Changing Rails consider_all_requests_local in RSpec fails

As many others I've been trying to change Rails.application.config.consider_all_requests_local and Rails.application.config.action_dispatch.show_exceptions inside my RSpec tests in order to test custom error pages in a Rails app. However this doesn't work. My code looked like this

feature 'Exceptions' do
  before do
    Rails.application.config.action_dispatch.show_exceptions = true
    Rails.application.config.consider_all_requests_local = false
  end

This works only if I execute exceptions_spec.rb alone. However when something else executes before that it fails. The config values are correctly updated but that doesn't seem to have effect.

The answer and solution comes from Henrik N.

action_dispatch.show_exceptions gets copied and cached in Rails.application.env_config, so even if you change Rails.application.config.action_dispatch.show_exceptions in this before block the value isn't what you expect when it's used in ActionDispatch::DebugExceptions.

In fact DebugExceptions uses env['action_dispatch.show_exceptions']. The correct code should look like this

before do
  method = Rails.application.method(:env_config)
  expect(Rails.application).to receive(:env_config).with(no_args) do
    method.call.merge(
      'action_dispatch.show_exceptions' => true,
      'action_dispatch.show_detailed_exceptions' => false,
      'consider_all_requests_local' => false
    )
  end
end

This allows the test to work regardless of the order of execution of spec files. I don't know why but I also had to leave show_detailed_exceptions otherwise I wasn't getting the desired results.