cache bug example

thing_controller.rb

require 'dalli'

class ThingController
  def create
    thing = Thing.new(params)
    thing.special_id = dalli.increment(:special_id, 1)
    thing.save!
  end
end

### Weird constraints...
# Because of some historical decisions
# the special id couldn't go below some really large number...
# something like 100,600
# 
# We started seeing bugs where `special_id` would become corrupt.
# We'd get varying unicode characters instead of numbers.
# We tracked down the bug for several days without being able to
# reproduce it locally.
#
# Why did we not use a more reliable counter? Say,
# one provided by our database? Hell if I know...
# It was a decision made so long ago no one was left
# who understood why it was made.

thing_controller_test.rb

require "spec_helper"
require "dalli"
describe "things with special ids" do

  # We had some test like this already
  describe "incrementing special_id" do
    it "should get incremented by 1" do
      n = 100600
      dalli.set(:special_id, n)
      # post to our controller with valid params
      expect { dalli.get(special_id) }.to eq(n + 1)
    end
  end
  
  # I added this generative test so that in CI we'd get
  # 1000 runs of the test every time
  # This seems like overkill in this example... but in
  # the real life version there was a reason it was preferable
  # to doing something like `1000.times do`. I no longer remember it.
  # It definitely did something more than just run the test repeatedly,
  # but I can't remember what the extra thing involved was here.
  generative do
    @n = 100600
    describe "incrementing special_id" do
      it "should get incremented by 1" do
        # post to our controller with valid params
        @n += 1
        expect { dalli.get(special_id) }.to eq(@n)
      end 
    end
  end
  
  # This worked... but it was *terribly* inefficient. It added
  # An extra 30 seconds to a minute to our build by itself.
  # So we really didn't want to leave it in but we did want
  # to find the bug semi-passively if possible while we focused
  # on other urgent things. So I wrote a test like this to "expire"
  # the other test.
  describe "temporary generative test" do
    it "should get removed on January 15th" do
      if Date.today > Date.new(2014,1,15)
        fail "Please remove the generative test in this file"
      end
    end
  end
end