Using Rails' Mounted route fails if it has a dot in path (at first part)

Steps to reproduce

1. Create a rails project.
2. Add grape as a dependency and install
3. Open routes.rb and add following:

# frozen_string_literal: true

class DomainConstraint
  def initialize(domain)
    @domain = domain.downcase
  end

  def matches?(request)
    Rails.logger.debug "Request for host: #{request.host}"
    @domain == request.host.downcase
  end
end

Rails.application.routes.draw do
  constraints DomainConstraint.new(domain) do
    mount API => '/' # this is grape API mounted in Rails via '/' path.
  end
end

Expected behavior

GET http://example.com/gencer/test
GET http://example.com/.gencer/test

Those two request should be passed to grape because I've mounted base path to Grape as '/'.

Actual behavior

Only

GET http://example.com/gencer/test

successfully passed to grape.

GET http://example.com/.gencer/test

throws 404 not found error.

No route matches [GET] "/.gencer/test"

Related Grape code to capture path:

get ':path', requirements: { path: /.*/ } do
   ...
end

Even constraint is not executed if I use dot .. I've added a logging to matches? method. If I use dot in path, log is not present.

The thing is, This was working in the past. Now it isn't. Something has changed but whats changed, that I don't know for sure.

System configuration

Rails version: 5.2.3

Ruby version: 2.6.3

Demo

Clone https://github.com/gencer/mount_test

run bundle install && rails s

Go to

GET http://localhost:3000/gencer/demo

It will show you

{ "success": true }

Now go to:

GET http://localhost:3000/.gencer/demo

You will get 404 / No route matched.

Refer: config/routes.rb

Update

This only happens if base path has a dot in it. For example this will success:

GET http://localhost:3000/demo/.gencer/demo

{ "success": true }

So, If path start with dot (first folder) than route does not match with any.

Reference: https://github.com/rails/rails/issues/36491

I am opening this issue here as well, because, it may be coming from grape and not rails. Just want to make sure.

Start with the mounted route and constraint in config/routes.rb, then run the mount_test reproduction with bundle install && rails s and compare the two documented requests. Trace why the constraint is skipped for a dot-prefixed first path segment, determining whether the behavior belongs to Grape or Rails; done means both paths reach the mounted API without the 404.