Use optimistic locking

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(build) do
         case latest_builds_status
         when 'pending' then enqueue
         when 'running' then run

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
 
@@ -32,12 +29,14 @@ 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)
+        created_builds_in_stage(index).select do |build|
+          Gitlab::OptimisticLocking.retry_lock(build) do |build|
             process_build(build, current_status)
           end
         end
       end
+    end
 
     def process_build(build, current_status)
       if valid_statuses_for_when(build.when).include?(current_status)

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, 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: 20161024042317) 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: 20161024042317) 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
+    def retry_lock(subject, &block)
+      while true do
+        begin
+          return yield subject
+        rescue StaleObjectError
+          subject.reload
+        end
+      end
+    end
+  end
+end