Preserve Bundle configuration during app generation (#34755)

When generating a new rails application (rails new) using a custom template that
includes gems from an authenticated source, the user has to provide credentials to
bundler.

One way to do this is by exporting environment variables, for example:

export BUNDLE_GITHUB__COM=user:pass: provides credentials for bundler to fetch
gems from github.com.

The problem this PR addresses is that we are currently scrubs all /BUNDLE_.*/
environment variables by wrapping our system calls in Bundler.with_clean_env.

We do this because we don't want our commands executed against the generated project
to use the generator's bundler environment (e.g. our gems): the generated project should
use it's own configuration.

The problem with Bundler.with_clean_env is that, on top of restoring environment
variables to their original state, it also scrubs any /BUNDLE_.*/ variables, which is harmful for authenticated gem sources.

This PR replaces Bundler.with_clean_env with Bundler.with_original_env, which only
restores environment variables to their initial state, without additional scrubbing.

railties/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Use original `bundler` environment variables during the process of generating a new rails project.
+
+    *Marco Costa*
+
 *   Send Active Storage analysis and purge jobs to dedicated queues by default.
 
     Analysis jobs now use the `:active_storage_analysis` queue, and purge jobs

railties/lib/rails/generators/app_base.rb

@@ -388,19 +388,21 @@ module Rails
         # its own vendored Thor, which could be a different version. Running both
         # things in the same process is a recipe for a night with paracetamol.
         #
-        # We unset temporary bundler variables to load proper bundler and Gemfile.
-        #
         # Thanks to James Tucker for the Gem tricks involved in this call.
         _bundle_command = Gem.bin_path("bundler", "bundle")
 
         require "bundler"
-        Bundler.with_clean_env do
-          full_command = %Q["#{Gem.ruby}" "#{_bundle_command}" #{command}]
-          if options[:quiet]
-            system(env, full_command, out: File::NULL)
-          else
-            system(env, full_command)
-          end
+        Bundler.with_original_env do
+          exec_bundle_command(_bundle_command, command, env)
+        end
+      end
+
+      def exec_bundle_command(bundle_command, command, env)
+        full_command = %Q["#{Gem.ruby}" "#{bundle_command}" #{command}]
+        if options[:quiet]
+          system(env, full_command, out: File::NULL)
+        else
+          system(env, full_command)
         end
       end
 

railties/test/generators/app_generator_test.rb

@@ -773,6 +773,24 @@ class AppGeneratorTest < Rails::Generators::TestCase
     assert_bundler_command_called("install")
   end
 
+  def test_generation_use_original_bundle_environment
+    generator([destination_root], skip_webpack_install: true)
+
+    mock_original_env = -> do
+      { "BUNDLE_RUBYONRAILS__ORG" => "user:pass" }
+    end
+
+    ensure_environment_is_set = -> *_args do
+      assert_equal "user:pass", ENV["BUNDLE_RUBYONRAILS__ORG"]
+    end
+
+    Bundler.stub :original_env, mock_original_env do
+      generator.stub :exec_bundle_command, ensure_environment_is_set do
+        quietly { generator.invoke_all }
+      end
+    end
+  end
+
   def test_dev_option
     generator([destination_root], dev: true, skip_webpack_install: true)