Reset bundler settings after locating Gemfile

bin/bundle

 #!/usr/bin/env ruby
 
-require 'bundler'
-
-ENV['BUNDLE_GEMFILE'] ||=
-  Bundler.settings[:gemfile] || File.expand_path('../Gemfile', __dir__)
+require_relative '../config/bundler_setup'
 
 load Gem.bin_path('bundler', 'bundle')

config/boot.rb

 # frozen_string_literal: true
 
-require 'bundler'
-
-ENV['BUNDLE_GEMFILE'] ||=
-  Bundler.settings[:gemfile] || File.expand_path('../Gemfile', __dir__)
-
-# Set up gems listed in the Gemfile.
-require 'bundler/setup' if File.exist?(ENV['BUNDLE_GEMFILE'])
+require_relative 'bundler_setup'
 require 'bootsnap/setup' if ENV['RAILS_ENV'] != 'production' || %w(1 yes true).include?(ENV['ENABLE_BOOTSNAP'])

config/bundler_setup.rb

 # frozen_string_literal: true
 
+# Instead of requiring 'bundle/setup' directly, we need the following
+# to make bundler more consistent when it comes to loading from
+# bundler config. See the following links for more context:
+# https://gitlab.com/gitlab-org/gitlab/-/issues/339939
+# https://github.com/rubygems/rubygems/pull/4892
+# https://github.com/rubygems/rubygems/issues/3363
 require 'bundler'
-ENV['BUNDLE_GEMFILE'] ||= Bundler.settings[:gemfile]
+ENV['BUNDLE_GEMFILE'] ||= Bundler.settings[:gemfile] || File.expand_path('../Gemfile', __dir__)
+Bundler::SharedHelpers.set_env('BUNDLE_GEMFILE', ENV['BUNDLE_GEMFILE'])
+
+if Bundler.respond_to?(:reset_settings_and_root!)
+  Bundler.reset_settings_and_root!
+else
+  # Bundler 2.1.4 does not have this method. Do the same as Bundler 2.2.26
+  # https://github.com/rubygems/rubygems/blob/bundler-v2.2.26/bundler/lib/bundler.rb#L612-L615
+  Bundler.instance_eval do
+    @settings = nil
+    @root = nil
+  end
+end
+
 require 'bundler/setup'