Preload runner tags for `UpdateBuildQueueService`

This makes us preload tags for this service
making this service to not have N+1 problem

app/models/ci/runner.rb

@@ -267,6 +267,16 @@ module Ci
       token[0...8] if token
     end
 
+    def tag_list
+      return super unless Feature.enabled?(:ci_preload_runner_tags, default_enabled: :yaml)
+
+      if tags.loaded?
+        tags.map(&:name)
+      else
+        super
+      end
+    end
+
     def has_tags?
       tag_list.any?
     end

app/services/ci/update_build_queue_service.rb

@@ -10,6 +10,8 @@ module Ci
 
     def tick_for(build, runners, metrics)
       runners = runners.with_recent_runner_queue
+      runners = runners.with_tags if Feature.enabled?(:ci_preload_runner_tags, default_enabled: :yaml)
+
       metrics.observe_active_runners(-> { runners.to_a.size })
 
       runners.each do |runner|

changelogs/unreleased/preload-runner-tags.yml

+---
+title: Preload runner tags for `UpdateBuildQueueService`
+merge_request: 55543
+author:
+type: performance

config/feature_flags/development/ci_preload_runner_tags.yml

+---
+name: ci_preload_runner_tags
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/55543
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/323243
+milestone: '13.10'
+type: development
+group: group::memory
+default_enabled: false

spec/services/ci/update_build_queue_service_spec.rb

@@ -115,4 +115,43 @@ RSpec.describe Ci::UpdateBuildQueueService do
       it_behaves_like 'does not refresh runner'
     end
   end
+
+  context 'avoids N+1 queries', :request_store do
+    let!(:build) { create(:ci_build, pipeline: pipeline, tag_list: %w[a b]) }
+    let!(:project_runner) { create(:ci_runner, :project, :online, projects: [project], tag_list: %w[a b c]) }
+
+    context 'when ci_preload_runner_tags and ci_reduce_queries_when_ticking_runner_queue are enabled' do
+      before do
+        stub_feature_flags(
+          ci_reduce_queries_when_ticking_runner_queue: true,
+          ci_preload_runner_tags: true
+        )
+      end
+
+      it 'does execute the same amount of queries regardless of number of runners' do
+        control_count = ActiveRecord::QueryRecorder.new { subject.execute(build) }.count
+
+        create_list(:ci_runner, 10, :project, :online, projects: [project], tag_list: %w[b c d])
+
+        expect { subject.execute(build) }.not_to exceed_all_query_limit(control_count)
+      end
+    end
+
+    context 'when ci_preload_runner_tags and ci_reduce_queries_when_ticking_runner_queue are disabled' do
+      before do
+        stub_feature_flags(
+          ci_reduce_queries_when_ticking_runner_queue: false,
+          ci_preload_runner_tags: false
+        )
+      end
+
+      it 'does execute more queries for more runners' do
+        control_count = ActiveRecord::QueryRecorder.new { subject.execute(build) }.count
+
+        create_list(:ci_runner, 10, :project, :online, projects: [project], tag_list: %w[b c d])
+
+        expect { subject.execute(build) }.to exceed_all_query_limit(control_count)
+      end
+    end
+  end
 end