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, 20 insertions, 0 deletions

diff --git a/actionmailer/test/mailers/delayed_mailer.rb b/actionmailer/test/mailers/delayed_mailer.rb
index 62d4baa434..e6211ef028 100644
--- a/actionmailer/test/mailers/delayed_mailer.rb
+++ b/actionmailer/test/mailers/delayed_mailer.rb
@@ -1,6 +1,26 @@
+require 'active_job/arguments'
+
+class DelayedMailerError < StandardError; end
+
 class DelayedMailer < ActionMailer::Base
+  cattr_accessor :last_error
+  cattr_accessor :last_rescue_from_instance
+
+  rescue_from DelayedMailerError do |error|
+    @@last_error = error
+    @@last_rescue_from_instance = self
+  end
+
+  rescue_from ActiveJob::DeserializationError do |error|
+    @@last_error = error
+    @@last_rescue_from_instance = self
+  end
 
   def test_message(*)
     mail(from: 'test-sender@test.com', to: 'test-receiver@test.com', subject: 'Test Subject', body: 'Test Body')
   end
+
+  def test_raise(klass_name)
+    raise klass_name.constantize, 'boom'
+  end
 end