my_obj_spec.rb

class MyObj

  def now(&callback)
    return callback.call @my_ivar if @my_ivar
    Dispatch::Queue.new("a").async do
      sleep 0.1 # simulate an http response
      Dispatch::Queue.main.sync do
        @my_ivar = { result: "real" }
        callback.call @my_ivar
      end
    end
  end

end

describe "Application 'rm_mocking_issue'" do
  before do
    @my_obj = MyObj.new

    # Removing this line allows the spec to pass.
    # Adding it causes the spec to hang.
    @my_obj.instance_variable_set("@my_ivar", { result: "fake" })
  end

  it "hangs" do
    @my_obj.now do |response|
      response.should.be.kind_of(Hash)
      resume
    end
    wait {}
  end

end