Don't rebuild the middleware stack when using with_routing

[Edouard-chin]

Motivation / Background

Fix #54595

Detail

Problem

When using the with_routing helper method in an integration test, we rebuild the middleware stack so that the entrypoint of our new stack is the new RouteSet object.

By rebuilding the middleware stack from scratch, we may end un-configuring a middleware from an application. We also break the contract of the Rails initialization process.

For instance, if you have a middleware that needs to be configured, after the Rails routes are loaded, this is what it would look like:

# config/application.rb

warden_config = nil

config.middleware.use Warden::Manager do |config|
  # The Warden middleware yields its config when the middleware is
  # instantiated. We can't configure warden yet as the routes
  # are not yet loaded.

  warden_config = config
end

initializer :configure_warden do
  ActiveSupport.on_load(:after_routes_loaded) do
    # The routes are now loaded, we can configure warden
  end
end

This snippet follows the order of the documented Rails initialization process where first the middleware stack is built and then the routes gets loaded.

---

By rebuilding the middleware stack, we unconfigure what was done in the initializer.

Solution

Since rebuilding the middleware stack has some caveats, my idea is to modify the middleware stack original entry point and redefine call so that it calls our new RouteSet middleware.
We have a reference to the original RouteSet middleware thanks to the application.routes method.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[byroot]

I'll try to review this tomorrow morning, it's a bit too hairy for tonight.

[quixoten]

Verified my test suite passes against this branch (and devise 4.9.4)

[Edouard-chin]

Thanks for reviewing! Also thank you @quixoten for helping debug the issue !