78 lines
3.0 KiB
Ruby
78 lines
3.0 KiB
Ruby
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class FixReblogsInFeeds < ActiveRecord::Migration[5.1]
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def up
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redis = Redis.current
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fm = FeedManager.instance
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# Old scheme:
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# Each user's feed zset had a series of score:value entries,
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# where "regular" statuses had the same score and value (their
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# ID). Reblogs had a score of the reblogging status' ID, and a
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# value of the reblogged status' ID.
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# New scheme:
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# The feed contains only entries with the same score and value.
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# Reblogs result in the reblogging status being added to the
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# feed, with an entry in a reblog tracking zset (where the score
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# is once again set to the reblogging status' ID, and the value
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# is set to the reblogged status' ID). This is safe for Redis'
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# float coersion because in this reblog tracking zset, we only
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# need the rebloggging status' ID to be able to stop tracking
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# entries after they have gotten too far down the feed, which
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# does not require an exact value.
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# This process reads all feeds and writes 3 times for each reblogs.
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# So we use Lua script to avoid overhead between Ruby and Redis.
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script = <<-LUA
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local timeline_key = KEYS[1]
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local reblog_key = KEYS[2]
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-- So, first, we iterate over the user's feed to find any reblogs.
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local items = redis.call('zrange', timeline_key, 0, -1, 'withscores')
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for i = 1, #items, 2 do
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local reblogged_id = items[i]
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local reblogging_id = items[i + 1]
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if (reblogged_id ~= reblogging_id) then
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-- The score and value don't match, so this is a reblog.
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-- (note that we're transitioning from IDs < 53 bits so we
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-- don't have to worry about the loss of precision)
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-- Remove the old entry
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redis.call('zrem', timeline_key, reblogged_id)
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-- Add a new one for the reblogging status
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redis.call('zadd', timeline_key, reblogging_id, reblogging_id)
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-- Track the fact that this was a reblog
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redis.call('zadd', reblog_key, reblogging_id, reblogged_id)
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end
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end
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LUA
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script_hash = redis.script(:load, script)
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# find_each is batched on the database side.
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User.includes(:account).find_each do |user|
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account = user.account
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timeline_key = fm.key(:home, account.id)
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reblog_key = fm.key(:home, account.id, 'reblogs')
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redis.evalsha(script_hash, [timeline_key, reblog_key])
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end
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end
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def down
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# We *deliberately* do nothing here. This means that reverting
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# this and the associated changes to the FeedManager code could
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# allow one superfluous reblog of any given status, but in the case
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# where things have gone wrong and a revert is necessary, this
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# appears preferable to requiring a database hit for every status
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# in every users' feed simply to revert.
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# Note that this is operating under the assumption that entries
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# with >53-bit IDs have already been entered. Otherwise, we could
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# just use the data in Redis to reverse this transition.
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end
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end
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