Prevent build race condition for same project

app/models/build.rb

@@ -10,6 +10,9 @@ class Build < ActiveRecord::Base
   scope :latest_sha, where("id IN(SELECT MAX(id) FROM #{self.table_name} group by sha)")
+  scope :running, where(status: "running")
+  scope :pending, where(status: "pending")
+
   state_machine :status, initial: :pending do
     event :run do
       transition pending: :running

app/models/project.rb

@@ -124,6 +124,10 @@ class Project < ActiveRecord::Base
   def schedule_id
     "project-#{id}"
   end
+
+  def no_running_builds?
+    builds.running.empty?
+  end
 end
@@ -141,22 +145,3 @@ end
 #  token       :string(255)
 #  default_ref :string(255)
 #
-
-# == Schema Information
-#
-# Table name: projects
-#
-#  id               :integer(4)      not null, primary key
-#  name             :string(255)     not null
-#  path             :string(255)     not null
-#  timeout          :integer(4)      default(1800), not null
-#  scripts          :text            default(""), not null
-#  created_at       :datetime        not null
-#  updated_at       :datetime        not null
-#  token            :string(255)
-#  default_ref      :string(255)
-#  gitlab_url       :string(255)
-#  always_build     :boolean(1)      default(FALSE), not null
-#  polling_interval :integer(4)
-#
-

lib/runner.rb

@@ -4,10 +4,8 @@ require 'timeout'
 class Runner
   include Sidekiq::Worker
-  TIMEOUT = 1800
   attr_accessor :project, :build, :output
-
   sidekiq_options queue: :runner
   def perform(build_id)
@@ -15,7 +13,16 @@ class Runner
     @project = @build.project
     @output = ''
-    run
+
+    if @project.no_running_builds?
+      run
+    else
+      run_later
+    end
+  end
+
+  def run_later
+    Runner.perform_in(2.minutes, @build.id)
   end
   def initialize