Disallow calling `#deliver_later` after local message modifications.

They would be lost when the delivery job is enqueued, otherwise.
Prevents a common, hard-to-find bug like:

```ruby
message = Notifier.welcome(user, foo)
message.message_id = my_generated_message_id
message.deliver_later
```

The message_id is silently lost here! *Only the mailer arguments are
passed to the delivery job.*

This raises an exception now.

Make modifications to the message within the mailer method or use a
custom Active Job to manage delivery instead of using #deliver_later.

1 files changed, 8 insertions, 0 deletions

diff --git a/actionmailer/test/message_delivery_test.rb b/actionmailer/test/message_delivery_test.rb
index b834cdd08c..f06d69369f 100644
--- a/actionmailer/test/message_delivery_test.rb
+++ b/actionmailer/test/message_delivery_test.rb
@@ -93,4 +93,12 @@ class MessageDeliveryTest < ActiveSupport::TestCase
       @mail.deliver_later(queue: :another_queue)
     end
   end
+
+  test 'deliver_later after accessing the message is disallowed' do
+    @mail.message # Load the message, which calls the mailer method.
+
+    assert_raise RuntimeError do
+      @mail.deliver_later
+    end
+  end
 end