Gab Social. All are welcome.

db/migrate/20170920032311_fix_reblogs_in_feeds.rb

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+class FixReblogsInFeeds < ActiveRecord::Migration[5.1]
+  def up
+    redis = Redis.current
+    fm = FeedManager.instance
+
+    # 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.
+
+    # This process reads all feeds and writes 3 times for each reblogs.
+    # So we use Lua script to avoid overhead between Ruby and Redis.
+    script = <<-LUA
+      local timeline_key = KEYS[1]
+      local reblog_key = KEYS[2]
+
+      -- So, first, we iterate over the user's feed to find any reblogs.
+      local items = redis.call('zrange', timeline_key, 0, -1, 'withscores')
+
+      for i = 1, #items, 2 do
+        local reblogged_id = items[i]
+        local reblogging_id = items[i + 1]
+        if (reblogged_id ~= reblogging_id) then
+
+          -- 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)
+
+          -- Remove the old entry
+          redis.call('zrem', timeline_key, reblogged_id)
+
+          -- Add a new one for the reblogging status
+          redis.call('zadd', timeline_key, reblogging_id, reblogging_id)
+
+          -- Track the fact that this was a reblog
+          redis.call('zadd', reblog_key, reblogging_id, reblogged_id)
+        end
+      end
+    LUA
+    script_hash = redis.script(:load, script)
+
+    # find_each is batched on the database side.
+    User.includes(:account).find_each do |user|
+      account = user.account
+
+      timeline_key = fm.key(:home, account.id)
+      reblog_key = fm.key(:home, account.id, 'reblogs')
+
+      redis.evalsha(script_hash, [timeline_key, reblog_key])
+    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