Deprecate mutating the default_url_options:

[Edouard-chin]

• Context

  This patch is about the default_url_options used to tweak the generated urls when calling for instance my_route_path inside controllers/views. There is another default_url_options at the RouteSet but this patch is not about this one.

  Problem

  Mutating the default_url_options is nowhere documented and the recommended way to customize generated urls is to override the default_url_options method and override it.

  The issue being that the Rails tests suite uses that pattern, and while I don't know whether this is also done in the wild, I picked the precautious path.

  Mutating the default_url_options can create super weird behaviour:

  class ApplicationController
    def hello_world
      redirect_to(root_path)
    end
  end
  
  class MyController < ApplicationController
    default_url_options[:lang] = "en"
  end

  In a autoloaded context, if a user hit the /hello_world path it will get redirected to /. Once the MyController is autoloaded , revisiting /hello_world will this time redirect to /?lang=en.

  Solution

  This patch freeze the default_url_options by default but still allows the hash to be mutated and trigger a deprecation message.

  This is done using a proxy which rescue from FrozenError anytime a mutation happens and duplicate the underlying hash.