ruby-grape-ivars-are-shared-through-mounts.rb

require "bundler/inline"

gemfile do
  source "https://rubygems.org"
  gem "grape"
  gem "webrick"
  gem "rackup"
  gem "builder"
end

require "rackup/handler/webrick"
require "net/http"

$gvar = nil

class Foo < Grape::API
  get '/' do
    @ivar = 1
    {ok: true}
  end
end

class Bar < Grape::API
  before do
    @ivar = 0
  end

  finally do
    $gvar = @ivar
  end

  mount Foo
end

Thread.new do
  Rackup::Handler::WEBrick.run(Bar, Port: 9292)
end

begin
  Net::HTTP.get('localhost', '/', 9292)
rescue => e
  sleep 1
  retry
end

puts "$gvar == 1 #=> #{$gvar == 1}"

I think that the before and finally blocks are shared to Foo by mount, and are evaluated in an instance of Foo.

(Updated)
It is a bit wrong.
finally seems to be evaluated in an anonymous subclass of Grape::Endpoint.