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, 14 insertions, 2 deletions

diff --git a/actionmailer/lib/action_mailer/message_delivery.rb b/actionmailer/lib/action_mailer/message_delivery.rb
index 5fcb5a0c88..d638057d72 100644
--- a/actionmailer/lib/action_mailer/message_delivery.rb
+++ b/actionmailer/lib/action_mailer/message_delivery.rb
@@ -18,6 +18,7 @@ module ActionMailer
       @mailer = mailer
       @mail_method = mail_method
       @args = args
+      @obj = nil
     end
 
     def __getobj__ #:nodoc:
@@ -91,8 +92,19 @@ module ActionMailer
     private
 
       def enqueue_delivery(delivery_method, options={})
-        args = @mailer.name, @mail_method.to_s, delivery_method.to_s, *@args
-        ActionMailer::DeliveryJob.set(options).perform_later(*args)
+        if @obj
+          raise "You've accessed the message before asking to deliver it " \
+            "later, so you may have made local changes that would be " \
+            "silently lost if we enqueued a job to deliver it. Why? Only " \
+            "the mailer method *arguments* are passed with the delivery job! " \
+            "Do not access the message in any way if you mean to deliver it " \
+            "later. Workarounds: 1. don't touch the message before calling " \
+            "#deliver_later, 2. only touch the message *within your mailer " \
+            "method*, or 3. use a custom Active Job instead of #deliver_later."
+        else
+          args = @mailer.name, @mail_method.to_s, delivery_method.to_s, *@args
+          ActionMailer::DeliveryJob.set(options).perform_later(*args)
+        end
       end
   end
 end