fix: ignore git env in vcs checkout

Author: pftg

lib/capybara/screenshot/diff/vcs.rb

@@ -9,21 +9,22 @@ module Diff
       module Vcs
         def self.checkout_vcs(root, screenshot_path, checkout_path)
           root_path = root.to_s
-          git_root, _, status = Open3.capture3("git", "-C", root_path, "rev-parse", "--show-toplevel")
+          git_env = { "GIT_DIR" => nil, "GIT_WORK_TREE" => nil, "GIT_INDEX_FILE" => nil }
+          git_root, _, status = Open3.capture3(git_env, "git", "-C", root_path, "rev-parse", "--show-toplevel")
           return false unless status.success?
 
           git_root = git_root.chomp
           vcs_file_path = Pathname.new(screenshot_path).expand_path.relative_path_from(Pathname.new(git_root)).to_s
 
           if Screenshot.use_lfs
             tmp_path = "#{checkout_path}.tmp"
-            success = system("git", "-C", root_path, "show", "HEAD:#{vcs_file_path}", out: tmp_path, err: File::NULL)
+            success = system(git_env, "git", "-C", root_path, "show", "HEAD:#{vcs_file_path}", out: tmp_path, err: File::NULL)
             if success
-              system("git", "-C", root_path, "lfs", "smudge", in: tmp_path, out: checkout_path.to_s, err: File::NULL)
+              system(git_env, "git", "-C", root_path, "lfs", "smudge", in: tmp_path, out: checkout_path.to_s, err: File::NULL)
             end
             File.delete(tmp_path) if File.exist?(tmp_path)
           else
-            success = system("git", "-C", root_path, "show", "HEAD:#{vcs_file_path}", out: checkout_path.to_s, err: File::NULL)
+            success = system(git_env, "git", "-C", root_path, "show", "HEAD:#{vcs_file_path}", out: checkout_path.to_s, err: File::NULL)
           end
 
           unless success

test/unit/vcs_test.rb

@@ -32,6 +32,30 @@ class VcsTest < ActiveSupport::TestCase
           assert base_screenshot_path.exist?
           assert_equal screenshot_path.size, base_screenshot_path.size
         end
+
+        test "#checkout_vcs ignores external git env overrides" do
+          screenshot_path = file_fixture("images/a.png")
+          base_screenshot_path = Pathname.new(@base_screenshot.path)
+
+          with_env("GIT_DIR" => "/tmp/does-not-exist", "GIT_WORK_TREE" => "/tmp/does-not-exist") do
+            assert Vcs.checkout_vcs(@tmp_dir, screenshot_path, base_screenshot_path),
+              "checkout_vcs failed with overridden git env: root=#{@tmp_dir}"
+          end
+
+          assert base_screenshot_path.exist?
+          assert_equal screenshot_path.size, base_screenshot_path.size
+        end
+
+        private
+
+        def with_env(vars)
+          previous = vars.transform_values { |_, _| nil }
+          vars.each_key { |key| previous[key] = ENV[key] }
+          vars.each { |key, value| ENV[key] = value }
+          yield
+        ensure
+          previous.each { |key, value| ENV[key] = value }
+        end
       end
     end
   end