Simple fork-safe proxy for Ruby

fork_safe_proxy.rb

class ForkSafeProxy < BasicObject
  class << self
    def unwrap(instance)
      instance.__send__(:__value__)
    end
  end

  def initialize(&initializer)
    @mutex = ::Thread::Mutex.new
    @initializer = initializer
    @value = nil
    @pid = nil
  end

  def method_missing(method, *args, &block)
    __value__.__send__(method, *args, &block)
  end

  private
  
  def __value__
    return @value if @pid == $$
    @mutex.synchronize do
      @value = @initializer.call
      @pid = $$
      $stdout.puts "# initialized: #{$$}"
      @value
    end
  end
end

class Runner
  def initialize
    @rand = ForkSafeProxy.new { Random.new }
    @rand = ForkSafeProxy.unwrap(@rand) if ENV['UNWRAP'] == '1'
  end

  def run
    puts_rand
    child_pids = 3.times.map { fork { puts_rand } }
    child_pids.each(&Process.method(:waitpid))
  end

  def puts_rand
    3.times { puts "#{$$}: #{@rand.rand(100)}" }
  end
end

Runner.new.run

=begin
$ ruby fork_safe_proxy.rb
# initialized: 11488
11488: 75
11488: 50
11488: 22
# initialized: 11517
11517: 55
11517: 77
11517: 49
# initialized: 11518
11518: 20
11518: 65
11518: 24
# initialized: 11519
11519: 90
11519: 96
11519: 67
=end

=begin
$ UNWRAP=1 ruby fork_safe_proxy.rb
# initialized: 12750
12750: 62
12750: 22
12750: 51
12779: 51
12779: 92
12779: 70
12780: 51
12780: 92
12780: 70
12781: 51
12781: 92
12781: 70
=end