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`.

2 files changed, 70 insertions, 1 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
diff --git a/actionmailer/test/message_delivery_test.rb b/actionmailer/test/message_delivery_test.rb
index f06d69369f..aaed94d519 100644
--- a/actionmailer/test/message_delivery_test.rb
+++ b/actionmailer/test/message_delivery_test.rb
@@ -12,16 +12,23 @@ class MessageDeliveryTest < ActiveSupport::TestCase
     ActionMailer::Base.deliver_later_queue_name = :test_queue
     ActionMailer::Base.delivery_method = :test
     ActiveJob::Base.logger = Logger.new(nil)
-    @mail = DelayedMailer.test_message(1, 2, 3)
     ActionMailer::Base.deliveries.clear
     ActiveJob::Base.queue_adapter.perform_enqueued_at_jobs = true
     ActiveJob::Base.queue_adapter.perform_enqueued_jobs = true
+
+    DelayedMailer.last_error = nil
+    DelayedMailer.last_rescue_from_instance = nil
+
+    @mail = DelayedMailer.test_message(1, 2, 3)
   end
 
   teardown do
     ActiveJob::Base.logger = @previous_logger
     ActionMailer::Base.delivery_method = @previous_delivery_method
     ActionMailer::Base.deliver_later_queue_name = @previous_deliver_later_queue_name
+
+    DelayedMailer.last_error = nil
+    DelayedMailer.last_rescue_from_instance = nil
   end
 
   test 'should have a message' do
@@ -101,4 +108,46 @@ class MessageDeliveryTest < ActiveSupport::TestCase
       @mail.deliver_later
     end
   end
+
+  test 'job delegates error handling to mailer' do
+    # Superclass not rescued by mailer's rescue_from RuntimeError
+    message = DelayedMailer.test_raise('StandardError')
+    assert_raise(StandardError) { message.deliver_later }
+    assert_nil DelayedMailer.last_error
+    assert_nil DelayedMailer.last_rescue_from_instance
+
+    # Rescued by mailer's rescue_from RuntimeError
+    message = DelayedMailer.test_raise('DelayedMailerError')
+    assert_nothing_raised { message.deliver_later }
+    assert_equal 'boom', DelayedMailer.last_error.message
+    assert_kind_of DelayedMailer, DelayedMailer.last_rescue_from_instance
+  end
+
+  class DeserializationErrorFixture
+    include GlobalID::Identification
+
+    def self.find(id)
+      raise 'boom, missing find'
+    end
+
+    attr_reader :id
+    def initialize(id = 1)
+      @id = id
+    end
+
+    def to_global_id(options = {})
+      super app: 'foo'
+    end
+  end
+
+  test 'job delegates deserialization errors to mailer class' do
+    # Inject an argument that can't be deserialized.
+    message = DelayedMailer.test_message(DeserializationErrorFixture.new)
+
+    # DeserializationError is raised, rescued, and delegated to the handler
+    # on the mailer class.
+    assert_nothing_raised { message.deliver_later }
+    assert_equal DelayedMailer, DelayedMailer.last_rescue_from_instance
+    assert_equal 'Error while trying to deserialize arguments: boom, missing find', DelayedMailer.last_error.message
+  end
 end