Minitest running twice when dealing with forks

minitest_twice_fork.rb

require 'minitest/autorun'

class TestSuite
  def self.before_run
    pid = fork do
      puts "on fork"
      exit 0
    end

    Process.detach pid
  end
end

class MyTestCase < Minitest::Test
  TestSuite.before_run
end

class TestTests < MyTestCase
  def test_aa
    puts "aaa"
  end
end

$ ruby minitest_twice_fork.rb
on fork
Run options: --seed 13402Run options: --seed 30729

# Running:



# Running:



Finished in 0.000856s, 0.0000 runs/s, 0.0000 assertions/s.

0 runs, 0 assertions, 0 failures, 0 errors, 0 skips
aaa
.

Finished in 0.001406s, 711.3979 runs/s, 0.0000 assertions/s.

1 runs, 0 assertions, 0 failures, 0 errors, 0 skips

The reason for this duplication is that Minitest uses #at_exit to write results (is the only way to deal with exits inside application); To workaround this, calls to Kernel#exit should be replaced by Kernel#exit!, which ignore callbacks).