Do not push status to feed if its reblog is already inserted (#6488)

A complemental change for precompute_feed_service_spec.rb also fixes its
random failure which is caused by the Snowlake randomization of the order
of an original status and its reblog.

app/lib/feed_manager.rb

@@ -220,6 +220,14 @@ class FeedManager
         return false
       end
     else
+      # A reblog may reach earlier than the original status because of the
+      # delay of the worker deliverying the original status, the late addition
+      # by merging timelines, and other reasons.
+      # If such a reblog already exists, just do not re-insert it into the feed.
+      rank = redis.zrevrank(reblog_key, status.id)
+
+      return false unless rank.nil?
+
       redis.zadd(timeline_key, status.id, status.id)
     end
 

spec/fabricators/list_fabricator.rb

 Fabricator(:list) do
-  account nil
+  account
   title   "MyString"
 end

spec/lib/feed_manager_spec.rb

@@ -157,7 +157,7 @@ RSpec.describe FeedManager do
     end
   end
 
-  describe '#push' do
+  describe '#push_to_home' do
     it 'trims timelines if they will have more than FeedManager::MAX_ITEMS' do
       account = Fabricate(:account)
       status = Fabricate(:status)
@@ -248,6 +248,39 @@ RSpec.describe FeedManager do
         expect(FeedManager.instance.push_to_home(account, reblogs.last)).to be true
       end
     end
+
+    it "does not push when the given status's reblog is already inserted" do
+      account = Fabricate(:account)
+      reblog = Fabricate(:status)
+      status = Fabricate(:status, reblog: reblog)
+      FeedManager.instance.push_to_home(account, status)
+
+      expect(FeedManager.instance.push_to_home(account, reblog)).to eq false
+    end
+  end
+
+  describe '#push_to_list' do
+    it "does not push when the given status's reblog is already inserted" do
+      list = Fabricate(:list)
+      reblog = Fabricate(:status)
+      status = Fabricate(:status, reblog: reblog)
+      FeedManager.instance.push_to_list(list, status)
+
+      expect(FeedManager.instance.push_to_list(list, reblog)).to eq false
+    end
+  end
+
+  describe '#merge_into_timeline' do
+    it "does not push source account's statuses whose reblogs are already inserted" do
+      account = Fabricate(:account, id: 0)
+      reblog = Fabricate(:status)
+      status = Fabricate(:status, reblog: reblog)
+      FeedManager.instance.push_to_home(account, status)
+
+      FeedManager.instance.merge_into_timeline(account, reblog.account)
+
+      expect(Redis.current.zscore("feed:home:0", reblog.id)).to eq nil
+    end
   end
 
   describe '#trim' do

spec/services/precompute_feed_service_spec.rb

@@ -9,14 +9,11 @@ RSpec.describe PrecomputeFeedService do
     let(:account) { Fabricate(:account) }
     it 'fills a user timeline with statuses' do
       account = Fabricate(:account)
-      followed_account = Fabricate(:account)
-      Fabricate(:follow, account: account, target_account: followed_account)
-      reblog = Fabricate(:status, account: followed_account)
-      status = Fabricate(:status, account: account, reblog: reblog)
+      status = Fabricate(:status, account: account)
 
       subject.call(account)
 
-      expect(Redis.current.zscore(FeedManager.instance.key(:home, account.id), reblog.id)).to be_within(0.1).of(status.id.to_f)
+      expect(Redis.current.zscore(FeedManager.instance.key(:home, account.id), status.id)).to be_within(0.1).of(status.id.to_f)
     end
 
     it 'does not raise an error even if it could not find any status' do