Limit `update_tracked_fields` to write to database once/hour

Every time a user logs in or out, the Trackable attributes are written to the
database. This is causing a lot of load on the database, for data that isn't
really critical.

So to avoid the database being hammered, add a Gitlab::ExclusiveLease before
writing trackable attributes to the database. This lease expires after an hour,
so only when the attributes were written more than an hour ago, they can be
written again. Otherwise they are ignored.

app/models/user.rb

@@ -40,6 +40,16 @@ class User < ActiveRecord::Base
   devise :lockable, :recoverable, :rememberable, :trackable,
     :validatable, :omniauthable, :confirmable, :registerable
 
+  # Limit trackable fields to update at most once every hour
+  alias_method :devise_update_tracked_fields!, :update_tracked_fields!
+
+  def update_tracked_fields!(request)
+    lease = Gitlab::ExclusiveLease.new("user_update_tracked_fields:#{id}", timeout: 1.hour.to_i)
+    return unless lease.try_obtain
+
+    devise_update_tracked_fields!(request)
+  end
+
   attr_accessor :force_random_password
 
   # Virtual attribute for authenticating by either username or email

changelogs/unreleased/tc-cache-trackable-attributes.yml

+---
+title: "Limit User's trackable attributes, like `current_sign_in_at`, to update at most once/hour"
+merge_request: 11053
+author:

spec/models/user_spec.rb

@@ -344,6 +344,25 @@ describe User, models: true do
     end
   end
 
+  describe '#update_tracked_fields!', :redis do
+    let(:request) { OpenStruct.new(remote_ip: "127.0.0.1") }
+    let(:user) { create(:user) }
+
+    it 'writes trackable attributes' do
+      expect do
+        user.update_tracked_fields!(request)
+      end.to change { user.reload.current_sign_in_at }
+    end
+
+    it 'does not write trackable attributes when called a second time within the hour' do
+      user.update_tracked_fields!(request)
+
+      expect do
+        user.update_tracked_fields!(request)
+      end.not_to change { user.current_sign_in_at }
+    end
+  end
+
   shared_context 'user keys' do
     let(:user) { create(:user) }
     let!(:key) { create(:key, user: user) }