config: use alternate delivery methods to configure ActionMailer

Previously `SENDINBLUE_BALANCING` was used only when
`SENDINBLUE_ENABLED` was *disabled* (otherwise only SendInBlue was ever
used).

This commit:

- Ensure that `SENDINBLUE_BALANCING` is used only when SendInBlue is
  *enabled* (which is more intuitive).
- Make it easier to add other delivery methods.

app/lib/mailtrap/smtp.rb

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

app/lib/sendinblue/smtp.rb

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

config/environments/production.rb

@@ -1,4 +1,6 @@
 require "active_support/core_ext/integer/time"
+require Rails.root.join("app/lib/mailtrap/smtp")
+require Rails.root.join("app/lib/sendinblue/smtp")
 
 Rails.application.configure do
   # Settings specified here will take precedence over those in config/application.rb.
@@ -76,8 +78,8 @@ Rails.application.configure do
   # config.action_mailer.raise_delivery_errors = false
 
   if ENV['MAILTRAP_ENABLED'] == 'enabled'
-    config.action_mailer.delivery_method = :smtp
-    config.action_mailer.smtp_settings = {
+    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',
@@ -85,17 +87,22 @@ Rails.application.configure do
       port: '2525',
       authentication: :cram_md5
     }
-  elsif 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
+    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
+    # (Actual delivery method will be selected at runtime by DynamicSmtpSettingsInterceptor)
     config.action_mailer.delivery_method = :mailjet
   end
 

lib/action_mailer/dynamic_smtp_settings_interceptor.rb

@@ -6,17 +6,21 @@
 
 class DynamicSmtpSettingsInterceptor
   def self.delivering_email(message)
-    if ENV['SENDINBLUE_BALANCING'] == 'enabled'
-      if rand(0..99) < ENV['SENDINBLUE_BALANCING_VALUE'].to_i
-        message.delivery_method.settings = {
-          user_name: ENV['SENDINBLUE_USER_NAME'],
-          password: ENV['SENDINBLUE_SMTP_KEY'],
-          address: 'smtp-relay.sendinblue.com',
-          domain: 'smtp-relay.sendinblue.com',
-          port: '587',
-          authentication: :cram_md5
-        }
-      end
+    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