class MemCache

  #
  # Try to acquire a global lock from memcached for a particular key. 
  # If successful, yield and set the key to the return value, then release
  # the lock. 
  #
  # Based on http://rubyurl.com/Sw7 , which I partially wrote.
  #

  def lock(key, lock_expiry = 30, retries = 5)
    retries.times do |count|
      response = CACHE.add("lock:#{key}", "Locked by #{Process.pid}", lock_expiry)
      if response == "STORED\r\n"
        begin
          value = yield(CACHE.get(key))
          CACHE.set(key, value)
          return value
        ensure 
          CACHE.delete("lock:#{key}")
        end
      else
        sleep((2**count) / 2.0)
      end
    end
    raise MemCacheError, "Couldn't acquire lock for #{key}"
  end
  
end