Support deferred Mailer deliver with deliver_later

Re-opens [#4224][]

By default, all `Devise::Models::Authenticatable`-initiated Action
Mailer deliveries are transmitted immediately (within the
request-response cycle).

Transmitting emails and interacting with any third-party services over
SMTP or HTTP risk service outages and other types of network-related
failures.

This commit adds support for deferring delivery to be done from an
Action Job background worker queue through the [deliver_later][] method.

```ruby
 # config/initializers/devise.rb
Devise.mailer_delivery_method = :deliver_later
```

[#4224]: #4224
[deliver_now]: https://edgeapi.rubyonrails.org/classes/ActionMailer/MessageDelivery.html#method-i-deliver_now
[deliver_later]: https://edgeapi.rubyonrails.org/classes/ActionMailer/MessageDelivery.html#method-i-deliver_later

CHANGELOG.md

@@ -5,6 +5,7 @@
 
 * enhancements
   * Removed deprecations warning output for `Devise::Models::Authenticatable::BLACKLIST_FOR_SERIALIZATION` (@soartec-lab)
+  * Added `Devise.mailer_delivery_method` configuration to enable deferring email delivery with `:deliver_later` [#5610](https://github.com/heartcombo/devise/pull/5610) [@seanpdoyle](https://github.com/seanpdoyle)
 
 * bug fixes
   * Respect locale set by controller in failure app. Devise will carry over the current I18n.locale option when triggering authentication, and will wrap the failure app call with it. [#5567](https://github.com/heartcombo/devise/pull/5567)

lib/devise.rb

@@ -166,6 +166,10 @@ module Test
   mattr_accessor :send_password_change_notification
   @@send_password_change_notification = false
 
+  # Used to control when to deliver Action Mailer emails
+  mattr_accessor :mailer_delivery_method
+  @@mailer_delivery_method = :deliver_now
+
   # Scoped views. Since it relies on fallbacks to render default views, it's
   # turned off by default.
   mattr_accessor :scoped_views

lib/devise/models/authenticatable.rb

@@ -192,7 +192,7 @@ def devise_mailer
       #
       def send_devise_notification(notification, *args)
         message = devise_mailer.send(notification, self, *args)
-        message.deliver_now
+        message.public_send(Devise.mailer_delivery_method)
       end
 
       def downcase_keys

lib/generators/templates/devise.rb

@@ -29,6 +29,9 @@
   # Configure the class responsible to send e-mails.
   # config.mailer = 'Devise::Mailer'
 
+  # Configure when to send e-mails.
+  # config.mailer_delivery_method = :deliver_now
+
   # Configure the parent class responsible to send e-mails.
   # config.parent_mailer = 'ActionMailer::Base'
 

test/mailers/mailer_test.rb

@@ -43,4 +43,20 @@ def computed_reply_to
     assert mail.from, "from@example.com"
     assert mail.reply_to, "reply_to@example.com"
   end
+
+  test "defaults to immediate delivery" do
+    create_user
+
+    assert_not_empty ActionMailer::Base.deliveries
+    assert_empty ActiveJob::Base.queue_adapter.enqueued_jobs
+  end
+
+  test "supports deferred delivery with Devise.mailer_delivery_method = :deliver_later" do
+    swap Devise, mailer_delivery_method: :deliver_later do
+      create_user
+
+      assert_empty ActionMailer::Base.deliveries
+      assert_not_empty ActiveJob::Base.queue_adapter.enqueued_jobs
+    end
+  end
 end