How to monkeypatch "GC.start"

Question

I want to do some experiments which involve hooking into invocations of GC.start.

Ruby tells me GC is not a class when I run this:

class GC
  def self.start
    puts "hello"
    super
  end
end

But running this, Ruby tells me GC.start has no superclass, so I think I'm not actually hooking into the original one, but just somehow taking over that name:

module GC
  def self.start
    puts "hello"
    super
  end
end
GC.start

How can I monkeypatch GC.start?

Answer 1

Let's first redefine GC::start so we can see when it is invoked.

module GC
  def self.start(full_mark: true, immediate_sweep: true)
    puts "old start, full_mark: #{full_mark}, " +
      "immediate_sweep: #{immediate_sweep}"
  end
end  

Here are two ways to obtain the desired result.

1. Use Module#prepend from within GC's singleton class

module X 
  def start(full_mark: true, immediate_sweep: true)
    puts "new start, full_mark: #{full_mark}, " +
      "immediate_sweep: #{immediate_sweep}"
    method(__method__).super_method.call(full_mark: full_mark,
      immediate_sweep: immediate_sweep)  
  end
end

module GC
  class << self
    prepend X
  end
end

GC.start(full_mark: 'cat')
new start, full_mark: cat, immediate_sweep: true
old start, full_mark: cat, immediate_sweep: true

Note:

GC.singleton_class.ancestors
  #=> [X, #<Class:GC>, Module, ...] 

Using Module#prepend within GC's singleton class is like GC.extend X except it places X ahead of GC's singleton class among GC's ancestors. See also Method#super_method, Object#method, Kernel#__method__ and Method#call.

Observe also that:

GC.singleton_class.public_send(:prepend, X)

can be used in place of:

module GC
  class << self
    prepend X
  end
end

2. Use aliasing

module GC
  class << self
    alias old_start start
  end

  def self.start(full_mark: true, immediate_sweep: true)
    puts "new start, full_mark: #{full_mark}, " +
      "immediate_sweep: #{immediate_sweep}"
    old_start(full_mark: full_mark, immediate_sweep: immediate_sweep)
  end
end

GC.start(full_mark: 'cat')
new start, full_mark: cat, immediate_sweep: true
old start, full_mark: cat, immediate_sweep: true

Aliasing was commonly used before Module#prepend made its debut in Ruby v2.0.