Fix #4515: Set List-Unsubscribe header on notification emails.

app/helpers/users_helper.rb

@@ -13,9 +13,11 @@ module UsersHelper
     user.dmails.active.unread.first
   end
 
-  def email_sig(user)
+  def disable_email_notifications_url(user)
     verifier = ActiveSupport::MessageVerifier.new(Danbooru.config.email_key, serializer: JSON, digest: "SHA256")
-    verifier.generate(user.id.to_s)
+    sig = verifier.generate(user.id.to_s)
+
+    maintenance_user_email_notification_url(user_id: user.id, sig: sig)
   end
 
   def email_verification_url(user)

app/mailers/application_mailer.rb

@@ -4,9 +4,14 @@
 #
 # @see https://guides.rubyonrails.org/action_mailer_basics.html
 class ApplicationMailer < ActionMailer::Base
+  helper :application
+  helper :users
+  include UsersHelper
+
   default from: "#{Danbooru.config.canonical_app_name} <#{Danbooru.config.contact_email}>", content_type: "text/html"
 
   def mail(user, require_verified_email:, **options)
+    headers["List-Unsubscribe"] = disable_email_notifications_url(user)
     message = super(to: user.email_address&.address, **options)
     message.perform_deliveries = user.can_receive_email?(require_verified_email: require_verified_email)
     message

app/mailers/user_mailer.rb

@@ -1,9 +1,6 @@
 # frozen_string_literal: true
 
 class UserMailer < ApplicationMailer
-  helper :application
-  helper :users
-
   # The email sent when a user receives a DMail.
   def dmail_notice(dmail)
     @dmail = dmail

app/views/user_mailer/dmail_notice.html.erb

@@ -7,6 +7,6 @@
   <%= format_text(@dmail.body, base_url: root_url) %>
 </div>
 
-<p><%= link_to "View message", dmail_url(@dmail, :host => Danbooru.config.hostname, :only_path => false) %> | <%= link_to "Unsubscribe", maintenance_user_email_notification_url(:user_id => @dmail.owner.id, :sig => email_sig(@dmail.owner), :host => Danbooru.config.hostname, :only_path => false) %></p>
+<p><%= link_to "View message", dmail_url(@dmail) %> | <%= link_to "Disable notifications", disable_email_notifications_url(@dmail.to) %></p>
 </body>
 </html>

test/unit/user_mailer_test.rb

@@ -10,6 +10,7 @@ class UserMailerTest < ActionMailer::TestCase
       should "work" do
         @dmail = create(:dmail, owner: @user, to: @user)
         mail = UserMailer.dmail_notice(@dmail)
+        assert_not_nil(mail.header["List-Unsubscribe"])
         assert_emails(1) { mail.deliver_now }
       end