Unsafe Ruby vars

output.sh

> ruby -v
ruby 2.1.1p76 (2014-02-24 revision 45161) [x86_64-darwin13.0]
> ruby test.rb
Run options: --seed 22679

# Running:

F.

Finished in 2.320485s, 0.8619 runs/s, 172.3778 assertions/s.

  1) Failure:
brute forcing#test_0001_breaks without synchronisation [tmp/test.rb:36]:
Expected: "1"
  Actual: "84"

2 runs, 400 assertions, 1 failures, 0 errors, 0 skips

test.rb

require 'minitest/autorun'


module Foo
  class << self
    attr_accessor :value
  end

  self.value = 'default'
end

def manipulate(new_value)
  previous = Foo.value
  begin
    Foo.value = new_value
    sleep 0.01 # pretend that we do something here, so that Foo.value may change
    return Foo.value # which should be 'new_vaue' on MRI and God knows what elsewhere
  ensure
    Foo.value = previous
  end
end


SYNC = Mutex.new

def safe_manipulate(new_value)
  SYNC.synchronize { manipulate(new_value) }
end


describe "brute forcing" do

  def run_test(&block)
    threads = (1..200).map do |n1|
      Thread.new do
        assert_equal n1.to_s, block.call(n1.to_s)
      end
    end
    threads.each(&:join)
  end


  it "breaks without synchronisation" do
    run_test { |val| manipulate val }
  end

  it "works fine with synchronisation" do
    run_test { |val| safe_manipulate val }
  end

end