From: Paul Brannan <pbrannan@...>
Date: 2007-09-04T22:20:03+09:00
Subject: Re: Resource Acquisition & Cleanup

I used to use an idiom like this:

class Rollback
  attr_reader :procs

  def initialize
    @procs = []
  end

  def undo(&b)
    @procs << b
  end

  def self.rollback
    rb = self.new
    begin
      yield rb
    rescue Exception
      rb.procs.reverse.each do |p|
        p.call rescue Exception
      end
      raise
    end
  end
end

Rollback.rollback do |rb|
  rb.undo { puts "1" }
  rb.undo { puts "2" }
  rb.undo { puts "3" }
  raise "FOO"
end

# => 3
# => 2
# => 1
# => RuntimeError: FOO

Looking back I think I could/should have picked a better name, but the
idea is that after you allocate each resource, call rb.undo to set up a
rollback procedure if an exception is raised later on.

I think I posted this somewhere to ruby-talk a few years ago, but I
couldn't find the post.

Paul