Fix a RactorLocalSingleton lazy mutex race

[havenwood]

RactorLocalSingleton lazily creates the mutex that guards instance creation, but that lazy creation isn't itself guarded. Multiple threads can each see nil, create their own mutex and synchronize on different locks.

This switches to using Ractor.store_if_absent, which makes the separate mutex and double-checked locking unnecessary. Ractor.store_if_absent had not been introduced yet when this code was initially written.

Reproduction:

require "singleton"

class Single
  include RactorLocalSingleton

  def initialize
    count = Ractor.current[:instances] || 0
    Ractor.current[:instances] = count.succ
    sleep 0.1
  end

  class << self
    private

    def set_mutex(v)
      sleep 0.01
      super
    end
  end
end

count = Ractor.new do
  Array.new(10) { Thread.new { Single.instance } }.each(&:join)
  Ractor.current[:instances]
end

pp count.value

Before patch:

10

After patch:

1