we only
      # need the rebloggging status' ID to be able to stop tracking
      # entries after they have gotten too far down the feed, which
      # does not require an exact value.

      # So, first, we iterate over the user's feed to find any reblogs.
      timeline_key = fm.key(:home, account.id)
      reblog_key = fm.key(:home, account.id, 'reblogs')
      redis.zrange(timeline_key, 0, -1, with_scores: true).each do |entry|
        next if entry[0] == entry[1]

        # The score and value don't match, so this is a reblog.
        # (note that we're transitioning from IDs < 53 bits so we
        # don't have to worry about the loss of precision)

        reblogged_id, reblogging_id = entry

        # Remove the old entry
        redis.zrem(timeline_key, reblogged_id)

        # Add a new one for the reblogging status
        redis.zadd(timeline_key, reblogging_id, reblogging_id)

        # Track the fact that this was a reblog
        redis.zadd(reblog_key, reblogging_id, reblogged_id)
      end
    end
  end

  def down
    # We *deliberately* do nothing here. This means that reverting
    # this and the associated changes to the FeedManager code could
    # allow one superfluous reblog of any given status, but in the case
    # where things have gone wrong and a revert is necessary, this
    # appears preferable to requiring a database hit for every status
    # in every users' feed simply to revert.

    # Note that this is operating under the assumption that entries
    # with >53-bit IDs have already been entered. Otherwise, we could
    # just use the data in Redis to reverse this transition.
  end
end