Thread Safety: Class Variables in Ruby

Question

Performing writes/reads on class variables in Ruby is not thread safe. Performing writes/reads on instance variables appears to be thread safe. That said, is it thread safe

Answer 1

Examples 2 and 3 are exactly the same. They are not thread-safety at all.

Please see the example Below.

class Foo
  def self.bar
    @bar ||= create_no
  end

  def self.create_no
    no = rand(10000)
    sleep 1
    no
  end
end

10.times.map do
  Thread.new do
    puts "bar is #{Foo.bar}"
  end
end.each(&:join)

It's result is not same. The result is same when using mutex as below.

class Foo
  @mutex = Mutex.new

  def self.bar
    @mutex.synchronize {
      @bar ||= create_no
    }
  end

  def self.create_no
    no = rand(10000)
    sleep 1
    no
  end
end

10.times.map do
  Thread.new do
    puts "bar is #{Foo.bar}"
  end
end.each(&:join)

It is run on CRuby 2.3.0.