[rubygems/rubygems] Fix rubygems update when non default `--install-dir` is configured

https://github.com/rubygems/rubygems/commit/9f3b21192d

lib/rubygems/commands/update_command.rb

@@ -173,10 +173,11 @@ command to remove old versions.
     highest_remote_gem.first
   end
 
-  def install_rubygems(version) # :nodoc:
+  def install_rubygems(spec) # :nodoc:
     args = update_rubygems_arguments
+    version = spec.version
 
-    update_dir = File.join Gem.dir, 'gems', "rubygems-update-#{version}"
+    update_dir = File.join spec.base_dir, 'gems', "rubygems-update-#{version}"
 
     Dir.chdir update_dir do
       say "Installing RubyGems #{version}" unless options[:silent]
@@ -290,9 +291,7 @@ command to remove old versions.
     installed_gems = update_gem('rubygems-update', version) if installed_gems.empty? || installed_gems.first.version != version
     return if installed_gems.empty?
 
-    version = installed_gems.first.version
-
-    install_rubygems version
+    install_rubygems installed_gems.first
   end
 
   def update_rubygems_arguments # :nodoc:

test/rubygems/test_gem_commands_update_command.rb

@@ -191,6 +191,37 @@ class TestGemCommandsUpdateCommand < Gem::TestCase
     assert_empty out
   end
 
+  def test_execute_system_update_installed_in_non_default_gem_path
+    rubygems_update_spec = quick_gem "rubygems-update", 9 do |s|
+      write_file File.join(@tempdir, 'setup.rb')
+
+      s.files += %w[setup.rb]
+    end
+
+    util_setup_spec_fetcher rubygems_update_spec
+
+    rubygems_update_package = Gem::Package.build rubygems_update_spec
+
+    gemhome2 = "#{@gemhome}2"
+
+    Gem::Installer.at(rubygems_update_package, :install_dir => gemhome2).install
+
+    Gem.use_paths @gemhome, [gemhome2, @gemhome]
+
+    @cmd.options[:args]          = []
+    @cmd.options[:system]        = true
+
+    use_ui @ui do
+      @cmd.execute
+    end
+
+    out = @ui.output.split "\n"
+    assert_equal "Installing RubyGems 9", out.shift
+    assert_equal "RubyGems system software updated", out.shift
+
+    assert_empty out
+  end
+
   def test_execute_system_specific
     spec_fetcher do |fetcher|
       fetcher.download 'rubygems-update', 8 do |s|