Action Mailer: Declarative exception handling with `rescue_from`.

Follows the same pattern as controllers and jobs. Exceptions raised in
delivery jobs (enqueued by `#deliver_later`) are also delegated to the
mailer's rescue_from handlers, so you can handle the DeserializationError
raised by delivery jobs:

```ruby
class MyMailer < ApplicationMailer
  rescue_from ActiveJob::DeserializationError do
    …
  end
```

ActiveSupport::Rescuable polish:
* Add the `rescue_with_handler` class method so exceptions may be
  handled at the class level without requiring an instance.
* Rationalize `exception.cause` handling. If no handler matches the
  exception, fall back to the handler that matches its cause.
* Handle exceptions raised elsewhere. Pass `object: …` to execute
  the `rescue_from` handler (e.g. a method call or a block to
  instance_exec) against a different object. Defaults to `self`.

1 files changed, 21 insertions, 0 deletions

diff --git a/actionmailer/lib/action_mailer/delivery_job.rb b/actionmailer/lib/action_mailer/delivery_job.rb
index 52772af2d3..d371c1b61a 100644
--- a/actionmailer/lib/action_mailer/delivery_job.rb
+++ b/actionmailer/lib/action_mailer/delivery_job.rb
@@ -3,11 +3,32 @@ require 'active_job'
 module ActionMailer
   # The <tt>ActionMailer::DeliveryJob</tt> class is used when you
   # want to send emails outside of the request-response cycle.
+  #
+  # Exceptions are rescued and handled by the mailer class.
   class DeliveryJob < ActiveJob::Base # :nodoc:
     queue_as { ActionMailer::Base.deliver_later_queue_name }
 
+    rescue_from StandardError, with: :handle_exception_with_mailer_class
+
     def perform(mailer, mail_method, delivery_method, *args) #:nodoc:
       mailer.constantize.public_send(mail_method, *args).send(delivery_method)
     end
+
+    private
+      # "Deserialize" the mailer class name by hand in case another argument
+      # (like a Global ID reference) raised DeserializationError.
+      def mailer_class
+        if mailer = Array(@serialized_arguments).first || Array(arguments).first
+          mailer.constantize
+        end
+      end
+
+      def handle_exception_with_mailer_class(exception)
+        if klass = mailer_class
+          klass.handle_exception exception
+        else
+          raise exception
+        end
+      end
   end
 end