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Testing Thread Safety in IRB


(Justin Leveck) #1

Hello I wrote this solution to a practice problem. I have received some feedback mentioning that it is not thread safe. Is there a way to test for this in IRB?

“If you spin up several threads and have the same Beer Song sing
different sets of verses at the same time, they’ll get in each-other’s
way. Same thing with a Singer.”

class Beer
  attr_reader :start_verse, :end_verse
 
  def sing(start_verse, end_verse = 0)
    @start_verse = start_verse
    @end_verse = end_verse
 
    requested_verses
  end
 
  def verse(verse_number)
    singer = Singer.new
    singer.recall_verse(verse_number)
  end
 
  private
 
  def requested_verses
    requested_verse_numbers = start_verse.downto(end_verse)
    requested_verses = ""
 
    requested_verse_numbers.each do |requested_verse_number|
      requested_verses << verse(requested_verse_number) + "\n"
    end
    requested_verses
  end
 
end
 
class Singer
  attr_accessor :bottles_of_beer, :verse_number
 
  def initialize
    @bottles_of_beer = nil
  end
 
  def recall_verse(verse_number)
    @bottles_of_beer = verse_number
    "#{dynamic_phrase_1} of beer on the wall, #{dynamic_phrase_2} of beer.\n#{dynamic_phrase_3}, #{dynamic_phrase_4} of beer on the wall.\n"
  end
 
  private
 
   def dynamic_phrase_1
    if zero_bottles?
      "No more bottles"
    else
      "#{bottles_of_beer} #{bottle_s}"
    end
  end
 
  def dynamic_phrase_2
    if zero_bottles?
      "no more #{bottle_s}"
    else
      "#{bottles_of_beer} #{bottle_s}"
    end
  end
 
  def dynamic_phrase_3
    one_it = 'one'
 
    if zero_bottles?
      'Go to the store and buy some more'
    else
      if one_bottle?
        one_it = 'it'
      end
      "Take #{one_it} down and pass it around"
    end
  end
 
  def dynamic_phrase_4
    if one_bottle?
      "no more bottles"
    elsif zero_bottles?
      "99 #{bottle_s}"
    else
      @bottles_of_beer -= 1
      "#{bottles_of_beer} #{bottle_s}"
    end
  end
 
  # This returns the correct plural or singular usage of bottle
  def bottle_s
    if bottles_of_beer > 1 || bottles_of_beer == 0
      'bottles'
    elsif one_bottle?
      'bottle'
    end
  end
 
  def one_bottle?
    bottles_of_beer == 1
  end
 
  def two_bottles?
    bottles_of_beer == 2
  end
 
  def zero_bottles?
    bottles_of_beer == 0
  end
 
end

(George Brocklehurst) #3

Most threading problems happen when Ruby switches threads at an inconvenient moment, leaving things in an inconsistent state. In your case, switching thread in the middle of any of the dynamic_phrase methods to another thread that called recall_verse on the same Singer would cause problems: When Ruby switched back to the original thread the value of @bottles_of_beer would have changed and the rest of the verse would be inconsistent.

The reason threading bugs are so tricky is that they are unpredictable. There are many different ways to interleave the code between multiple threads, and most of them will work just fine. This makes it very difficult to write a simple test (in IRB or otherwise) to catch them: You could run the code multiple times and never see the bug.

You might find this article by Jessie Storimer helpful in understanding the potential problems: Does the GIL Make Your Ruby Code Thread-Safe?

I know it doesn’t answer your question directly, but I hope it’s helpful all the same.


(Justin Leveck) #4

Thanks for the reply. This article is very interesting.