fix(mailer): observers on balancer and balanced delivery methods

- Les observers doivent être déclarés *avant*
  le chargement de nos delivery methods (Sendinblue & Dolist),
  sinon ils seront ignorés par ces derniers

- dans le balancer, on ne peut pas appeler `.deliver` une seconde fois,
  sinon les observers (et interceptors) sont invoqués deux fois.

app/lib/balancer_delivery_method.rb

@@ -25,7 +25,16 @@ class BalancerDeliveryMethod
   def deliver!(mail)
     balanced_delivery_method = delivery_method(mail)
     ApplicationMailer.wrap_delivery_behavior(mail, balanced_delivery_method)
-    mail.deliver
+
+    # Because we don't want to invoke observers or interceptors twice,
+    # we can't call again `mail.deliver` here to send the email with balanced method
+    # (it was first called before by deliver_now in ActiveJob or application code, which leads us here).
+    #
+    # Instead, we directly deliver the email from the handler (set by the wrapper above)
+    # like Mail::Message.deliver does.
+    #
+    # See https://github.com/mikel/mail/blob/199a76bed3fc518508b46135691914a1cfd8bff8/lib/mail/message.rb#L250
+    mail.delivery_handler.deliver_mail(mail) { mail.send :do_delivery }
   end
 
   private

config/initializers/mail_observers.rb

@@ -1,3 +1,6 @@
-Rails.application.configure do
-  config.action_mailer.observers = ['EmailDeliveryObserver']
+# Must be registered *before* loading custom delivery methods
+# otherwise the observer won't be invoked.
+#
+ActiveSupport.on_load(:action_mailer) do |mailer|
+  mailer.register_observer EmailDeliveryObserver
 end

spec/lib/balancer_delivery_method_spec.rb

@@ -83,6 +83,24 @@ RSpec.describe BalancerDeliveryMethod do
     end
   end
 
+  context 'when observers are configured' do
+    let(:observer) { double("Observer") }
+
+    before do
+      allow(observer).to receive(:delivered_email)
+      ActionMailer::Base.register_observer(observer)
+    end
+
+    after do
+      ActionMailer::Base.unregister_observer(observer)
+    end
+
+    it 'invoke the observer exactly once' do
+      mail = ExampleMailer.greet('Joshua').deliver_now
+      expect(observer).to have_received(:delivered_email).with(mail).once
+    end
+  end
+
   # Helpers
 
   def have_been_delivered_using(delivery_class)