Lock compact_index vendoring against parallel races

The vendored compact_index install ran without any coordination, so two
test setups starting at once could both write into tmp/compact_index/ at
the same time. The skip guard also checked a single file, which meant an
interrupted download leaving only lib/compact_index.rb behind would be
treated as a complete vendor tree on the next run.

Take an exclusive file lock around the install and only skip the download
once every expected file is present, removing the tree under the same lock
when COMPACT_INDEX_REF forces a refresh.

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

Author: hsbt

spec/support/rubygems_ext.rb

@@ -84,26 +84,34 @@ def install_test_deps
     def install_vendored_compact_index
       target_root = Path.tmp_root.join("compact_index")
       require "fileutils"
+      FileUtils.mkdir_p(Path.tmp_root)
 
-      if ENV["COMPACT_INDEX_REF"]
-        FileUtils.rm_rf(target_root)
-      elsif File.exist?(target_root.join("lib/compact_index.rb"))
-        return
-      end
-
-      require "open-uri"
-      ref = ENV["COMPACT_INDEX_REF"] || "7c68a7b39761c61a66f9299f85b889ec39afc02c"
-      %w[
+      files = %w[
         lib/compact_index.rb
         lib/compact_index/dependency.rb
         lib/compact_index/gem.rb
         lib/compact_index/gem_version.rb
         lib/compact_index/versions_file.rb
-      ].each do |path|
-        url = "https://raw.githubusercontent.com/rubygems/rubygems.org/#{ref}/#{path}"
-        target = target_root.join(path)
-        FileUtils.mkdir_p(File.dirname(target))
-        File.write(target, URI.parse(url).open(&:read))
+      ]
+
+      # Serialize installs so parallel test setups don't race on the same
+      # vendor tree, and only skip the download when every file is present so
+      # an interrupted run can't leave a partial copy behind.
+      File.open(Path.tmp_root.join("compact_index.lock"), File::CREAT | File::RDWR) do |lock|
+        lock.flock(File::LOCK_EX)
+
+        FileUtils.rm_rf(target_root) if ENV["COMPACT_INDEX_REF"]
+
+        next if files.all? {|path| File.exist?(target_root.join(path)) }
+
+        require "open-uri"
+        ref = ENV["COMPACT_INDEX_REF"] || "7c68a7b39761c61a66f9299f85b889ec39afc02c"
+        files.each do |path|
+          url = "https://raw.githubusercontent.com/rubygems/rubygems.org/#{ref}/#{path}"
+          target = target_root.join(path)
+          FileUtils.mkdir_p(File.dirname(target))
+          File.write(target, URI.parse(url).open(&:read))
+        end
       end
     end