JRuby should handle this - I think it's a bug in packing rather than a JRuby limitation.

Specifically it might be related to how Duration defers a lot to its @value - see https://github.com/rails/rails/blob/51278579477eb7ee20fe2aba53b4b13203791b22/activesupport/lib/active_support/duration.rb#L37-L44 and https://github.com/rails/rails/blob/51278579477eb7ee20fe2aba53b4b13203791b22/activesupport/lib/active_support/duration.rb#L102-L104.

The JRuby pack code isn't something I know well and it's quite complicated, but I would guess that MRI is using a more general type check and method lookup in converting the object to a Fixnum and maybe JRuby is doing something a little more rigid.

I couldn't immediately reduce the Duration class to reproduce in a simpler test case.

Thanks very much for the bug report.

Minimal test case:

class Test
  def method_missing(method, *args, &block)
    14.send(method, *args, &block)
  end
end

p [Test.new].pack('N')

Duration doesn't defer respond_to to @value like it does method_missing.

It looks like JRuby checks respond_to before calling to_i https://github.com/jruby/jruby/blob/master/core/src/main/java/org/jruby/util/TypeConverter.java#L60, which also seems to be what MRI does (via rb_check_funcall), so I'm not sure how it works in MRI.

@chrisseaton It looks like that issue may have been fixed since Rails 4.2.0 beta 2. See this commit - rails/rails@f976751 . Let me run the same tests against Rails master and see if the problem is resolved.

Ok - we still have a different in behaviour for my test case though - so whether or not that Rails class works there is still a bug here.

So it doesn't work with master rails, even though the respond_to? call is properly deferred. From my tests:

irb(main):019:0> require 'active_support/core_ext/numeric/time'
=> false
irb(main):020:0> [7.days].pack('N')
TypeError: can't convert ActiveSupport::Duration into Integer
   from org/jruby/RubyArray.java:4125:in `pack'
   ...
irb(main):022:0* 7.days.respond_to?(:to_i)
=> true
irb(main):023:0* 7.days.respond_to?(:to_int)
=> true

I'll wait for someone from the core team to weigh in on this - they will probably understand quicker than I how this conversion to integer is supposed to work and what the problem is likely to be.

@chrisseaton Sounds good. Thanks for investigating.

This smells like it might be related to #1856

this bug has been fixed, somewhere along the line of JRuby 1.7.x releases :

jruby-1.7.25 :001 > require 'active_support/core_ext/numeric/time'
 => true 
jruby-1.7.25 :002 > ActiveSupport::VERSION::STRING
 => "4.2.8" 
jruby-1.7.25 :005 > 7.days.class
 => ActiveSupport::Duration 
jruby-1.7.25 :006 > [7.days].pack('N')
 => "\x00\t:\x80"