Add google tag manager

This adds the google tag manager scripts on the sign in/up and trial
sign up page. To align with our privacy policy the script only gets
activated when the `google_tag_manager_id` is set in the config on
GitLab.com and when there is no signed in user.

app/helpers/auth_helper.rb

@@ -151,6 +151,13 @@ module AuthHelper
     current_user.allow_password_authentication_for_web? && !current_user.password_automatically_set?
   end
 
+  def google_tag_manager_enabled?
+    Gitlab.com? &&
+      extra_config.has_key?('google_tag_manager_id') &&
+      extra_config.google_tag_manager_id.present? &&
+      !current_user
+  end
+
   extend self
 end
 

app/views/devise/registrations/new.html.haml

 - page_title _("Sign up")
 - add_page_specific_style 'page_bundles/signup'
+- content_for :page_specific_javascripts do
+  = render "layouts/google_tag_manager_head"
+= render "layouts/google_tag_manager_body"
 
 .signup-page
   = render 'devise/shared/signup_box',

app/views/devise/sessions/new.html.haml

 - page_title _("Sign in")
+- content_for :page_specific_javascripts do
+  = render "layouts/google_tag_manager_head"
+= render "layouts/google_tag_manager_body"
 
 #signin-container
   - if any_form_based_providers_enabled?

app/views/layouts/_google_tag_manager_body.html.haml

+- return unless google_tag_manager_enabled?
+
+<noscript><iframe src="https://www.googletagmanager.com/ns.html?id=#{extra_config.google_tag_manager_id}"
+height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript>

app/views/layouts/_google_tag_manager_head.html.haml

+- if google_tag_manager_enabled?
+  = javascript_tag nonce: true do
+    :plain
+      (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
+      new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
+      j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
+      'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
+      })(window,document,'script','dataLayer','#{extra_config.google_tag_manager_id}');

changelogs/unreleased/223768-add-google-analytics-tracking-to-sign-in-pages.yml

+---
+title: Add Google Tag Manger to sign in/up and trial pages
+merge_request: 38395
+author:
+type: changed

config/gitlab.yml.example

@@ -1213,6 +1213,9 @@ production: &base
     ## Google analytics. Uncomment if you want it
     # google_analytics_id: '_your_tracking_id'
 
+    ## Google tag manager
+    # google_tag_manager_id: '_your_tracking_id'
+
     ## Piwik analytics.
     # piwik_url: '_your_piwik_url'
     # piwik_site_id: '_your_piwik_site_id'

ee/app/views/trial_registrations/new.html.haml

 - page_title _('Start a Free Gold Trial')
 - add_page_specific_style 'page_bundles/signup'
+- content_for :page_specific_javascripts do
+  = render "layouts/google_tag_manager_head"
+= render "layouts/google_tag_manager_body"
 
 %h2.center.pt-6.pb-4.gl-mb-0
   = _('Start a Free Gold Trial')

ee/app/views/trials/select.html.haml

 - page_title _('Start a Free Gold Trial')
+- content_for :page_specific_javascripts do
+  = render "layouts/google_tag_manager_head"
+= render "layouts/google_tag_manager_body"
 
 %h3.center.pt-6
   = _('Almost there')

spec/helpers/auth_helper_spec.rb

@@ -260,4 +260,41 @@ RSpec.describe AuthHelper do
       end
     end
   end
+
+  describe '#google_tag_manager_enabled?' do
+    let(:is_gitlab_com) { true }
+    let(:user) { nil }
+
+    before do
+      allow(Gitlab).to receive(:com?).and_return(is_gitlab_com)
+      stub_config(extra: { google_tag_manager_id: 'key' })
+      allow(helper).to receive(:current_user).and_return(user)
+    end
+
+    subject(:google_tag_manager_enabled?) { helper.google_tag_manager_enabled? }
+
+    context 'on gitlab.com and a key set without a current user' do
+      it { is_expected.to be_truthy }
+    end
+
+    context 'when not on gitlab.com' do
+      let(:is_gitlab_com) { false }
+
+      it { is_expected.to be_falsey }
+    end
+
+    context 'when current user is set' do
+      let(:user) { instance_double('User') }
+
+      it { is_expected.to be_falsey }
+    end
+
+    context 'when no key is set' do
+      before do
+        stub_config(extra: {})
+      end
+
+      it { is_expected.to be_falsey }
+    end
+  end
 end