This behave differently in jruby and MRI

queues.rb

#! /usr/bin/env ruby

@queues = 10.times.map { Array.new }

# An enumerator that yields `nil` if none of the queues has any items, but
# otherwise round-robins across them.
@foo = Enumerator.new do |enum|
  loop do
    found = false
    @queues.each do |queue|
      job = queue.pop
      unless job.nil?
        found = true
        enum.yield(job)
      end
    end
    enum.yield(nil) unless found
  end
end

# Pop up to 'count' objects from 
def pop(count)
  jobs = []
  while jobs.length < count
    job = @foo.next
    break if job.nil?
    jobs.push(job)
  end
  jobs
end

# First, verify that the there are no items available
puts "Pop before put: #{pop(5)}"
@queues[5].push(1)
# Now, I'd expect the item to be immediately available after putting it, but in
# jRuby, it's not yielded until the second pop.
puts "First pop after put: #{pop(5)}"
puts "Second pop after put: #{pop(5)}"

# jruby 1.7.9 (1.9.3p392) 2013-12-06 87b108a on Java HotSpot(TM) 64-Bit Server VM 1.7.0_45-b18 [darwin-x86_64]
# Pop before put: []
# First pop after put: []
# Second pop after put: [1]

# ruby 2.0.0p247 (2013-06-27 revision 41674) [universal.x86_64-darwin13]
# Pop before put: []
# First pop after put: [1]
# Second pop after put: []

# ruby 1.9.3p429 (2013-05-15 revision 40747) [x86_64-darwin13.0.0]
# Pop before put: []
# First pop after put: [1]
# Second pop after put: []