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, 17 insertions, 4 deletions

diff --git a/activesupport/test/rescuable_test.rb b/activesupport/test/rescuable_test.rb
index bd43ad0797..e42e6d2973 100644
--- a/activesupport/test/rescuable_test.rb
+++ b/activesupport/test/rescuable_test.rb
@@ -3,9 +3,6 @@ require 'abstract_unit'
 class WraithAttack < StandardError
 end
 
-class NuclearExplosion < StandardError
-end
-
 class MadRonon < StandardError
 end
 
@@ -19,6 +16,10 @@ module WeirdError
 end
 
 class Stargate
+  # Nest this so the 'NuclearExplosion' handler needs a lexical const_get
+  # to find it.
+  class NuclearExplosion < StandardError; end
+
   attr_accessor :result
 
   include ActiveSupport::Rescuable
@@ -57,6 +58,14 @@ class Stargate
     raise MadRonon.new("dex")
   end
 
+  def fall_back_to_cause
+    # This exception is the cause and has a handler.
+    ronanize
+  rescue
+    # This is the exception we'll handle that doesn't have a cause.
+    raise 'unhandled RuntimeError with a handleable cause'
+  end
+
   def weird
     StandardError.new.tap do |exc|
       def exc.weird?
@@ -74,7 +83,6 @@ class Stargate
   def sos_first
     @result = 'sos_first'
   end
-
 end
 
 class CoolStargate < Stargate
@@ -127,4 +135,9 @@ class RescuableTest < ActiveSupport::TestCase
     result = @cool_stargate.send(:rescue_handlers).collect(&:first)
     assert_equal expected, result
   end
+
+  def test_rescue_falls_back_to_exception_cause
+    @stargate.dispatch :fall_back_to_cause
+    assert_equal 'unhandled RuntimeError with a handleable cause', @stargate.result
+  end
 end