g.rb

require 'fiber'
Thread.abort_on_exception = true

class Foo
  def initialize(a_val)
    puts "Step 1"
    @io_fiber = Fiber.new do |calling_fiber|
      puts "Step 4"
      some_val = calling_fiber.transfer(a_val)
      puts "Step 7"
      2
    end
    puts "Step 2"
  end
  
  def complete_setup
    # calls into block above, so Fiber.current becomes +calling_fiber+
    puts "Step 3"
    value = @io_fiber.transfer(Fiber.current)
    puts "Step 5"
    value
  end
  
  def enqueue(b_val)
    puts "Step 6"
    value = @io_fiber.transfer(b_val)
    puts "Step 8"
    value
  end
end

f1 = Foo.new(16)
puts f1.complete_setup # should output 16
puts f1.enqueue(18) # should output 2

t2 = Thread.new do
  puts "Starting steps over from new thread, using new Foo object and fibers"
  f2 = Foo.new(25)
  puts f2.complete_setup # should output 25
  puts f2.enqueue(33) # should output 2
end

sleep 2

jruby_output.txt

Charless-Air:ruby-io cremes$ ruby -v
jruby 9.1.15.0 (2.3.3) 2017-12-07 929fde8 Java HotSpot(TM) 64-Bit Server VM 25.144-b01 on 1.8.0_144-b01 +jit [darwin-x86_64]
Charless-Air:ruby-io cremes$ ruby g.rb
Step 1
Step 2
Step 3
Step 4
FiberError: fiber called across threads
         __transfer__ at org/jruby/ext/fiber/ThreadFiber.java:186
  block in initialize at g.rb:9

mri_output.txt

cremes$ ruby -v
ruby 2.5.0p0 (2017-12-25 revision 61468) [x86_64-darwin16]
Charless-Air:ruby-io cremes$ ruby g.rb
Step 1
Step 2
Step 3
Step 4
Step 5
16
Step 6
Step 7
Step 8
2
Starting steps over from new thread, using new Foo object and fibers
Step 1
Step 2
Step 3
Step 4
Step 5
25
Step 6
Step 7
Step 8
2

rbx_output.txt

Charless-Air:ruby-io cremes$ ruby -v
rubinius 3.86.c109 (2.3.1 65a8656f 2017-12-29 5.0.0) [x86_64-darwin16.7.0]
cremes$ ruby g.rb
Step 1
Step 2
Step 3
Step 4
Step 5
16
Step 6
Step 7
Step 8
2
Starting steps over from new thread, using new Foo object and fibers
Step 1
Step 2
Step 3
Step 4
                call # Proc at core/proc.rb:20
   { } in initialize # Foo at g.rb:9

attempt to transfer fiber across threads: current thread: 6, objid: 0, Fiber Thread: 1, objid: 0, Fiber objid: 0 (FiberError)

An exception occurred running g.rb