Remove lease from Event#reset_project_activity

Per GitLab.com's performance metrics this method could take up to 5
seconds of wall time to complete, while only taking 1-2 milliseconds of
CPU time. Removing the Redis lease in favour of conditional updates
allows us to work around this.

A slight drawback is that this allows for multiple threads/processes to
try and update the same row. However, only a single thread/process will
ever win since the UPDATE query uses a WHERE condition to only update
rows that were not updated in the last hour.

Fixes gitlab-org/gitlab-ce#22473

CHANGELOG

@@ -35,6 +35,7 @@ v 8.13.0 (unreleased)
   - Add missing values to linter !6276 (Katarzyna Kobierska Ula Budziszewska)
   - Fix Long commit messages overflow viewport in file tree
   - Revert avoid touching file system on Build#artifacts?
+  - Stop using a Redis lease when updating the project activity timestamp whenever a new event is created
   - Add broadcast messages and alerts below sub-nav
   - Better empty state for Groups view
   - Update ruby-prof to 0.16.2. !6026 (Elan Ruusamäe)

app/models/event.rb

@@ -328,13 +328,15 @@ class Event < ActiveRecord::Base
   def reset_project_activity
     return unless project
 
-    # Don't even bother obtaining a lock if the last update happened less than
-    # 60 minutes ago.
+    # Don't bother updating if we know the project was updated recently.
     return if recent_update?
 
-    return unless try_obtain_lease
-
-    project.update_column(:last_activity_at, created_at)
+    # At this point it's possible for multiple threads/processes to try to
+    # update the project. Only one query should actually perform the update,
+    # hence we add the extra WHERE clause for last_activity_at.
+    Project.unscoped.where(id: project_id).
+      where('last_activity_at > ?', RESET_PROJECT_ACTIVITY_INTERVAL.ago).
+      update_all(last_activity_at: created_at)
   end
 
   private
@@ -342,11 +344,4 @@ class Event < ActiveRecord::Base
   def recent_update?
     project.last_activity_at > RESET_PROJECT_ACTIVITY_INTERVAL.ago
   end
-
-  def try_obtain_lease
-    Gitlab::ExclusiveLease.
-      new("project:update_last_activity_at:#{project.id}",
-          timeout: RESET_PROJECT_ACTIVITY_INTERVAL.to_i).
-      try_obtain
-  end
 end

spec/models/event_spec.rb

@@ -173,13 +173,11 @@ describe Event, models: true do
       it 'updates the project' do
         project.update(last_activity_at: 1.year.ago)
 
-        expect_any_instance_of(Gitlab::ExclusiveLease).
-          to receive(:try_obtain).and_return(true)
+        create_event(project, project.owner)
 
-        expect(project).to receive(:update_column).
-          with(:last_activity_at, a_kind_of(Time))
+        project.reload
 
-        create_event(project, project.owner)
+        project.last_activity_at <= 1.minute.ago
       end
     end
   end

spec/models/project_spec.rb

@@ -308,8 +308,7 @@ describe Project, models: true do
   end
 
   describe 'last_activity methods' do
-    let(:timestamp) { Time.now - 2.hours }
-    let(:project) { create(:project, created_at: timestamp, updated_at: timestamp) }
+    let(:project) { create(:project, last_activity_at: 2.hours.ago) }
 
     describe 'last_activity' do
       it 'alias last_activity to last_event' do
@@ -321,7 +320,6 @@ describe Project, models: true do
 
     describe 'last_activity_date' do
       it 'returns the creation date of the project\'s last event if present' do
-        expect_any_instance_of(Event).to receive(:try_obtain_lease).and_return(true)
         new_event = create(:event, project: project, created_at: Time.now)
 
         expect(project.last_activity_at.to_i).to eq(new_event.created_at.to_i)