Merge branch 'use-optimistic-locking' into 'master'

Use optimistic locking

## What does this MR do?
Removes the usage of pessimistic locking in favor of optimistic which is way cheaper and doesn't block database operation.

Since this is very simple change it should be safe. If we receive `StaleObjectError` message we will reload object a retry operations in lock.

However, I still believe that we need this one: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7005 as this will reduce a load on Database and FS.
This changes a behavior from:

### Pesimistic locking (previous behavior)

#### For updating
1. SELECT * FOR UPDATE (other updates wait on this)
2. we update ci_pipeline
3. latest_build_status
4. enqueue: (use: transition :created -> :pending)
5. [state_machine] we are in  state created, we can go to pending
6. [state_machine] ci_pipeline.status = created
7. [state_machine] ci_pipeline.save
8. [state_machine] after_transition: (if for success): PipelineSuccessWorker on Sidekiq
9. release DB lock

#### If no update is required
1. SELECT * FOR UPDATE (other updates wait on this)
2. we update ci_pipeline
3. latest_build_status
4. we are in pending, we can't transition to pending, because it's forbidden
5. release DB lock

### Optimistic locking (implemented by this MR)

#### For updating
1. latest_build_status
2. enqueue: (use `transition :created -> :pending`)
3. [state_machine] we are in state created, we can go to pending
4. [state_machine] ci_pipeline.status = created
5. [state_machine] ci_pipeline.save
6. [state_machine] [save] where(lock_version: ci_pipeline.lock_version).update_all(status: :created, updated_at: Time.now)
7. [state_machine] [save] unless we_updated_row then raise ObjectInconsistentError

#### If no update is required
1. we update ci_pipeline
2. latest_build_status
3. we are in pending, we can't transition to pending, because it's forbidden

## Why was this MR needed?
We have been seeing a number of problems when we migrated Pipeline/Build processing to Sidekiq. Especially we started seeing a lot of blocking queries.

We used a pessimistic locking which doesn't seem to be required. This effectively allows us to fix our issues with blocked queries by using more efficient method of operation.

## What are the relevant issue numbers?
Issues: https://gitlab.com/gitlab-com/infrastructure/issues/623 and https://gitlab.com/gitlab-com/infrastructure/issues/584, but also there's a bunch of Merge Requests that try to improve behavior of scheduled jobs.

cc @pcarranza @yorickpeterse @stanhu

See merge request !7040

CHANGELOG.md

@@ -91,6 +91,7 @@ Please view this file on the master branch, on stable branches it's out of date.
   - Updating verbiage on git basics to be more intuitive
   - Fix project_feature record not generated on project creation
   - Clarify documentation for Runners API (Gennady Trafimenkov)
+  - Use optimistic locking for pipelines and builds
   - The instrumentation for Banzai::Renderer has been restored
   - Change user & group landing page routing from /u/:username to /:username
   - Added documentation for .gitattributes files

app/models/ci/pipeline.rb

@@ -30,23 +30,23 @@ module Ci
       end
 
       event :run do
-        transition any => :running
+        transition any - [:running] => :running
       end
 
       event :skip do
-        transition any => :skipped
+        transition any - [:skipped] => :skipped
       end
 
       event :drop do
-        transition any => :failed
+        transition any - [:failed] => :failed
       end
 
       event :succeed do
-        transition any => :success
+        transition any - [:success] => :success
       end
 
       event :cancel do
-        transition any => :canceled
+        transition any - [:canceled] => :canceled
       end
 
       # IMPORTANT
@@ -260,7 +260,7 @@ module Ci
     end
 
     def update_status
-      with_lock do
+      Gitlab::OptimisticLocking.retry_lock(self) do
         case latest_builds_status
         when 'pending' then enqueue
         when 'running' then run

app/models/commit_status.rb

@@ -73,16 +73,16 @@ class CommitStatus < ActiveRecord::Base
       transition [:created, :pending, :running] => :canceled
     end
 
-    after_transition created: [:pending, :running] do |commit_status|
-      commit_status.update_attributes queued_at: Time.now
+    before_transition created: [:pending, :running] do |commit_status|
+      commit_status.queued_at = Time.now
     end
 
-    after_transition [:created, :pending] => :running do |commit_status|
-      commit_status.update_attributes started_at: Time.now
+    before_transition [:created, :pending] => :running do |commit_status|
+      commit_status.started_at = Time.now
     end
 
-    after_transition any => [:success, :failed, :canceled] do |commit_status|
-      commit_status.update_attributes finished_at: Time.now
+    before_transition any => [:success, :failed, :canceled] do |commit_status|
+      commit_status.finished_at = Time.now
     end
 
     after_transition do |commit_status, transition|

app/services/ci/process_pipeline_service.rb

@@ -10,7 +10,6 @@ module Ci
         create_builds!
       end
 
-      @pipeline.with_lock do
       new_builds =
         stage_indexes_of_created_builds.map do |index|
           process_stage(index)
@@ -18,10 +17,8 @@ module Ci
 
       @pipeline.update_status
 
-        # Return a flag if a when builds got enqueued
       new_builds.flatten.any?
     end
-    end
 
     private
 
@@ -32,9 +29,11 @@ module Ci
     def process_stage(index)
       current_status = status_for_prior_stages(index)
 
-      created_builds_in_stage(index).select do |build|
       if HasStatus::COMPLETED_STATUSES.include?(current_status)
-          process_build(build, current_status)
+        created_builds_in_stage(index).select do |build|
+          Gitlab::OptimisticLocking.retry_lock(build) do |subject|
+            process_build(subject, current_status)
+          end
         end
       end
     end

app/services/ci/register_build_service.rb

@@ -28,17 +28,14 @@ module Ci
 
       if build
         # In case when 2 runners try to assign the same build, second runner will be declined
-        # with StateMachines::InvalidTransition in run! method.
-        build.with_lock do
+        # with StateMachines::InvalidTransition or StaleObjectError when doing run! or save method.
         build.runner_id = current_runner.id
-          build.save!
         build.run!
       end
-      end
 
       build
 
-    rescue StateMachines::InvalidTransition
+    rescue StateMachines::InvalidTransition, ActiveRecord::StaleObjectError
       nil
     end
 

db/migrate/20161021114307_add_lock_version_to_build_and_pipelines.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddLockVersionToBuildAndPipelines < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  def change
+    add_column :ci_builds, :lock_version, :integer
+    add_column :ci_commits, :lock_version, :integer
+  end
+end

db/schema.rb

@@ -189,6 +189,7 @@ ActiveRecord::Schema.define(version: 20161025231710) do
     t.text "yaml_variables"
     t.datetime "queued_at"
     t.string "token"
+    t.integer "lock_version"
   end
 
   add_index "ci_builds", ["commit_id", "stage_idx", "created_at"], name: "index_ci_builds_on_commit_id_and_stage_idx_and_created_at", using: :btree
@@ -219,6 +220,7 @@ ActiveRecord::Schema.define(version: 20161025231710) do
     t.datetime "finished_at"
     t.integer "duration"
     t.integer "user_id"
+    t.integer "lock_version"
   end
 
   add_index "ci_commits", ["gl_project_id", "sha"], name: "index_ci_commits_on_gl_project_id_and_sha", using: :btree

lib/gitlab/optimistic_locking.rb

+module Gitlab
+  module OptimisticLocking
+    extend self
+
+    def retry_lock(subject, retries = 100, &block)
+      loop do
+        begin
+          ActiveRecord::Base.transaction do
+            return block.call(subject)
+          end
+        rescue ActiveRecord::StaleObjectError
+          retries -= 1
+          raise unless retries >= 0
+          subject.reload
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/import_export/safe_model_attributes.yml

@@ -178,6 +178,7 @@ Ci::Pipeline:
 - finished_at
 - duration
 - user_id
+- lock_version
 CommitStatus:
 - id
 - project_id
@@ -217,6 +218,7 @@ CommitStatus:
 - yaml_variables
 - queued_at
 - token
+- lock_version
 Ci::Variable:
 - id
 - project_id

spec/lib/gitlab/optimistic_locking_spec.rb

+require 'spec_helper'
+
+describe Gitlab::OptimisticLocking, lib: true do
+  describe '#retry_lock' do
+    let!(:pipeline) { create(:ci_pipeline) }
+    let!(:pipeline2) { Ci::Pipeline.find(pipeline.id) }
+
+    it 'does not reload object if state changes' do
+      expect(pipeline).not_to receive(:reload)
+      expect(pipeline).to receive(:succeed).and_call_original
+
+      described_class.retry_lock(pipeline) do |subject|
+        subject.succeed
+      end
+    end
+
+    it 'retries action if exception is raised' do
+      pipeline.succeed
+
+      expect(pipeline2).to receive(:reload).and_call_original
+      expect(pipeline2).to receive(:drop).twice.and_call_original
+
+      described_class.retry_lock(pipeline2) do |subject|
+        subject.drop
+      end
+    end
+
+    it 'raises exception when too many retries' do
+      expect(pipeline).to receive(:drop).twice.and_call_original
+
+      expect do
+        described_class.retry_lock(pipeline, 1) do |subject|
+          subject.lock_version = 100
+          subject.drop
+        end
+      end.to raise_error(ActiveRecord::StaleObjectError)
+    end
+  end
+end

spec/models/ci/pipeline_spec.rb

@@ -138,32 +138,26 @@ describe Ci::Pipeline, models: true do
 
   describe 'state machine' do
     let(:current) { Time.now.change(usec: 0) }
-    let(:build) { create_build('build1', current, 10) }
-    let(:build_b) { create_build('build2', current, 20) }
-    let(:build_c) { create_build('build3', current + 50, 10) }
+    let(:build) { create_build('build1', 0) }
+    let(:build_b) { create_build('build2', 0) }
+    let(:build_c) { create_build('build3', 0) }
 
     describe '#duration' do
       before do
-        pipeline.update(created_at: current)
-
-        travel_to(current + 5) do
-          pipeline.run
-          pipeline.save
-        end
-
         travel_to(current + 30) do
-          build.success
+          build.run!
+          build.success!
+          build_b.run!
+          build_c.run!
         end
 
         travel_to(current + 40) do
-          build_b.drop
+          build_b.drop!
         end
 
         travel_to(current + 70) do
-          build_c.success
+          build_c.success!
         end
-
-        pipeline.drop
       end
 
       it 'matches sum of builds duration' do
@@ -455,7 +449,9 @@ describe Ci::Pipeline, models: true do
         context 'when all builds succeed' do
           before do
             build_a.success
-            build_b.success
+
+            # We have to reload build_b as this is in next stage and it gets triggered by PipelineProcessWorker
+            build_b.reload.success
           end
 
           it 'receives a success event once' do

spec/requests/api/builds_spec.rb

@@ -277,6 +277,7 @@ describe API::API, api: true do
 
       context 'with regular branch' do
         before do
+          pipeline.reload
           pipeline.update(ref: 'master',
                           sha: project.commit('master').sha)
 
@@ -288,6 +289,7 @@ describe API::API, api: true do
 
       context 'with branch name containing slash' do
         before do
+          pipeline.reload
           pipeline.update(ref: 'improve/awesome',
                           sha: project.commit('improve/awesome').sha)
         end

spec/services/ci/register_build_service_spec.rb

@@ -101,11 +101,11 @@ module Ci
           it 'equalises number of running builds' do
             # after finishing the first build for project 1, get a second build from the same project
             expect(service.execute(shared_runner)).to eq(build1_project1)
-            build1_project1.success
+            build1_project1.reload.success
             expect(service.execute(shared_runner)).to eq(build2_project1)
 
             expect(service.execute(shared_runner)).to eq(build1_project2)
-            build1_project2.success
+            build1_project2.reload.success
             expect(service.execute(shared_runner)).to eq(build2_project2)
             expect(service.execute(shared_runner)).to eq(build1_project3)
             expect(service.execute(shared_runner)).to eq(build3_project1)

spec/services/merge_requests/merge_when_build_succeeds_service_spec.rb

@@ -147,6 +147,7 @@ describe MergeRequests::MergeWhenBuildSucceedsService do
         expect(MergeWorker).not_to receive(:perform_async)
 
         build.success
+        test.reload
         test.drop
       end
 
@@ -154,6 +155,7 @@ describe MergeRequests::MergeWhenBuildSucceedsService do
         expect(MergeWorker).to receive(:perform_async)
 
         build.success
+        test.reload
         test.success
       end
     end