def set(x)
  Thread.current[:x] = x
end

def say
  puts "#{Thread.current.object_id} #{Thread.current[:x].inspect}"
end

set 'ok'
say

Thread.new do
  say
  set 't1'
  say
end.join

class ContextCopyingThread < Thread
  def initialize
    self[:x] = Thread.current[:x].dup
    super
  end
end

t2 = ContextCopyingThread.new do
  say
  set 't2'
  say
end

puts 'waiting...'

t2.join