Add new configuration setting to enable/disable HTML emails.

This new global setting will allow admins to specify if HTML emails should be sent or not,
this is basically useful when system administrators want to save some disk space by avoiding
emails in HTML format and using only the Plain Text version.

app/controllers/admin/application_settings_controller.rb

@@ -112,6 +112,7 @@ class Admin::ApplicationSettingsController < Admin::ApplicationController
       :koding_enabled,
       :koding_url,
       :email_author_in_body,
+      :html_emails_enabled,
       :repository_checks_enabled,
       :metrics_packet_size,
       :send_user_confirmation_email,

app/views/admin/application_settings/_form.html.haml

@@ -443,7 +443,16 @@
             Some email servers do not support overriding the email sender name.
             Enable this option to include the name of the author of the issue,
             merge request or comment in the email body instead.
-
+    .form-group
+      .col-sm-offset-2.col-sm-10
+        .checkbox
+          = f.label :html_emails_enabled do
+            = f.check_box :html_emails_enabled
+            Enable HTML emails
+          .help-block
+            By default GitLab sends emails in HTML and plain text formats so mail
+            clients can choose what format to use. Disable this option if you only
+            want to send emails in plain text format.
   %fieldset
     %legend Automatic Git repository housekeeping
     .form-group

changelogs/unreleased/7749-add-setting-to-disable-html-emails.yml

+title: Add setting to enable/disable HTML emails
+merge_request: 7749
+author:

config/initializers/email_template_interceptor.rb

+# Interceptor in lib/email_template_interceptor.rb
+ActionMailer::Base.register_interceptor(EmailTemplateInterceptor)

db/migrate/20161128161412_add_html_emails_enabled_to_application_settings.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddHtmlEmailsEnabledToApplicationSettings < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  # When a migration requires downtime you **must** uncomment the following
+  # constant and define a short and easy to understand explanation as to why the
+  # migration requires downtime.
+  # DOWNTIME_REASON = ''
+
+  # When using the methods "add_concurrent_index" or "add_column_with_default"
+  # you must disable the use of transactions as these methods can not run in an
+  # existing transaction. When using "add_concurrent_index" make sure that this
+  # method is the _only_ method called in the migration, any other changes
+  # should go in a separate migration. This ensures that upon failure _only_ the
+  # index creation fails and can be retried or reverted easily.
+  #
+  # To disable transactions uncomment the following line and remove these
+  # comments:
+  # disable_ddl_transaction!
+
+  def change
+    add_column :application_settings, :html_emails_enabled, :boolean, default: true
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20161118183841) do
+ActiveRecord::Schema.define(version: 20161128161412) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -106,6 +106,7 @@ ActiveRecord::Schema.define(version: 20161118183841) do
     t.integer "housekeeping_incremental_repack_period", default: 10, null: false
     t.integer "housekeeping_full_repack_period", default: 50, null: false
     t.integer "housekeeping_gc_period", default: 200, null: false
+    t.boolean "html_emails_enabled", default: true
   end
 
   create_table "audit_events", force: :cascade do |t|

lib/email_template_interceptor.rb

+# Read about interceptors in http://guides.rubyonrails.org/action_mailer_basics.html#intercepting-emails
+class EmailTemplateInterceptor
+  include Gitlab::CurrentSettings
+
+  def self.delivering_email(message)
+    # Remove HTML part if HTML emails are disabled.
+    unless current_application_settings.html_emails_enabled
+      message.part.delete_if do |part|
+        part.content_type.try(:start_with?, 'text/html')
+      end
+    end
+  end
+end

spec/mailers/notify_spec.rb

@@ -1099,4 +1099,38 @@ describe Notify do
       is_expected.to have_body_text /#{diff_path}/
     end
   end
+
+  describe 'HTML emails setting' do
+    let(:project) { create(:project) }
+    let(:user) { create(:user) }
+    let(:multipart_mail) { Notify.project_was_moved_email(project.id, user.id, "gitlab/gitlab") }
+
+    context 'when disabled' do
+      it 'only sends the text template' do
+        stub_application_setting(html_emails_enabled: false)
+
+        EmailTemplateInterceptor.delivering_email(multipart_mail)
+
+        expect(multipart_mail).to have_part_with('text/plain')
+        expect(multipart_mail).not_to have_part_with('text/html')
+      end
+    end
+
+    context 'when enabled' do
+      it 'sends a multipart message' do
+        stub_application_setting(html_emails_enabled: true)
+
+        EmailTemplateInterceptor.delivering_email(multipart_mail)
+
+        expect(multipart_mail).to have_part_with('text/plain')
+        expect(multipart_mail).to have_part_with('text/html')
+      end
+    end
+
+    matcher :have_part_with do |expected|
+      match do |actual|
+        actual.body.parts.any? { |part| part.content_type.try(:match, %r(#{expected})) }
+      end
+    end
+  end
 end