Eager autoload mail gem when eager load is true (#32808)

* Eager autoload mail gem when eager load is true

We had a production issue where our Sidekiq worker threads all became
deadlocked while autoloading a file within the mail gem, required via
ActionMailer, despite setting our Rails applicaiton to eager load.
`Mail.eager_autoload!` exists and works great, ActionMailer just doesn't
call it during eager loading. Adding it to the ActionMailer Railtie's
eager_load_namespaces takes care of calling `Mail.eager_autoload!`
during the `eager_load!` initializer.

* 'Mail' isn't defined yet, use before_eager_load instead

* Make sure mail is loaded

* Move eager load of Mail into ActionMailer.eager_load!

[Samuel Cochran + Rafael Mendonça França]

2 files changed, 11 insertions, 0 deletions

diff --git a/actionmailer/CHANGELOG.md b/actionmailer/CHANGELOG.md
index 9fb2e44210..6d91d4fbd6 100644
--- a/actionmailer/CHANGELOG.md
+++ b/actionmailer/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Ensure mail gem is eager autoloaded when eager load is true to prevent thread deadlocks.
+
+    *Samuel Cochran*
+
 *   Perform email jobs in `assert_emails`.
 
     *Gannon McGibbon*
diff --git a/actionmailer/lib/action_mailer.rb b/actionmailer/lib/action_mailer.rb
index fabbdd1b25..69eae65d60 100644
--- a/actionmailer/lib/action_mailer.rb
+++ b/actionmailer/lib/action_mailer.rb
@@ -52,6 +52,13 @@ module ActionMailer
   autoload :TestHelper
   autoload :MessageDelivery
   autoload :DeliveryJob
+
+  def self.eager_load!
+    super
+
+    require "mail"
+    Mail.eager_autoload!
+  end
 end
 
 autoload :Mime, "action_dispatch/http/mime_type"