Reduce queries when ticking runner queue

When UpdateBuildQueueService runs it iterates already
through the available runners for a given project.
In order to `tick_runner_queue` for each runner we
checked whether the `can_pick?` the build which
was checking again whether the runner was in the list
of available runner for the given project.

This change removes this redundant query which is very
expensive given that it's a UNION of 3 expensive
subqueries.

app/models/ci/runner.rb

@@ -253,9 +253,10 @@ module Ci
     end
 
     def can_pick?(build)
-      return false if self.ref_protected? && !build.protected?
-
-      assignable_for?(build.project_id) && accepting_tags?(build)
+      #  Run `matches_build?` checks before, since they are cheaper than
+      # `assignable_for?`.
+      #
+      matches_build?(build) && assignable_for?(build.project_id)
     end
 
     def only_for?(project)
@@ -305,8 +306,10 @@ module Ci
     end
 
     def pick_build!(build)
-      if can_pick?(build)
-        tick_runner_queue
+      if Feature.enabled?(:ci_reduce_queries_when_ticking_runner_queue, self, default_enabled: :yaml)
+        tick_runner_queue if matches_build?(build)
+      else
+        tick_runner_queue if can_pick?(build)
       end
     end
 
@@ -370,6 +373,13 @@ module Ci
       end
     end
 
+    # TODO: choose a better name and consider splitting this method into two
+    def matches_build?(build)
+      return false if self.ref_protected? && !build.protected?
+
+      accepting_tags?(build)
+    end
+
     def accepting_tags?(build)
       (run_untagged? || build.has_tags?) && (build.tag_list - tag_list).empty?
     end

changelogs/unreleased/fp-reduce-queries-when-ticking-runner-queue.yml

+---
+title: Reduce queries when ticking runner queue
+merge_request: 55496
+author:
+type: performance

config/feature_flags/development/ci_reduce_queries_when_ticking_runner_queue.yml

+---
+name: ci_reduce_queries_when_ticking_runner_queue
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/55496
+rollout_issue_url: 
+milestone: '13.10'
+type: development
+group: group::continuous integration
+default_enabled: false

spec/models/ci/runner_spec.rb

@@ -842,27 +842,50 @@ RSpec.describe Ci::Runner do
   end
 
   describe '#pick_build!' do
+    let(:build) { create(:ci_build) }
+    let(:runner) { create(:ci_runner) }
+
     context 'runner can pick the build' do
       it 'calls #tick_runner_queue' do
-        ci_build = build(:ci_build)
-        runner = build(:ci_runner)
-        allow(runner).to receive(:can_pick?).with(ci_build).and_return(true)
-
         expect(runner).to receive(:tick_runner_queue)
 
-        runner.pick_build!(ci_build)
+        runner.pick_build!(build)
       end
     end
 
     context 'runner cannot pick the build' do
-      it 'does not call #tick_runner_queue' do
-        ci_build = build(:ci_build)
-        runner = build(:ci_runner)
-        allow(runner).to receive(:can_pick?).with(ci_build).and_return(false)
+      before do
+        build.tag_list = [:docker]
+      end
 
+      it 'does not call #tick_runner_queue' do
         expect(runner).not_to receive(:tick_runner_queue)
 
-        runner.pick_build!(ci_build)
+        runner.pick_build!(build)
+      end
+    end
+
+    context 'build picking improvement enabled' do
+      before do
+        stub_feature_flags(ci_reduce_queries_when_ticking_runner_queue: true)
+      end
+
+      it 'does not check if the build is assignable to a runner' do
+        expect(runner).not_to receive(:can_pick?)
+
+        runner.pick_build!(build)
+      end
+    end
+
+    context 'build picking improvement disabled' do
+      before do
+        stub_feature_flags(ci_reduce_queries_when_ticking_runner_queue: false)
+      end
+
+      it 'checks if the build is assignable to a runner' do
+        expect(runner).to receive(:can_pick?).and_call_original
+
+        runner.pick_build!(build)
       end
     end
   end

spec/services/ci/update_build_queue_service_spec.rb

@@ -26,6 +26,24 @@ RSpec.describe Ci::UpdateBuildQueueService do
       end
 
       it_behaves_like 'refreshes runner'
+
+      it 'avoids running redundant queries' do
+        expect(Ci::Runner).not_to receive(:owned_or_instance_wide)
+
+        subject.execute(build)
+      end
+
+      context 'when feature flag ci_reduce_queries_when_ticking_runner_queue is disabled' do
+        before do
+          stub_feature_flags(ci_reduce_queries_when_ticking_runner_queue: false)
+        end
+
+        it 'runs redundant queries using `owned_or_instance_wide` scope' do
+          expect(Ci::Runner).to receive(:owned_or_instance_wide).and_call_original
+
+          subject.execute(build)
+        end
+      end
     end
   end