emails: fix one-click unsubscription.

Fix it so that emails are (hopefully) able to show the one-click
unsubscribe button in Gmail and other mail providers that support the
List-Unsubscribe header. This way users can unsubscribe instead of
marking emails as spam.

* Add the List-Unsubscribe-Post header.
* Fix the disable email notifications endpoint to support POST as well as DELETE requests.
* Fix the disable email notifications endpoint to disable XSRF protection (we don't need
  users to be logged in because we use a signed URL instead).

https://www.rfc-editor.org/rfc/rfc8058#section-3.1
https://www.rfc-editor.org/rfc/rfc8058#section-8.1

app/controllers/maintenance/user/email_notifications_controller.rb

@@ -7,8 +7,9 @@ module Maintenance
 
       respond_to :html, :json, :xml
 
-      before_action :validate_sig, :only => [:destroy]
+      before_action :validate_sig, only: [:create, :destroy]
-      rescue_from VerificationError, with: :render_verification_error
+      skip_forgery_protection only: [:create, :destroy]
+      rescue_with VerificationError, status: 403
 
       def show
       end
@@ -16,15 +17,23 @@ module Maintenance
       def destroy
         @user = ::User.find(params[:user_id])
         @user.update!(receive_email_notifications: false)
-        respond_with(@user)
+
+        # https://www.rfc-editor.org/rfc/rfc8058#section-3.1
+        #
+        # A mail receiver can do a one-click unsubscription by performing an HTTPS POST to the HTTPS URI in the
+        # List-Unsubscribe header. It sends the key/value pair in the List-Unsubscribe-Post header as the request body.
+        # The List-Unsubscribe-Post header MUST contain the single key/value pair "List-Unsubscribe=One-Click".
+        # The mail sender MUST NOT return an HTTPS redirect
+        if params["List-Unsubscribe"] == "One-Click"
+          head 200
+        else
+          respond_with(@user)
+        end
       end
+      alias_method :create, :destroy
 
       private
 
-      def render_verification_error
-        render plain: "", status: 403
-      end
-
       def validate_sig
         verifier = ActiveSupport::MessageVerifier.new(Danbooru.config.email_key, digest: "SHA256", serializer: JSON)
         calculated_sig = verifier.generate(params[:user_id].to_s)

app/mailers/application_mailer.rb

@@ -11,7 +11,14 @@ class ApplicationMailer < ActionMailer::Base
   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)
+    # https://www.rfc-editor.org/rfc/rfc8058#section-3.1
+    #
+    # A mail receiver can do a one-click unsubscription by performing an HTTPS POST to the HTTPS URI in the
+    # List-Unsubscribe header. It sends the key/value pair in the List-Unsubscribe-Post header as the request body.
+    # The List-Unsubscribe-Post header MUST contain the single key/value pair "List-Unsubscribe=One-Click".
+    headers["List-Unsubscribe"] = "<#{disable_email_notifications_url(user)}>"
+    headers["List-Unsubscribe-Post"] = "List-Unsubscribe=One-Click"
+
     message = super(to: user.email_address&.address, **options)
     message.perform_deliveries = user.can_receive_email?(require_verified_email: require_verified_email)
     message

config/routes.rb

@@ -49,7 +49,7 @@ Rails.application.routes.draw do
   namespace :maintenance do
     namespace :user do
       resource :count_fixes, only: [:new, :create]
-      resource :email_notification, :only => [:show, :destroy]
+      resource :email_notification, only: [:show, :create, :destroy]
       resource :deletion, :only => [:show, :destroy]
     end
   end

test/functional/maintenance/user/email_notifications_controller.rb

@@ -19,14 +19,21 @@ module Maintenance
 
         context "#destroy" do
           should "disable email notifications" do
-            delete_auth maintenance_user_email_notification_path(user_id: @user.id, sig: @sig), @user
+            delete maintenance_user_email_notification_path(user_id: @user.id, sig: @sig)
+
+            assert_response :success
+            assert_equal(false, @user.reload.receive_email_notifications)
+          end
+
+          should "disable email notifications from a one-click unsubscribe" do
+            post maintenance_user_email_notification_path(user_id: @user.id, sig: @sig), params: { "List-Unsubscribe": "One-Click" }
 
             assert_response :success
             assert_equal(false, @user.reload.receive_email_notifications)
           end
 
           should "not disable email notifications when given an incorrect signature" do
-            delete_auth maintenance_user_email_notification_path(user_id: @user.id, sig: "foo"), @user
+            delete maintenance_user_email_notification_path(user_id: @user.id, sig: "foo")
 
             assert_response 403
             assert_equal(true, @user.reload.receive_email_notifications)