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

4 files changed, 27 insertions, 31 deletions

diff --git a/app/models/ci/pipeline.rb b/app/models/ci/pipeline.rb
index adda3b8f40c..d3432632899 100644
--- a/app/models/ci/pipeline.rb
+++ b/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
diff --git a/app/models/commit_status.rb b/app/models/commit_status.rb
index 4cb3a69416e..d159fc6c5c7 100644
--- a/app/models/commit_status.rb
+++ b/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|
diff --git a/app/services/ci/process_pipeline_service.rb b/app/services/ci/process_pipeline_service.rb
index d3dd30b2588..8face432d97 100644
--- a/app/services/ci/process_pipeline_service.rb
+++ b/app/services/ci/process_pipeline_service.rb
@@ -10,17 +10,14 @@ module Ci
         create_builds!
       end
 
-      @pipeline.with_lock do
-        new_builds =
-          stage_indexes_of_created_builds.map do |index|
-            process_stage(index)
-          end
+      new_builds =
+        stage_indexes_of_created_builds.map do |index|
+          process_stage(index)
+        end
 
-        @pipeline.update_status
+      @pipeline.update_status
 
-        # Return a flag if a when builds got enqueued
-        new_builds.flatten.any?
-      end
+      new_builds.flatten.any?
     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)
+      if HasStatus::COMPLETED_STATUSES.include?(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
diff --git a/app/services/ci/register_build_service.rb b/app/services/ci/register_build_service.rb
index 6973191b203..74b5ebf372b 100644
--- a/app/services/ci/register_build_service.rb
+++ b/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
-          build.runner_id = current_runner.id
-          build.save!
-          build.run!
-        end
+        # with StateMachines::InvalidTransition or StaleObjectError when doing run! or save method.
+        build.runner_id = current_runner.id
+        build.run!
       end
 
       build
 
-    rescue StateMachines::InvalidTransition
+    rescue StateMachines::InvalidTransition, ActiveRecord::StaleObjectError
       nil
     end