Relativize a plugin path only when it is really under the root

The save side matched the root as a string prefix, so an absolute path
that starts with the root but climbs above it was written back relative,
and the filter above then dropped it on the next load. Asking the same
question both ways leaves it absolute and loadable.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: hsbt

lib/bundler/plugin/index.rb

@@ -231,7 +231,7 @@ def escaping_plugins(index, base)
       end
 
       # Expanded here, not by the caller: what gets stored stays joined, because
-      # the rest of the class matches it against Plugin.root as written.
+      # #installed_in_plugin_root? matches it against Plugin.root as written.
       def contained_in?(path, base)
         path = File.expand_path(path)
         base = File.expand_path(base)
@@ -264,12 +264,9 @@ def relativize_path(path, base)
         pathname = Pathname.new(path)
         return path unless pathname.absolute?
 
-        base_path = Pathname.new(base)
-        if pathname == base_path || pathname.to_s.start_with?(base_path.to_s + File::SEPARATOR)
-          pathname.relative_path_from(base_path).to_s
-        else
-          path
-        end
+        return path unless contained_in?(pathname.to_s, base)
+
+        pathname.relative_path_from(Pathname.new(base)).to_s
       end
 
       def absolutize_path(path, base)

spec/bundler/plugin/index_spec.rb

@@ -277,6 +277,15 @@
       expect(new_index.load_paths(plugin_name)).to eq([plugin_root.join("~nosuchuser", "lib").to_s])
     end
 
+    it "keeps an absolute path that only looks like it is under the plugin root" do
+      escaping_path = File.join(Bundler::Plugin.root.to_s, "..", "..", "escaping-plugin")
+
+      index.register_plugin("escaping-plugin", escaping_path, [File.join(escaping_path, "lib")], [], [], [])
+
+      new_index = Index.new
+      expect(new_index.installed?("escaping-plugin")).to eq(escaping_path)
+    end
+
     it "keeps paths outside the plugin root as absolute" do
       outside_path = tmp.join("outside", "external-plugin")
       FileUtils.mkdir_p(outside_path.join("lib"))