Refresh project authorizations using a Redis lease

When I proposed using serializable transactions I was hoping we would be
able to refresh data of individual users concurrently. Unfortunately
upon closer inspection it was revealed this was not the case. This could
result in a lot of queries failing due to serialization errors,
overloading the database in the process (given enough workers trying to
update the target table).

To work around this we're now using a Redis lease that is cancelled upon
completion. This ensures we can update the data of different users
concurrently without overloading the database.

The code will try to obtain the lease until it succeeds, waiting at
least 1 second between retries. This is necessary as we may otherwise
end up _not_ updating the data which is not an option.

app/models/user.rb

@@ -445,13 +445,12 @@ class User < ActiveRecord::Base
   end
 
   def refresh_authorized_projects
-    loop do
-      begin
-        Gitlab::Database.serialized_transaction do
+    transaction do
       project_authorizations.delete_all
 
-          # project_authorizations_union can return multiple records for the same project/user with
-          # different access_level so we take row with the maximum access_level
+      # project_authorizations_union can return multiple records for the same
+      # project/user with different access_level so we take row with the maximum
+      # access_level
       project_authorizations.connection.execute <<-SQL
       INSERT INTO project_authorizations (user_id, project_id, access_level)
       SELECT user_id, project_id, MAX(access_level) AS access_level
@@ -459,13 +458,8 @@ class User < ActiveRecord::Base
       GROUP BY user_id, project_id
       SQL
 
-          update_column(:authorized_projects_populated, true) unless authorized_projects_populated
-        end
-
-        break
-      # In the event of a concurrent modification Rails raises StatementInvalid.
-      # In this case we want to keep retrying until the transaction succeeds
-      rescue ActiveRecord::StatementInvalid
+      unless authorized_projects_populated
+        update_column(:authorized_projects_populated, true)
       end
     end
   end

app/workers/authorized_projects_worker.rb

@@ -2,14 +2,33 @@ class AuthorizedProjectsWorker
   include Sidekiq::Worker
   include DedicatedSidekiqQueue
 
+  LEASE_TIMEOUT = 1.minute.to_i
+
   def self.bulk_perform_async(args_list)
     Sidekiq::Client.push_bulk('class' => self, 'args' => args_list)
   end
 
   def perform(user_id)
     user = User.find_by(id: user_id)
-    return unless user
 
+    refresh(user) if user
+  end
+
+  def refresh(user)
+    lease_key = "refresh_authorized_projects:#{user.id}"
+    lease = Gitlab::ExclusiveLease.new(lease_key, timeout: LEASE_TIMEOUT)
+
+    until uuid = lease.try_obtain
+      # Keep trying until we obtain the lease. If we don't do so we may end up
+      # not updating the list of authorized projects properly. To prevent
+      # hammering Redis too much we'll wait for a bit between retries.
+      sleep(1)
+    end
+
+    begin
       user.refresh_authorized_projects
+    ensure
+      Gitlab::ExclusiveLease.cancel(lease_key, uuid)
+    end
   end
 end

changelogs/unreleased/refresh-authorizations-with-lease.yml

+---
+title: Use a Redis lease for updating authorized projects
+merge_request: 7733
+author: 

db/fixtures/development/04_project.rb

 require 'sidekiq/testing'
-require './db/fixtures/support/serialized_transaction'
 
 Sidekiq::Testing.inline! do
   Gitlab::Seeder.quiet do

db/fixtures/development/06_teams.rb

 require 'sidekiq/testing'
-require './db/fixtures/support/serialized_transaction'
 
 Sidekiq::Testing.inline! do
   Gitlab::Seeder.quiet do

db/fixtures/development/17_cycle_analytics.rb

 require 'sidekiq/testing'
 require './spec/support/test_env'
-require './db/fixtures/support/serialized_transaction'
 
 class Gitlab::Seeder::CycleAnalytics
   def initialize(project, perf: false)

db/fixtures/support/serialized_transaction.rb

-require 'gitlab/database'
-
-module Gitlab
-  module Database
-    def self.serialized_transaction
-      connection.transaction { yield }
-    end
-  end
-end

lib/gitlab/database.rb

@@ -35,13 +35,6 @@ module Gitlab
       order
     end
 
-    def self.serialized_transaction
-      opts = {}
-      opts[:isolation] = :serializable unless Rails.env.test? && connection.transaction_open?
-
-      connection.transaction(opts) { yield }
-    end
-
     def self.random
       Gitlab::Database.postgresql? ? "RANDOM()" : "RAND()"
     end

spec/models/user_spec.rb

@@ -1349,4 +1349,31 @@ describe User, models: true do
       expect(projects).to be_empty
     end
   end
+
+  describe '#refresh_authorized_projects', redis: true do
+    let(:project1) { create(:empty_project) }
+    let(:project2) { create(:empty_project) }
+    let(:user) { create(:user) }
+
+    before do
+      project1.team << [user, :reporter]
+      project2.team << [user, :guest]
+
+      user.project_authorizations.delete_all
+      user.refresh_authorized_projects
+    end
+
+    it 'refreshes the list of authorized projects' do
+      expect(user.project_authorizations.count).to eq(2)
+    end
+
+    it 'sets the authorized_projects_populated column' do
+      expect(user.authorized_projects_populated).to eq(true)
+    end
+
+    it 'stores the correct access levels' do
+      expect(user.project_authorizations.where(access_level: Gitlab::Access::GUEST).exists?).to eq(true)
+      expect(user.project_authorizations.where(access_level: Gitlab::Access::REPORTER).exists?).to eq(true)
+    end
+  end
 end

spec/workers/authorized_projects_worker_spec.rb

 require 'spec_helper'
 
 describe AuthorizedProjectsWorker do
+  let(:worker) { described_class.new }
+
   describe '#perform' do
     it "refreshes user's authorized projects" do
       user = create(:user)
 
-      expect(User).to receive(:find_by).with(id: user.id).and_return(user)
-      expect(user).to receive(:refresh_authorized_projects)
+      expect(worker).to receive(:refresh).with(an_instance_of(User))
 
-      described_class.new.perform(user.id)
+      worker.perform(user.id)
     end
 
-    context "when user is not found" do
+    context "when the user is not found" do
       it "does nothing" do
-        expect_any_instance_of(User).not_to receive(:refresh_authorized_projects)
+        expect(worker).not_to receive(:refresh)
 
-        described_class.new.perform(999_999)
+        described_class.new.perform(-1)
+      end
     end
   end
+
+  describe '#refresh', redis: true do
+    it 'refreshes the authorized projects of the user' do
+      user = create(:user)
+
+      expect(user).to receive(:refresh_authorized_projects)
+
+      worker.refresh(user)
+    end
   end
 end