rails just *cannot* run in threads. doing even simple things will bork it in strange and magical ways

a.rb

class A
  attr_accessor :dsl
  attr_accessor :state

  def initialize
    @state = []

    @dsl = self.class.dsl

    sleep(rand)

    instance_eval(&@dsl) if @dsl

    sleep(rand)
  end

  def A.dsl(&block)
    if block
      @dsl = block
    end

    sleep(rand)

    defined?(@dsl) && @dsl
  end
end

b.rb

class B < A
  B.dsl do
    @state.push Thread.current
  end
end

c.rb

threads =
  []

5.times do
  threads <<
    Thread.new do
      Thread.current.abort_on_exception = true

      (sleep(rand) && load('a.rb')) unless defined?(A) # initial rails autoload of base class
      (sleep(rand) && load('b.rb')) unless defined?(B) # subsequent app load of a needed subclass of base

      42.times do
        b = B.new
        raise 'borked' if b.state != [Thread.current]
      end
    end
end

threads.map{|t| t.join}

__END__

run this via and watch it bork:

  ~> while true;do ruby c.rb || say 'borked' && echo 'borked'; done

  borked
  borked
  borked
  borked
  borked
  borked
  ...

couple of notes:

 - ruby's own require / load are NOT thread safe
 - yet rails makes an attempt to implement an autoload that will be on top if it
 - it ain't.  well, it might be, but to find out you'd have to just write code
 with 'A.foo' vs 'A.foo if defined?(A)', meaning you'd have to avoid all
 'defined?' tests and trigger the `const_missing` handler rails uses to
 attempt to work around mt loading issues.  theoretically possible but
 *extremely* fragile.

      
ref: