Fix #2619 - When redis feed is empty, fall back to database (#3721)

* Fix #2619 - When redis feed is empty, fall back to database

* Use redis value to return feed from database only while RegenerationWorker
hasn't finished running

* Fix specs

* Replace usage of reject!

app/controllers/concerns/user_tracking_concern.rb

@@ -17,7 +17,7 @@ module UserTrackingConcern
     current_user.update_tracked_fields!(request)
 
     # Regenerate feed if needed
-    RegenerationWorker.perform_async(current_user.account_id) if user_needs_feed_update?
+    regenerate_feed! if user_needs_feed_update?
   end
 
   def user_needs_sign_in_update?
@@ -27,4 +27,9 @@ module UserTrackingConcern
   def user_needs_feed_update?
     current_user.last_sign_in_at < REGENERATE_FEED_DAYS.days.ago
   end
+
+  def regenerate_feed!
+    Redis.current.setnx("account:#{current_user.account_id}:regeneration", true) == 1 && Redis.current.expire("account:#{current_user.account_id}:regeneration", 3_600 * 24)
+    RegenerationWorker.perform_async(current_user.account_id)
+  end
 end

app/models/feed.rb

@@ -7,15 +7,28 @@ class Feed
   end
 
   def get(limit, max_id = nil, since_id = nil)
+    if redis.exists("account:#{@account.id}:regeneration")
+      from_database(limit, max_id, since_id)
+    else
+      from_redis(limit, max_id, since_id)
+    end
+  end
+
+  private
+
+  def from_redis(limit, max_id, since_id)
     max_id     = '+inf' if max_id.blank?
     since_id   = '-inf' if since_id.blank?
     unhydrated = redis.zrevrangebyscore(key, "(#{max_id}", "(#{since_id}", limit: [0, limit], with_scores: true).map(&:last).map(&:to_i)
     status_map = Status.where(id: unhydrated).cache_ids.map { |s| [s.id, s] }.to_h
-
     unhydrated.map { |id| status_map[id] }.compact
   end
 
-  private
+  def from_database(limit, max_id, since_id)
+    Status.as_home_timeline(@account)
+          .paginate_by_max_id(limit, max_id, since_id)
+          .reject { |status| FeedManager.instance.filter?(:home, status, @account.id) }
+  end
 
   def key
     FeedManager.instance.key(@type, @account.id)

app/services/precompute_feed_service.rb

@@ -17,6 +17,8 @@ class PrecomputeFeedService < BaseService
       statuses.each do |status|
         process_status(status)
       end
+
+      redis.del("account:#{@account.id}:regeneration")
     end
   end
 

spec/models/feed_spec.rb

@@ -2,20 +2,20 @@ require 'rails_helper'
 
 RSpec.describe Feed, type: :model do
   describe '#get' do
-    it "gets statuses with ids in the range, maintining the order from Redis" do
+    it 'gets statuses with ids in the range, maintining the order from Redis' do
       account = Fabricate(:account)
       Fabricate(:status, account: account, id: 1)
       Fabricate(:status, account: account, id: 2)
       Fabricate(:status, account: account, id: 3)
       Fabricate(:status, account: account, id: 10)
-      redis = double(zrevrangebyscore: [["val2", 2.0], ["val1", 1.0], ["val3", 3.0], ["deleted", 4.0]])
+      redis = double(zrevrangebyscore: [['val2', 2.0], ['val1', 1.0], ['val3', 3.0], ['deleted', 4.0]], exists: false)
       allow(Redis).to receive(:current).and_return(redis)
 
-      feed = Feed.new("type", account)
+      feed = Feed.new(:home, account)
       results = feed.get(3)
 
       expect(results.map(&:id)).to eq [2, 1, 3]
-      expect(results.first.attributes.keys).to eq ["id", "updated_at"]
+      expect(results.first.attributes.keys).to eq %w(id updated_at)
     end
   end
 end