[ruby/rubygems] Respect BUNDLE_VERSION config at Gem::BundlerVersionFinder

If we use "system" variable in BUNDLE_VERSION on Bundler configuration,
we can use bundler version provided by system installation.

But the current logic returns the first activated version of bundler
like 2.7.2. It makes to confuse users.

ruby/rubygems@4eb66d9549

Author: hsbt

lib/rubygems/bundler_version_finder.rb

@@ -2,6 +2,8 @@
 
 module Gem::BundlerVersionFinder
   def self.bundler_version
+    return if bundle_config_version == "system"
+
     v = ENV["BUNDLER_VERSION"]
     v = nil if v&.empty?
 
@@ -78,4 +80,33 @@ def self.lockfile_contents
     File.read(lockfile)
   end
   private_class_method :lockfile_contents
+
+  def self.bundle_config_version
+    config_file = bundler_config_file
+    return unless config_file && File.file?(config_file)
+
+    contents = File.read(config_file)
+    contents =~ /^BUNDLE_VERSION:\s*["']?([^"'\s]+)["']?\s*$/
+
+    $1
+  end
+  private_class_method :bundle_config_version
+
+  def self.bundler_config_file
+    # see Bundler::Settings#global_config_file and local_config_file
+    # global
+    if ENV["BUNDLE_CONFIG"] && !ENV["BUNDLE_CONFIG"].empty?
+      ENV["BUNDLE_CONFIG"]
+    elsif ENV["BUNDLE_USER_CONFIG"] && !ENV["BUNDLE_USER_CONFIG"].empty?
+      ENV["BUNDLE_USER_CONFIG"]
+    elsif ENV["BUNDLE_USER_HOME"] && !ENV["BUNDLE_USER_HOME"].empty?
+      ENV["BUNDLE_USER_HOME"] + "config"
+    elsif Gem.user_home && !Gem.user_home.empty?
+      Gem.user_home + ".bundle/config"
+    else
+      # local
+      "config"
+    end
+  end
+  private_class_method :bundler_config_file
 end

test/rubygems/helper.rb

@@ -335,6 +335,9 @@ def setup
     ENV["XDG_STATE_HOME"] = nil
     ENV["SOURCE_DATE_EPOCH"] = nil
     ENV["BUNDLER_VERSION"] = nil
+    ENV["BUNDLE_CONFIG"] = nil
+    ENV["BUNDLE_USER_CONFIG"] = nil
+    ENV["BUNDLE_USER_HOME"] = nil
     ENV["RUBYGEMS_PREVENT_UPDATE_SUGGESTION"] = "true"
 
     @current_dir = Dir.pwd

test/rubygems/test_gem_bundler_version_finder.rb

@@ -2,6 +2,7 @@
 
 require_relative "helper"
 require "rubygems/bundler_version_finder"
+require "tempfile"
 
 class TestGemBundlerVersionFinder < Gem::TestCase
   def setup
@@ -56,6 +57,75 @@ def test_bundler_version_with_bundle_update_bundler
     assert_nil bvf.bundler_version
   end
 
+  def test_bundler_version_with_bundle_config
+    config_content = <<~CONFIG
+      BUNDLE_VERSION: "system"
+    CONFIG
+
+    Tempfile.create("bundle_config") do |f|
+      f.write(config_content)
+      f.flush
+
+      bvf.stub(:bundler_config_file, f.path) do
+        assert_nil bvf.bundler_version
+      end
+    end
+  end
+
+  def test_bundler_version_with_bundle_config_single_quoted
+    config_with_single_quoted_version = <<~CONFIG
+      BUNDLE_VERSION: 'system'
+    CONFIG
+
+    Tempfile.create("bundle_config") do |f|
+      f.write(config_with_single_quoted_version)
+      f.flush
+
+      bvf.stub(:bundler_config_file, f.path) do
+        assert_nil bvf.bundler_version
+      end
+    end
+  end
+
+  def test_bundler_version_with_bundle_config_version
+    ENV["BUNDLER_VERSION"] = "1.1.1.1"
+
+    config_content = <<~CONFIG
+      BUNDLE_VERSION: "1.2.3"
+    CONFIG
+
+    Tempfile.create("bundle_config") do |f|
+      f.write(config_content)
+      f.flush
+
+      bvf.stub(:bundler_config_file, f.path) do
+        assert_equal v("1.1.1.1"), bvf.bundler_version
+      end
+    end
+  end
+
+  def test_bundler_version_with_bundle_config_non_existent_file
+    bvf.stub(:bundler_config_file, "/non/existent/path") do
+      assert_nil bvf.bundler_version
+    end
+  end
+
+  def test_bundler_version_with_bundle_config_without_version
+    config_without_version = <<~CONFIG
+      BUNDLE_JOBS: "8"
+      BUNDLE_GEM__TEST: "minitest"
+    CONFIG
+
+    Tempfile.create("bundle_config") do |f|
+      f.write(config_without_version)
+      f.flush
+
+      bvf.stub(:bundler_config_file, f.path) do
+        assert_nil bvf.bundler_version
+      end
+    end
+  end
+
   def test_bundler_version_with_lockfile
     bvf.stub(:lockfile_contents, "") do
       assert_nil bvf.bundler_version