Clean up the compact index tmpdir when the cache is not writable

When the gem home is not writable, compact_index_cache_dir falls back
to Dir.mktmpdir but never removed it, leaking a directory under the
system temp on every gem command. Remove it at process exit.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hsbt

lib/rubygems/source.rb

@@ -299,7 +299,10 @@ def compact_index_cache_dir(index_uri)
         "#{index_uri.host}%#{index_uri.port}", *escaped_path.split("/").reject(&:empty?)
     else
       require "tmpdir"
-      Dir.mktmpdir "gem_compact_index"
+      require "fileutils"
+      dir = Dir.mktmpdir "gem_compact_index"
+      at_exit { FileUtils.rm_rf dir }
+      dir
     end
   end
 

test/rubygems/test_gem_source.rb

@@ -195,6 +195,24 @@ def test_load_specs_compact_index_skips_invalid_version
     assert_equal %w[a-1], released
   end
 
+  def test_compact_index_cache_dir_removes_tmpdir_at_exit
+    source = Gem::Source.new @gem_repo
+    def source.update_cache?
+      false
+    end
+
+    cleanup = nil
+    source.define_singleton_method(:at_exit) {|&block| cleanup = block }
+
+    dir = source.send(:compact_index_cache_dir, Gem::URI(@gem_repo))
+
+    assert_path_exist dir
+    refute_nil cleanup, "expected a cleanup hook to be registered"
+
+    cleanup.call
+    assert_path_not_exist dir
+  end
+
   def test_load_specs_falls_back_to_marshal_index
     # no compact index data set up, only the Marshal indexes from setup
     released = @source.load_specs(:released).map(&:full_name)