Preload number of warnings in every stage in a pipeline

This makes it possible to avoid N+1 queries when loading pipelines
table.

app/controllers/projects/pipelines_controller.rb

@@ -26,7 +26,7 @@ class Projects::PipelinesController < Projects::ApplicationController
     @finished_count = limited_pipelines_count(project, 'finished')
     @pipelines_count = limited_pipelines_count(project)
 
-    Gitlab::Ci::Pipeline::Preloader.preload(@project, @pipelines)
+    Gitlab::Ci::Pipeline::Preloader.preload(@pipelines)
 
     respond_to do |format|
       format.html

app/models/ci/pipeline.rb

@@ -424,7 +424,7 @@ module Ci
 
     def number_of_warnings
       BatchLoader.for(id).batch(default_value: 0) do |pipeline_ids, loader|
-        Build.where(commit_id: pipeline_ids)
+        ::Ci::Build.where(commit_id: pipeline_ids)
           .latest
           .failed_but_allowed
           .group(:commit_id)

app/models/ci/stage.rb

@@ -89,7 +89,18 @@ module Ci
     end
 
     def has_warnings?
-      statuses.latest.failed_but_allowed.any?
+      number_of_warnings.positive?
+    end
+
+    def number_of_warnings
+      BatchLoader.for(id).batch(default_value: 0) do |stage_ids, loader|
+        ::Ci::Build.where(stage_id: stage_ids)
+          .latest
+          .failed_but_allowed
+          .group(:stage_id)
+          .count
+          .each { |id, amount| loader.call(id, amount) }
+      end
     end
 
     def detailed_status(current_user)

lib/gitlab/ci/pipeline/preloader.rb

@@ -6,7 +6,7 @@ module Gitlab
       # Class for preloading data associated with pipelines such as commit
       # authors.
       module Preloader
-        def self.preload(project, pipelines)
+        def self.preload(pipelines)
           # This ensures that all the pipeline commits are eager loaded before we
           # start using them.
           pipelines.each(&:commit)
@@ -20,6 +20,11 @@ module Gitlab
             # that Ci::Pipeline#has_warnings? doesn't execute any additional
             # queries.
             pipeline.number_of_warnings
+
+            # This preloads the number of warnings for every stage, ensuring
+            # that Ci::Stage#has_warnings? doesn't execute any additional
+            # queries.
+            pipeline.stages.each { |stage| stage.number_of_warnings }
           end
         end
       end

spec/controllers/projects/pipelines_controller_spec.rb

@@ -17,35 +17,27 @@ describe Projects::PipelinesController do
 
   describe 'GET index.json' do
     before do
-      %w(pending running created success).each_with_index do |status, index|
-        sha = project.commit("HEAD~#{index}")
-
-        pipeline = create(:ci_empty_pipeline, status: status,
-                                              project: project,
-                                              sha: sha)
-
-
-        create_build(pipeline, 'test', 1, 'unit')
-        create_build(pipeline, 'test', 1, 'feature')
-        create_build(pipeline, 'review', 2, 'staging')
-        create_build(pipeline, 'deploy', 3, 'production')
-      end
+      %w(pending running running success canceled)
+        .each_with_index do |status, index|
+          create_pipeline(status, project.commit("HEAD~#{index}"))
+        end
     end
 
     it 'returns JSON with serialized pipelines', :request_store do
-      queries = ActiveRecord::QueryRecorder.new { get_pipelines_index_json }
+      queries = ActiveRecord::QueryRecorder.new do
+        get_pipelines_index_json
+      end
 
       expect(response).to have_gitlab_http_status(:ok)
       expect(response).to match_response_schema('pipeline')
 
       expect(json_response).to include('pipelines')
-      expect(json_response['pipelines'].count).to eq 4
-      expect(json_response['count']['all']).to eq '4'
-      expect(json_response['count']['running']).to eq '1'
+      expect(json_response['pipelines'].count).to eq 5
+      expect(json_response['count']['all']).to eq '5'
+      expect(json_response['count']['running']).to eq '2'
       expect(json_response['count']['pending']).to eq '1'
-      expect(json_response['count']['finished']).to eq '1'
-      puts queries.log
-      expect(queries.count).to be < 25
+      expect(json_response['count']['finished']).to eq '2'
+      expect(queries.count).to be < 32
     end
 
     it 'does not include coverage data for the pipelines' do
@@ -67,8 +59,19 @@ describe Projects::PipelinesController do
                   format: :json
     end
 
+    def create_pipeline(status, sha)
+      pipeline = create(:ci_empty_pipeline, status: status,
+                                            project: project,
+                                            sha: sha)
+
+      create_build(pipeline, 'build', 1, 'build')
+      create_build(pipeline, 'test', 2, 'test')
+      create_build(pipeline, 'deploy', 3, 'deploy')
+    end
+
     def create_build(pipeline, stage, stage_idx, name)
-      create(:ci_build, pipeline: pipeline, stage: stage, stage_idx: stage_idx, name: name)
+      status = %w[created running pending success failed canceled].sample
+      create(:ci_build, pipeline: pipeline, stage: stage, stage_idx: stage_idx, name: name, status: status)
     end
   end