1 files changed, 69 insertions, 0 deletions
diff --git a/app/services/ci/queue/pending_builds_strategy.rb b/app/services/ci/queue/pending_builds_strategy.rb
new file mode 100644
index 00000000000..55d5cb96a0a
--- /dev/null
+++ b/app/services/ci/queue/pending_builds_strategy.rb
@@ -0,0 +1,69 @@
+# frozen_string_literal: true
+
+module Ci
+  module Queue
+    class PendingBuildsStrategy
+      attr_reader :runner
+
+      def initialize(runner)
+        @runner = runner
+      end
+
+      # rubocop:disable CodeReuse/ActiveRecord
+      def builds_for_shared_runner
+        relation = new_builds
+          # don't run projects which have not enabled shared runners and builds
+          .joins('INNER JOIN projects ON ci_pending_builds.project_id = projects.id')
+          .where(projects: { shared_runners_enabled: true, pending_delete: false })
+          .joins('LEFT JOIN project_features ON ci_pending_builds.project_id = project_features.project_id')
+          .where('project_features.builds_access_level IS NULL or project_features.builds_access_level > 0')
+
+        if Feature.enabled?(:ci_queueing_disaster_recovery_disable_fair_scheduling, runner, type: :ops, default_enabled: :yaml)
+          # if disaster recovery is enabled, we fallback to FIFO scheduling
+          relation.order('ci_pending_builds.build_id ASC')
+        else
+          # 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
+          relation
+            .with(running_builds_for_shared_runners_cte.to_arel)
+            .joins("LEFT JOIN project_builds ON ci_pending_builds.project_id = project_builds.project_id")
+            .order(Arel.sql('COALESCE(project_builds.running_builds, 0) ASC'), 'ci_pending_builds.build_id ASC')
+        end
+      end
+
+      def builds_matching_tag_ids(relation, ids)
+        relation.merge(CommitStatus.matches_tag_ids(ids, table: 'ci_pending_builds', column: 'build_id'))
+      end
+
+      def builds_with_any_tags(relation)
+        relation.merge(CommitStatus.with_any_tags(table: 'ci_pending_builds', column: 'build_id'))
+      end
+
+      def order(relation)
+        relation.order('build_id ASC')
+      end
+
+      def new_builds
+        ::Ci::PendingBuild.all
+      end
+
+      def build_ids(relation)
+        relation.pluck(:build_id)
+      end
+
+      private
+
+      def running_builds_for_shared_runners_cte
+        running_builds = ::Ci::RunningBuild
+          .instance_type
+          .group(:project_id)
+          .select(:project_id, 'COUNT(*) AS running_builds')
+
+        ::Gitlab::SQL::CTE
+          .new(:project_builds, running_builds, materialized: true)
+      end
+      # rubocop:enable CodeReuse/ActiveRecord
+    end
+  end
+end