Fair usage of Shared Runners

CHANGELOG

@@ -19,6 +19,7 @@ v 8.9.0 (unreleased)
   - Added descriptions to notification settings dropdown
   - Improve note validation to prevent errors when creating invalid note via API
   - Reduce number of fog gem dependencies
+  - Implement a fair usage of shared runners
   - Remove project notification settings associated with deleted projects
   - Fix 404 page when viewing TODOs that contain milestones or labels in different projects
   - Redesign navigation for project pages

app/services/ci/register_build_service.rb

@@ -7,15 +7,15 @@ module Ci
 
       builds =
         if current_runner.shared?
-          # don't run projects which have not enables shared runners
-          builds.joins(:project).where(projects: { builds_enabled: true, shared_runners_enabled: true })
+          # this returns builds that are ordered by number of running builds
+          # we prefer projects that don't use shared runners at all
+          builds.joins("JOIN (#{projects_with_builds_for_shared_runners.to_sql}) AS projects ON ci_builds.gl_project_id=projects.gl_project_id").
+            order('projects.running_builds ASC', 'ci_builds.id ASC')
         else
-          # do run projects which are only assigned to this runner
-          builds.where(project: current_runner.projects.where(builds_enabled: true))
+          # do run projects which are only assigned to this runner (FIFO)
+          builds.where(project: current_runner.projects.where(builds_enabled: true)).order('created_at ASC')
         end
 
-      builds = builds.order('created_at ASC')
-
       build = builds.find do |build|
         build.can_be_served?(current_runner)
       end
@@ -35,5 +35,18 @@ module Ci
     rescue StateMachines::InvalidTransition
       nil
     end
+
+    private
+
+    def projects_with_builds_for_shared_runners
+      Ci::Build.running_or_pending.
+        joins(:project).where(projects: { builds_enabled: true, shared_runners_enabled: true }).
+        group(:gl_project_id).
+        select(:gl_project_id, "count(case when status = 'running' AND runner_id = (#{shared_runners.to_sql}) then 1 end) as running_builds")
+    end
+
+    def shared_runners
+      Ci::Runner.shared.select(:id)
+    end
   end
 end

spec/services/ci/register_build_service_spec.rb

@@ -50,6 +50,46 @@ module Ci
           project.update(shared_runners_enabled: true)
         end
 
+        context 'for multiple builds' do
+          let!(:project2) { create :empty_project, shared_runners_enabled: true }
+          let!(:pipeline2) { create :ci_pipeline, project: project2 }
+          let!(:project3) { create :empty_project, shared_runners_enabled: true }
+          let!(:pipeline3) { create :ci_pipeline, project: project3 }
+          let!(:build1_project1) { pending_build }
+          let!(:build2_project1) { FactoryGirl.create :ci_build, pipeline: pipeline }
+          let!(:build3_project1) { FactoryGirl.create :ci_build, pipeline: pipeline }
+          let!(:build1_project2) { FactoryGirl.create :ci_build, pipeline: pipeline2 }
+          let!(:build2_project2) { FactoryGirl.create :ci_build, pipeline: pipeline2 }
+          let!(:build1_project3) { FactoryGirl.create :ci_build, pipeline: pipeline3 }
+
+          it 'prefers projects without builds first' do
+            # it gets for one build from each of the projects
+            expect(service.execute(shared_runner)).to eq(build1_project1)
+            expect(service.execute(shared_runner)).to eq(build1_project2)
+            expect(service.execute(shared_runner)).to eq(build1_project3)
+
+            # then it gets a second build from each of the projects
+            expect(service.execute(shared_runner)).to eq(build2_project1)
+            expect(service.execute(shared_runner)).to eq(build2_project2)
+
+            # in the end the third build
+            expect(service.execute(shared_runner)).to eq(build3_project1)
+          end
+
+          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
+            expect(service.execute(shared_runner)).to eq(build2_project1)
+
+            expect(service.execute(shared_runner)).to eq(build1_project2)
+            build1_project2.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)
+          end
+        end
+
         context 'shared runner' do
           let(:build) { service.execute(shared_runner) }