Spec involving Mutex passes in 3.8 but fails in 3.9

[johann-koebbe-vericity]

Subject of the issue

This comes from a project that hasn't been updated in a few years, but I was recently bringing into the modern day. A spec involving the use of Mutex.new passes in 3.8 but fails in 3.9. I believe it is related to the change in rspec-mocks 3.9 that improves thread safety by using Mutex.

Your environment

• Ruby version: ruby 2.7.0p0 (currently, but experienced in 2.5 and 2.6 as well)
• rspec-mocks version: 3.8.2 (working), 3.9.1 (failing)

Steps to reproduce

class Mootex
  def mootex
    @mootex ||= Mutex.new
  end
end

RSpec.describe Mootex do
  describe 'memoization' do
    subject { described_class.new }

    it 'creates a mootex when one is not available' do
      subject.instance_variable_set('@mootex', nil)
      expect(Mutex).to receive(:new)
      subject.mootex
    end

    it 'does not create a mootex when one is available' do
      subject.instance_variable_set('@mootex', 'mootex')
      expect(Mutex).not_to receive(:new)
      subject.mootex
    end
  end
end

Expected behavior

Both specs should pass.

Actual behavior

  1) Mootex memoization creates a mootex when one is not available
     Failure/Error: @mootex ||= Mutex.new

     NoMethodError:
       undefined method `synchronize' for #<Object:0x0000000002376430>

[pirj]

I guess there's some overlap with Support::Mutex.new, but making expectations on ::Mutex doesn't change the situation. 🤔

[JonRowe]

This happens because we use Mutex when it is available to make mocks threadsafe, there is no real Support::Mutex except on 1.8.7, but we've done this by simply assigning the class to the constant.

See rspec/rspec-support#411

[JonRowe]

Closing here now, as the fix is over in rspec-support, however as a work around you could add and_call_original to generate mutexes.

[johann-koebbe-vericity]

@JonRowe Thanks for the quick reply/fix! I tried

    it 'creates a mootex when one is not available' do
      subject.instance_variable_set('@mootex', nil)
      expect(Mutex).to receive(:new).and_call_original
      subject.mootex
    end

but got the same error. It's not a big deal, though, since you already have a fix in place. I'll just leave the project pinned at 3.8.x until the fix hits the wild.

Many thanks!