Remove the internal_gem Gemfile.lock before the test starts:

[Edouard-chin]

• The test suite of this gem is broken since Bundler 2.5.0 with this Make lockfiles generated on macOS include a lock for Linux by default ruby/rubygems#5700.

  TL;DR is that the Gemfile.lock from the internal_gem doesn't match its Gemfile which now causes problems.

  ---

  When the sqlite3 gem gets stripped from the internal_gem Gemfile, the resulting Gemfile.lock (that gets created when running bundle update), will not contain sqlite3 for any platforms.

  When your test starts and the "internal_app" gets generated, this condition inside the Gemfile

  engine_cart/lib/engine_cart/gemfile_stanza.rb

  Lines 12 to 15 in b89464c

  	file = File.expand_path('Gemfile', ENV['ENGINE_CART_DESTINATION'] || ENV['RAILS_ROOT'] || File.expand_path('#{EngineCart.destination}', File.dirname(__FILE__)))
  	if File.exist?(file)
  	begin
  	eval_gemfile file

  gets evaluated and Bundler tries to evaluates the Gemfile of the app.

  We are now in a situation where the Gemfile of the internal gem doesn't match at all what's in the Gemfile.lock (since at the time the Gemfile.lock was generated, the eval_gemfile didn't get evaluated since no internal_app existed).

  With the change introduced in #5700, the initial generation of the Gemfile.lock adds all linux platforms (this is in the case your Gemfile includes a gem that comes with precompiled binaries like nokogiri). Now, when the Gemfile.lock contains a PLATFORM section, it checks whether all gem in the Gemfile are locked for those specific platforms.

[jrochkind]

Thank you very much @Edouard-chin , kind of you to send, with explanation.