Sure, monkey patching is great and all. That period of disbelief, followed by increasing exasperation as the victim maintenance programmer discovers that an object is behaving differently than it’s source code says it should, is satisfying. But sooner or later he or she wises up and greps through the codebase, discovers where you re-opened the class in question, and the game is up.

The fact of the matter is that monkeys simply aren’t very stealthy. They are easy to find when you know what you are looking for. When you really want to catch a coder by surprise, a monkey doesn’t cut it. What you need is a Ninja:.

And so, today I’m unleashing the technique of Ninja-Patching, along with a reference implementation. Ninja-Patching is silent, untraceable, precise, unpredictable, and always deadly. And unlike monkey patching, which usually happens at startup, Ninja-Patching happens when you least expect it. Here’s an example use:

  require 'ninja'
  Ninja.hire(Enumerable) do def self.to_s "PWNED by Ninjas!!!" end

That’s all you have to do. A silent assassin has been hired, and will ruthlessly hunt down its target. In this case, the target is the first object found in ObjectSpace which is a kind of Enumerable. Which one? Hard to say. Ninjas are, like I said, unpredictable. Once the target is acquired, the Ninja will wait some random amount of time in order to throw off the trail and instill a false sense of security. Then, without warning, the Ninja will attack! The block given to Ninja.hire will be executed in the context of the target object. And then it’s all over but the crying.

Here’s the implementation:

CODE = <<'END_CODE'
class Ninja
  def self.hire(target_description, &instructions)
    self.new(target_description, instructions)
  end

  private

  # target_description can be either an object which responds to +#===+, or a
  # Proc which returns true or false.
  def initialize(target_description, instructions)
    @target_description = if target_description.kind_of?(Proc) then
                            target_description
                          else
                            lambda {|obj| target_description === obj}
                          end
    @instructions       = instructions
    @target             = acquire_target(@target_description)
    if @target.equal?(self)
      $stderr.puts "Never double-cross a Ninja!"
      exit
    elsif @target
      stalk(@target)
    else
      raise "No such object found!"
    end
  end

  def acquire_target(target_description)
    ObjectSpace.each_object do |object|
      if target_description.call(object)
        return object
      end
    end
    nil
  end

  def stalk(target)
    Thread.new do
      sleep(rand(60))
      attack!(target)
    end
  end

  def attack!(target)
    target.instance_eval(&@instructions)
  end
end
END_CODE

# Using eval conceals the Ninja in the stack trace
eval CODE

This code is released under the Ninja Public License (NPL), which releases me from all liability should ninja.rb turn on you and assasinate you in your Kernel#sleep(). Warning: ninja.rb has a known vulnerability to chosen_one.rb.