Got an intermittent failure in your tests for a source file? Try this

heisenbug_detector.rb

# Run this inside Pry or irb and let it find a randomisation seed that will
# cause your tests to fail. This assumes that you're running tests for a single
# test file, which you pass in as the argument to `run_tests`.
#
# Why don't we collect a whole bunch of failing seeds? You'll need to run this
# each time you make a material change to the code being run by the tests, or to
# the tests themselves; with that in mind, take one step at a time.
#
# A quick hack, but feedback appreciated.

$used_seeds ||= []

def run_tests(filename)
  seeds = Array.new(2000) { rand(65536) };
  seeds.each do |seed|
    # cmdline = %{ruby -e 'require "#{filename}"' -- --seed #{seed}}
    cmdline = 'bin/rake test'
    unless system(cmdline)
      puts "Failed seed was #{seed}"
      break seed
    end
    $used_seeds << seed
  end
end