config: simplify mailer configuration again

Move everything to initializers, and replace the email settings
interceptor by a BalancerDeliveryMethod.

It has the advantage that it can be configured entirely from the
`config/environment.rb` file, without an extra file to look at.

app/lib/balancer_delivery_method.rb

@@ -0,0 +1,38 @@
+# A Mail delivery method that randomly balances the actual delivery between different
+# methods.
+#
+# Usage:
+#
+# ```ruby
+#   ActionMailer::Base.add_delivery_method :balancer, BalancerDeliveryMethod
+#   config.action_mailer.balancer_settings = {
+#     smtp: 25,
+#     sendmail: 75
+#   }
+#   config.action_mailer.delivery_method = :balancer
+# ```
+#
+# Be sure to restart your server when you modify this file.
+class BalancerDeliveryMethod
+  # Allows configuring the random number generator used for selecting a delivery method,
+  # mostly for testing purposes.
+  mattr_accessor :random, default: Random.new
+
+  def initialize(settings)
+    @delivery_methods = settings
+  end
+
+  def deliver!(mail)
+    balanced_delivery_method = delivery_method(mail)
+    ApplicationMailer.wrap_delivery_behavior(mail, balanced_delivery_method)
+    mail.deliver
+  end
+
+  private
+
+  def delivery_method(mail)
+    @delivery_methods
+      .flat_map { |delivery_method, weight| [delivery_method] * weight }
+      .sample(random: self.class.random)
+  end
+end

app/lib/mailtrap/smtp.rb

@@ -1,4 +0,0 @@
-module Mailtrap
-  class Smtp < ::Mail::SMTP
-  end
-end

app/lib/sendinblue/smtp.rb

@@ -1,4 +0,0 @@
-module Sendinblue
-  class Smtp < ::Mail::SMTP
-  end
-end

config/environments/development.rb

@@ -75,40 +75,13 @@ Rails.application.configure do
   config.assets.raise_runtime_errors = true
 
   # Action Mailer settings
-
+  config.action_mailer.delivery_method = :letter_opener
-  if ENV['SENDINBLUE_ENABLED'] == 'enabled'
-    config.action_mailer.delivery_method = :smtp
-    config.action_mailer.smtp_settings = {
-      user_name: Rails.application.secrets.sendinblue[:username],
-      password: Rails.application.secrets.sendinblue[:smtp_key],
-      address: 'smtp-relay.sendinblue.com',
-      domain: 'smtp-relay.sendinblue.com',
-      port: '587',
-      authentication: :cram_md5
-    }
-  else
-    # https://usehelo.com
-    if ENV['HELO_ENABLED'] == 'enabled'
-      config.action_mailer.delivery_method = :smtp
-      config.action_mailer.smtp_settings = {
-        user_name: 'demarches-simplifiees',
-        password: '',
-        address: '127.0.0.1',
-        domain: '127.0.0.1',
-        port: ENV.fetch('HELO_PORT', '2525'),
-        authentication: :plain
-      }
-    else
-      config.action_mailer.delivery_method = :letter_opener_web
-    end
 
   config.action_mailer.default_url_options = {
     host: 'localhost',
     port: 3000
   }
-
   config.action_mailer.asset_host = "http://" + ENV['APP_HOST']
-  end
 
   Rails.application.routes.default_url_options = {
     host: 'localhost',

config/environments/production.rb

@@ -1,6 +1,5 @@
 require "active_support/core_ext/integer/time"
-require Rails.root.join("app/lib/mailtrap/smtp")
+require Rails.root.join("app/lib/balancer_delivery_method")
-require Rails.root.join("app/lib/sendinblue/smtp")
 
 Rails.application.configure do
   # Settings specified here will take precedence over those in config/application.rb.
@@ -78,31 +77,20 @@ Rails.application.configure do
   # config.action_mailer.raise_delivery_errors = false
 
   if ENV['MAILTRAP_ENABLED'] == 'enabled'
-    ActionMailer::Base.add_delivery_method :mailtrap, Mailtrap::Smtp
-    config.action_mailer.mailtrap_settings = {
-      user_name: Rails.application.secrets.mailtrap[:username],
-      password: Rails.application.secrets.mailtrap[:password],
-      address: 'smtp.mailtrap.io',
-      domain: 'smtp.mailtrap.io',
-      port: '2525',
-      authentication: :cram_md5
-    }
     config.action_mailer.delivery_method = :mailtrap
-  elsif
-    if ENV['SENDINBLUE_ENABLED'] == 'enabled'
-      ActionMailer::Base.add_delivery_method :sendinblue, Sendinblue::Smtp
-      config.action_mailer.sendinblue_settings = {
-        user_name: Rails.application.secrets.sendinblue[:username],
-        password: Rails.application.secrets.sendinblue[:smtp_key],
-        address: 'smtp-relay.sendinblue.com',
-        domain: 'smtp-relay.sendinblue.com',
-        port: '587',
-        authentication: :cram_md5
-      }
-    end
 
-    # Default delivery method
+  elsif ENV['SENDINBLUE_ENABLED'] == 'enabled' && ENV['SENDINBLUE_BALANCING'] == 'enabled'
-    # (Actual delivery method will be selected at runtime by DynamicSmtpSettingsInterceptor)
+    ActionMailer::Base.add_delivery_method :balancer, BalancerDeliveryMethod
+    config.action_mailer.balancer_settings = {
+      sendinblue: ENV.fetch('SENDINBLUE_BALANCING_VALUE').to_i,
+      mailjet: 100 - ENV.fetch('SENDINBLUE_BALANCING_VALUE').to_i
+    }
+    config.action_mailer.delivery_method = :balancer
+
+  elsif ENV['SENDINBLUE_ENABLED'] == 'enabled'
+    config.action_mailer.delivery_method = :sendinblue
+
+  else
     config.action_mailer.delivery_method = :mailjet
   end
 

config/initializers/dynamic_smtp_settings_interceptor.rb

@@ -1,14 +0,0 @@
-# We want to register an interceptor, but we can't make the action idempotent
-# (because there's no way to peek at the currently registered interceptors).
-#
-# To make zeitwerk happy, instead signal that we don't want the
-# DynamicSmtpSettingsInterceptor constant to be auto-loaded, by:
-# - adding it to a non-autoloaded-path (/lib),
-# - requiring it explicitely.
-#
-# See https://guides.rubyonrails.org/autoloading_and_reloading_constants.html#autoloading-when-the-application-boots
-require 'action_mailer/dynamic_smtp_settings_interceptor'
-
-ActiveSupport.on_load(:action_mailer) do
-  ActionMailer::Base.register_interceptor DynamicSmtpSettingsInterceptor
-end

config/initializers/helo.rb

@@ -0,0 +1,17 @@
+if ENV['HELO_ENABLED'] == 'enabled'
+  ActiveSupport.on_load(:action_mailer) do
+    module Helo
+      class SMTP < ::Mail::SMTP; end
+    end
+
+    ActionMailer::Base.add_delivery_method :helo, Helo::SMTP
+    ActionMailer::Base.helo_settings = {
+      user_name: 'demarches-simplifiees',
+      password: '',
+      address: '127.0.0.1',
+      domain: '127.0.0.1',
+      port: ENV.fetch('HELO_PORT', '2525'),
+      authentication: :plain
+    }
+  end
+end

config/initializers/mailtrap.rb

@@ -0,0 +1,17 @@
+if ENV.fetch('MAILTRAP_ENABLED') == 'enabled'
+  ActiveSupport.on_load(:action_mailer) do
+    module Mailtrap
+      class SMTP < ::Mail::SMTP; end
+    end
+
+    ActionMailer::Base.add_delivery_method :mailtrap, Mailtrap::SMTP
+    ActionMailer::Base.mailtrap_settings = {
+      user_name: Rails.application.secrets.mailtrap[:username],
+      password: Rails.application.secrets.mailtrap[:password],
+      address: 'smtp.mailtrap.io',
+      domain: 'smtp.mailtrap.io',
+      port: '2525',
+      authentication: :cram_md5
+    }
+  end
+end

config/initializers/sendinblue.rb

@@ -1,5 +1,23 @@
+if ENV.fetch('SENDINBLUE_ENABLED') == 'enabled'
   require 'sib-api-v3-sdk'
 
-SibApiV3Sdk.configure do |config|
+  ActiveSupport.on_load(:action_mailer) do
-  config.api_key['api-key'] = ENV.fetch('SENDINBLUE_API_V3_KEY', '')
+    module Sendinblue
+      class SMTP < ::Mail::SMTP; end
+    end
+
+    ActionMailer::Base.add_delivery_method :sendinblue, Sendinblue::SMTP
+    ActionMailer::Base.sendinblue_settings = {
+      user_name: Rails.application.secrets.sendinblue[:username],
+      password: Rails.application.secrets.sendinblue[:smtp_key],
+      address: 'smtp-relay.sendinblue.com',
+      domain: 'smtp-relay.sendinblue.com',
+      port: '587',
+      authentication: :cram_md5
+    }
+  end
+
+  SibApiV3Sdk.configure do |config|
+    config.api_key['api-key'] = Rails.application.secrets.sendinblue[:api_v3_key]
+  end
 end

lib/action_mailer/dynamic_smtp_settings_interceptor.rb

@@ -1,26 +0,0 @@
-# Note: this class is instanciated when being added as an interceptor
-# during the app initialization.
-#
-# If you edit this file in development env, you will need to restart
-# the app to see the changes.
-
-class DynamicSmtpSettingsInterceptor
-  def self.delivering_email(message)
-    if balance_to_sendinblue?
-      ApplicationMailer.wrap_delivery_behavior(message, :sendinblue)
-    end
-    # fallback to the default delivery method
-  end
-
-  private
-
-  def self.balance_to_sendinblue?
-    if ENV.fetch('SENDINBLUE_ENABLED') != 'enabled'
-      false
-    elsif ENV.fetch('SENDINBLUE_BALANCING') == 'enabled'
-      rand(0..99) < ENV.fetch('SENDINBLUE_BALANCING_VALUE').to_i
-    else
-      true
-    end
-  end
-end

spec/lib/balancer_delivery_method_spec.rb

@@ -0,0 +1,93 @@
+RSpec.describe BalancerDeliveryMethod do
+  class ExampleMailer < ApplicationMailer
+    def greet(name)
+      mail(to: "smtp_to", from: "smtp_from", body: "Hello #{name}")
+    end
+  end
+
+  class TestMail
+    def self.deliveries
+      @deliveries ||= []
+    end
+
+    def self.deliveries=(val)
+      @deliveries = val
+    end
+
+    attr_accessor :settings
+
+    def initialize(values)
+      @settings = values.dup
+    end
+
+    def deliver!(mail)
+      Mail::CheckDeliveryParams.check(mail)
+      self.class.deliveries << mail
+    end
+  end
+
+  class MockSmtp < TestMail; end
+
+  class MockSendmail < TestMail; end
+
+  class FixedSequence
+    def initialize(sequence)
+      @enumerator = sequence.each
+    end
+
+    def rand(_)
+      @enumerator.next
+    end
+  end
+
+  before do
+    ActionMailer::Base.add_delivery_method :mock_smtp, MockSmtp
+    ActionMailer::Base.add_delivery_method :mock_sendmail, MockSendmail
+    ActionMailer::Base.add_delivery_method :balancer, BalancerDeliveryMethod
+
+    ExampleMailer.delivery_method = :balancer
+  end
+
+  context 'when a single delivery method is provided' do
+    before do
+      ActionMailer::Base.balancer_settings = { mock_smtp: 10 }
+    end
+
+    it 'sends emails to the selected delivery method' do
+      mail = ExampleMailer.greet('Joshua').deliver_now
+      expect(mail).to have_been_delivered_using(MockSmtp)
+    end
+  end
+
+  context 'when multiple delivery methods are provided' do
+    before do
+      ActionMailer::Base.balancer_settings = { mock_smtp: 10, mock_sendmail: 5 }
+
+      rng_sequence = [3, 14, 1]
+      BalancerDeliveryMethod.random = FixedSequence.new(rng_sequence)
+    end
+
+    after do
+      BalancerDeliveryMethod.random = Random.new
+    end
+
+    it 'sends emails randomly, given the provided weights' do
+      mail1 = ExampleMailer.greet('Lucia').deliver_now
+      expect(mail1).to have_been_delivered_using(MockSmtp)
+
+      mail2 = ExampleMailer.greet('Damian').deliver_now
+      expect(mail2).to have_been_delivered_using(MockSendmail)
+
+      mail3 = ExampleMailer.greet('Rahwa').deliver_now
+      expect(mail3).to have_been_delivered_using(MockSmtp)
+    end
+  end
+
+  # Helpers
+
+  def have_been_delivered_using(delivery_class)
+    satisfy("have been delivered using #{delivery_class}") do |mail|
+      delivery_class.deliveries.include?(mail)
+    end
+  end
+end