1 files changed, 85 insertions, 0 deletions
diff --git a/app/services/ci/register_job_service.rb b/app/services/ci/register_job_service.rb
new file mode 100644
index 00000000000..0ab9042bf24
--- /dev/null
+++ b/app/services/ci/register_job_service.rb
@@ -0,0 +1,85 @@
+module Ci
+  # This class responsible for assigning
+  # proper pending build to runner on runner API request
+  class RegisterJobService
+    include Gitlab::CurrentSettings
+
+    attr_reader :runner
+
+    Result = Struct.new(:build, :valid?)
+
+    def initialize(runner)
+      @runner = runner
+    end
+
+    def execute
+      builds =
+        if runner.shared?
+          builds_for_shared_runner
+        else
+          builds_for_specific_runner
+        end
+
+      valid = true
+
+      builds.find do |build|
+        next unless runner.can_pick?(build)
+
+        begin
+          # In case when 2 runners try to assign the same build, second runner will be declined
+          # with StateMachines::InvalidTransition or StaleObjectError when doing run! or save method.
+          build.runner_id = runner.id
+          build.run!
+
+          return Result.new(build, true)
+        rescue StateMachines::InvalidTransition, ActiveRecord::StaleObjectError
+          # We are looping to find another build that is not conflicting
+          # It also indicates that this build can be picked and passed to runner.
+          # If we don't do it, basically a bunch of runners would be competing for a build
+          # and thus we will generate a lot of 409. This will increase
+          # the number of generated requests, also will reduce significantly
+          # how many builds can be picked by runner in a unit of time.
+          # In case we hit the concurrency-access lock,
+          # we still have to return 409 in the end,
+          # to make sure that this is properly handled by runner.
+          valid = false
+        end
+      end
+
+      Result.new(nil, valid)
+    end
+
+    private
+
+    def builds_for_shared_runner
+      new_builds.
+        # don't run projects which have not enabled shared runners and builds
+        joins(:project).where(projects: { shared_runners_enabled: true }).
+        joins('LEFT JOIN project_features ON ci_builds.gl_project_id = project_features.project_id').
+        where('project_features.builds_access_level IS NULL or project_features.builds_access_level > 0').
+
+        # Implement fair scheduling
+        # this returns builds that are ordered by number of running builds
+        # we prefer projects that don't use shared runners at all
+        joins("LEFT JOIN (#{running_builds_for_shared_runners.to_sql}) AS project_builds ON ci_builds.gl_project_id=project_builds.gl_project_id").
+        order('COALESCE(project_builds.running_builds, 0) ASC', 'ci_builds.id ASC')
+    end
+
+    def builds_for_specific_runner
+      new_builds.where(project: runner.projects.with_builds_enabled).order('created_at ASC')
+    end
+
+    def running_builds_for_shared_runners
+      Ci::Build.running.where(runner: Ci::Runner.shared).
+        group(:gl_project_id).select(:gl_project_id, 'count(*) AS running_builds')
+    end
+
+    def new_builds
+      Ci::Build.pending.unstarted
+    end
+
+    def shared_runner_build_limits_feature_enabled?
+      ENV['DISABLE_SHARED_RUNNER_BUILD_MINUTES_LIMIT'].to_s != 'true'
+    end
+  end
+end