test-threads.rb

class ThreadTest

  def initialize
    # kicks off the thread, and only afterwards assigns thread to @t
    @t = Thread.new { joiner }
  end

  def joiner
    # without this sleep, will sometimes crash, 
    # as @t is not yet the thread, so will try to call @t.join on nil
    # sleep 0.5
    if @t == Thread.current
      puts "I'm the same thread!"
    else
      @t.join
      puts "that's not me, I'm joining!"
    end
  end
  
end


test = ThreadTest.new
result = test.joiner
# output with sleep at line 11:
# I'm the same thread!
# that's not me, I'm joining!
# 
# output without sleep at line 11 is sometimes the above, and sometimes the following:
# test-threads.rb:15:in `joiner': undefined method `join' for nil:NilClass (NoMethodError)
#
# Happens in 1.8.7, 1.9.1, JRuby