Load repository language from the DB if detected

The repository charts page used to detect the repository language for
each request that was made to the page. Given the detection is an
expensive operation and the same data is stored in the database the
database is now serving the request.

The same goes for an API endpoint that serves the languages.

When a repository is empty or non-existent the languages will always be
empty. And the language detection RPC isn't requested.

Closes: https://gitlab.com/gitlab-org/gitlab-ce/issues/47390

app/controllers/projects/graphs_controller.rb

@@ -45,7 +45,14 @@ class Projects::GraphsController < Projects::ApplicationController
   end
 
   def get_languages
-    @languages = @project.repository.languages
+    @languages =
+      if @project.repository_languages.present?
+        @project.repository_languages.map do |lang|
+          { value: lang.share, label: lang.name, color: lang.color, highlight: lang.color }
+        end
+      else
+        @project.repository.languages
+      end
   end
 
   def fetch_graph

changelogs/unreleased/zj-load-languages-from-database.yml

+---
+title: Load repository language from the database if detected before
+merge_request: 25518
+author:
+type: performance

lib/api/projects.rb

@@ -386,8 +386,12 @@ module API
 
       desc 'Get languages in project repository'
       get ':id/languages' do
+        if user_project.repository_languages.present?
+          user_project.repository_languages.map { |l| [l.name, l.share] }.to_h
+        else
           user_project.repository.languages.map { |language| language.values_at(:label, :value) }.to_h
         end
+      end
 
       desc 'Remove a project'
       delete ":id" do

spec/controllers/projects/graphs_controller_spec.rb

@@ -24,4 +24,20 @@ describe Projects::GraphsController do
       expect(response).to redirect_to action: :charts
     end
   end
+
+  describe 'charts' do
+    context 'when languages were previously detected' do
+      let!(:repository_language) { create(:repository_language, project: project) }
+
+      it 'sets the languages properly' do
+        get(:charts, params: { namespace_id: project.namespace.path, project_id: project.path, id: 'master' })
+
+        expect(assigns[:languages]).to eq(
+          [value: repository_language.share,
+           label: repository_language.name,
+           color: repository_language.color,
+           highlight: repository_language.color])
+      end
+    end
+  end
 end

spec/requests/api/projects_spec.rb

@@ -4,6 +4,15 @@ require 'spec_helper'
 shared_examples 'languages and percentages JSON response' do
   let(:expected_languages) { project.repository.languages.map { |language| language.values_at(:label, :value)}.to_h }
 
+  before do
+    allow(project.repository).to receive(:languages).and_return(
+      [{ value: 66.69, label: "Ruby", color: "#701516", highlight: "#701516" },
+       { value: 22.98, label: "JavaScript", color: "#f1e05a", highlight: "#f1e05a" },
+       { value: 7.91, label: "HTML", color: "#e34c26", highlight: "#e34c26" },
+       { value: 2.42, label: "CoffeeScript", color: "#244776", highlight: "#244776" }]
+    )
+  end
+
   it 'returns expected language values' do
     get api("/projects/#{project.id}/languages", user)
 
@@ -11,6 +20,23 @@ shared_examples 'languages and percentages JSON response' do
     expect(json_response).to eq(expected_languages)
     expect(json_response.count).to be > 1
   end
+
+  context 'when the languages were detected before' do
+    before do
+      Projects::DetectRepositoryLanguagesService.new(project, project.owner).execute
+    end
+
+    it 'returns the detection from the database' do
+      # Allow this to happen once, so the expected languages can be determined
+      expect(project.repository).to receive(:languages).once
+
+      get api("/projects/#{project.id}/languages", user)
+
+      expect(response).to have_gitlab_http_status(:ok)
+      expect(json_response).to eq(expected_languages)
+      expect(json_response.count).to be > 1
+    end
+  end
 end
 
 describe API::Projects do