Example utility for doing synchronized updates of instance variables.

test.rb

module Atomically
  GLOBAL_MUTEX = Mutex.new
  def atomically(varname, &block)
    # check first to avoid locking if possible
    instance_variable_get(varname) || ___atomically_update___(varname, &block)
  end
  
  def ___atomically_update___(varname)
    # lock and do ||= update
    ___mutex___.synchronize do
      instance_variable_get(varname) || instance_variable_set(varname, yield)
    end
  end

  def ___mutex___
    # check first to avoid locking if possible
    @___mutex___ || ___mutex_init___
  end

  def ___mutex_init___
    # global lock to instantiate mutex for this object...kinda heavy
    GLOBAL_MUTEX.synchronize do
      @___mutex___ ||= Mutex.new
    end
  end
end

class A
  include Atomically
  attr_reader :a
  @@lock = Mutex.new
  @@incrementer = 0
 
  def a
    sleep 1
    atomically(:@a) { @@lock.synchronize{@@incrementer += 1} }
  end
 
  def incrementer
    @@incrementer
  end
end
 
a = A.new
 
100.times.map do
  Thread.start{a.a}
end.each &:join
 
100.times.map do
  Thread.start{A.new.a}
end.each &:join
 
p a.a, a.incrementer, A.new.a, a.incrementer