Appending to arrays is not thread-safe in Ruby.

array-is-not-thread-safe.rb

array = []

5.times.map do
  Thread.new do
    1000.times do
      array << nil
    end
  end
end.each(&:join)

puts array.size

$ ruby pushing_nil.rb
5000

$ jruby pushing_nil.rb
4446

$ rbx pushing_nil.rb
3088

MRI implementation works as expected because of the GIL (internal mutex).

Making this code atomic to ensure that it can't be interrupted until it's finished. The simplest way to make an operation like this atomic is to use a lock.

array = []
mutex = Mutex.new

5.times.map do
  Thread.new do

    mutex.synchronize do
      1000.times do
        array << nil
      end
    end

  end
end.each(&:join)

puts array.size