Automatically switch from open to approved registrations in absence of moderators (#29318)

app/mailers/admin_mailer.rb

@@ -61,6 +61,12 @@ class AdminMailer < ApplicationMailer
     end
   end
 
+  def auto_close_registrations
+    locale_for_account(@me) do
+      mail subject: default_i18n_subject(instance: @instance)
+    end
+  end
+
   private
 
   def process_params

app/views/admin_mailer/auto_close_registrations.text.erb

@@ -0,0 +1,3 @@
+<%= raw t('admin_mailer.auto_close_registrations.body', instance: @instance) %>
+
+<%= raw t('application_mailer.view')%> <%= admin_settings_registrations_url %>

app/workers/scheduler/auto_close_registrations_scheduler.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+class Scheduler::AutoCloseRegistrationsScheduler
+  include Sidekiq::Worker
+  include Redisable
+
+  sidekiq_options retry: 0
+
+  # Automatically switch away from open registrations if no
+  # moderator had any activity in that period of time
+  OPEN_REGISTRATIONS_MODERATOR_THRESHOLD = 1.week + UserTrackingConcern::SIGN_IN_UPDATE_FREQUENCY
+
+  def perform
+    return if Rails.configuration.x.email_domains_whitelist.present? || ENV['DISABLE_AUTOMATIC_SWITCHING_TO_APPROVED_REGISTRATIONS'] == 'true'
+    return unless Setting.registrations_mode == 'open'
+
+    switch_to_approval_mode! unless active_moderators?
+  end
+
+  private
+
+  def active_moderators?
+    User.those_who_can(:manage_reports).exists?(current_sign_in_at: OPEN_REGISTRATIONS_MODERATOR_THRESHOLD.ago...)
+  end
+
+  def switch_to_approval_mode!
+    Setting.registrations_mode = 'approved'
+
+    User.those_who_can(:view_devops).includes(:account).find_each do |user|
+      AdminMailer.with(recipient: user.account).auto_close_registrations.deliver_later
+    end
+  end
+end