Add List-Id to notification emails

app/helpers/emails_helper.rb

@@ -98,4 +98,29 @@ module EmailsHelper
 
     "#{string} on #{Gitlab.config.gitlab.host}"
   end
+
+  def create_list_id_string(project, list_id_max_length = 255)
+    project_path_as_domain = project.full_path.downcase
+      .split('/').reverse.join('/')
+      .gsub(%r{[^a-z0-9\/]}, '-')
+      .gsub(%r{\/+}, '.')
+      .gsub(/(\A\.+|\.+\z)/, '')
+
+    max_domain_length = list_id_max_length - Gitlab.config.gitlab.host.length - project.id.to_s.length - 2
+
+    if max_domain_length < 3
+      return project.id.to_s + "..." + Gitlab.config.gitlab.host
+    end
+
+    if project_path_as_domain.length > max_domain_length
+      project_path_as_domain = project_path_as_domain.slice(0, max_domain_length)
+
+      last_dot_index = project_path_as_domain[0..-2].rindex(".")
+      last_dot_index ||= max_domain_length - 2
+
+      project_path_as_domain = project_path_as_domain.slice(0, last_dot_index).concat("..")
+    end
+
+    project.id.to_s + "." + project_path_as_domain + "." + Gitlab.config.gitlab.host
+  end
 end

app/mailers/notify.rb

@@ -3,6 +3,7 @@
 class Notify < BaseMailer
   include ActionDispatch::Routing::PolymorphicRoutes
   include GitlabRoutingHelper
+  include EmailsHelper
 
   include Emails::Issues
   include Emails::MergeRequests
@@ -194,6 +195,7 @@ class Notify < BaseMailer
     headers['X-GitLab-Project'] = @project.name
     headers['X-GitLab-Project-Id'] = @project.id
     headers['X-GitLab-Project-Path'] = @project.full_path
+    headers['List-Id'] = "#{@project.full_path} <#{create_list_id_string(@project)}>"
   end
 
   def add_unsubscription_headers_and_links

changelogs/unreleased/53493-list-id-email-header.yml

+---
+title: Add project identifier as List-Id email Header to ease filtering
+merge_request: 22817
+author: Olivier Crête
+type: added

doc/workflow/notifications.md

@@ -135,6 +135,7 @@ Notification emails include headers that provide extra content about the notific
 | X-GitLab-Pipeline-Id        | Only in pipeline emails, the ID of the pipeline the notification is for |
 | X-GitLab-Reply-Key          | A unique token to support reply by email                                |
 | X-GitLab-NotificationReason | The reason for being notified. "mentioned", "assigned", etc             |
+| List-Id                     | The path of the project in a RFC 2919 mailing list identifier useful for email organization, for example, with GMail filters |
 
 #### X-GitLab-NotificationReason
 

spec/helpers/emails_helper_spec.rb

@@ -73,4 +73,59 @@ describe EmailsHelper do
       end
     end
   end
+
+  describe '#create_list_id_string' do
+    using RSpec::Parameterized::TableSyntax
+
+    where(:full_path, :list_id_path) do
+      "01234"  | "01234"
+      "5/0123" | "012.."
+      "45/012" | "012.."
+      "012"    | "012"
+      "23/01"  | "01.23"
+      "2/01"   | "01.2"
+      "234/01" | "01.."
+      "4/2/0"  | "0.2.4"
+      "45/2/0" | "0.2.."
+      "5/23/0" | "0.."
+      "0-2/5"  | "5.0-2"
+      "0_2/5"  | "5.0-2"
+      "0.2/5"  | "5.0-2"
+    end
+
+    with_them do
+      it 'ellipcizes different variants' do
+        project = double("project")
+        allow(project).to receive(:full_path).and_return(full_path)
+        allow(project).to receive(:id).and_return(12345)
+        # Set a max length that gives only 5 chars for the project full path
+        max_length = "12345..#{Gitlab.config.gitlab.host}".length + 5
+        list_id = create_list_id_string(project, max_length)
+
+        expect(list_id).to eq("12345.#{list_id_path}.#{Gitlab.config.gitlab.host}")
+        expect(list_id).to satisfy { |s| s.length <= max_length }
+      end
+    end
+  end
+
+  describe 'Create realistic List-Id identifier' do
+    using RSpec::Parameterized::TableSyntax
+
+    where(:full_path, :list_id_path) do
+      "gitlab-org/gitlab-ce" | "gitlab-ce.gitlab-org"
+      "project-name/subproject_name/my.project" | "my-project.subproject-name.project-name"
+    end
+
+    with_them do
+      it 'Produces the right List-Id' do
+        project = double("project")
+        allow(project).to receive(:full_path).and_return(full_path)
+        allow(project).to receive(:id).and_return(12345)
+        list_id = create_list_id_string(project)
+
+        expect(list_id).to eq("12345.#{list_id_path}.#{Gitlab.config.gitlab.host}")
+        expect(list_id).to satisfy { |s| s.length <= 255 }
+      end
+    end
+  end
 end

spec/support/shared_examples/notify_shared_examples.rb

 shared_context 'gitlab email notification' do
-  set(:project) { create(:project, :repository) }
+  set(:project) { create(:project, :repository, name: 'a-known-name') }
   set(:recipient) { create(:user, email: 'recipient@example.com') }
 
   let(:gitlab_sender_display_name) { Gitlab.config.gitlab.email_display_name }
@@ -62,9 +62,11 @@ end
 shared_examples 'an email with X-GitLab headers containing project details' do
   it 'has X-GitLab-Project headers' do
     aggregate_failures do
+      full_path_as_domain = "#{project.name}.#{project.namespace.path}"
       is_expected.to have_header('X-GitLab-Project', /#{project.name}/)
       is_expected.to have_header('X-GitLab-Project-Id', /#{project.id}/)
       is_expected.to have_header('X-GitLab-Project-Path', /#{project.full_path}/)
+      is_expected.to have_header('List-Id', "#{project.full_path} <#{project.id}.#{full_path_as_domain}.#{Gitlab.config.gitlab.host}>")
     end
   end
 end