Fix un-reblogged status being at wrong position in the home timeline (#5418)

We've changed un-reblogging behavior when we implement Snowflake, to insert un-reblogged status at the position reblogging status existed.

However, our API expects home timeline is ordered by status ids, and max_id/since_id filters by zset score. Due to this, un-reblogged status appears as a last item of result set, and timeline expansion may skips many statuses.

So this reverts that change...reblogged status inserted at corresponding position to its id.

app/lib/feed_manager.rb

@@ -198,10 +198,11 @@ class FeedManager
       # 2. Remove the reblogged status from the `:reblogs` zset.
       redis.zrem(reblog_key, status.reblog_of_id)
 
-      # 3. Add the reblogged status to the feed using the reblogging
-      # status' ID as its score, and the reblogged status' ID as its
-      # value.
-      redis.zadd(timeline_key, status.id, status.reblog_of_id)
+      # 3. Add the reblogged status to the feed.
+      # Note that we can't use old score in here
+      # and it must be an ID of corresponding status
+      # because we need to filter timeline by status ID.
+      redis.zadd(timeline_key, status.reblog_of_id, status.reblog_of_id)
 
       # 4. Remove the reblogging status from the feed (as normal)
     end

spec/lib/feed_manager_spec.rb

@@ -233,19 +233,22 @@ RSpec.describe FeedManager do
   describe '#unpush' do
     it 'leaves a reblogged status when deleting the reblog' do
       account = Fabricate(:account)
-      reblogged = Fabricate(:status)
+      reblogged = Fabricate(:status, id: Mastodon::Snowflake.id_at(2.day.ago.utc))
+      other_status = Fabricate(:status, id: Mastodon::Snowflake.id_at(1.day.ago.utc))
       status = Fabricate(:status, reblog: reblogged)
 
+      FeedManager.instance.push('type', account, other_status)
       FeedManager.instance.push('type', account, status)
 
       # The reblogging status should show up under normal conditions.
-      expect(Redis.current.zrange("feed:type:#{account.id}", 0, -1)).to eq [status.id.to_s]
+      expect(Redis.current.zrange("feed:type:#{account.id}", 0, -1)).to eq [other_status.id.to_s, status.id.to_s]
 
       FeedManager.instance.unpush('type', account, status)
 
       # Because we couldn't tell if the status showed up any other way,
       # we had to stick the reblogged status in by itself.
-      expect(Redis.current.zrange("feed:type:#{account.id}", 0, -1)).to eq [reblogged.id.to_s]
+      # And it must be ordered by status ids.
+      expect(Redis.current.zrange("feed:type:#{account.id}", 0, -1)).to eq [reblogged.id.to_s, other_status.id.to_s]
     end
 
     it 'sends push updates' do