Add method that checks if path exists in `StringPath`

app/controllers/projects/artifacts_controller.rb

@@ -17,7 +17,7 @@ class Projects::ArtifactsController < Projects::ApplicationController
   def browse
     return render_404 unless build.artifacts?
     @path = build.artifacts_metadata_string_path(params[:path] || './')
-    return render_404 if @path.universe.empty?
+    return render_404 unless @path.exists?
   end
 
   private

lib/gitlab/string_path.rb

@@ -19,6 +19,10 @@ module Gitlab
       @path
     end
 
+    def exists?
+      @path == './' || @universe.include?(@path)
+    end
+
     def absolute?
       @path.start_with?('/')
     end

spec/lib/gitlab/string_path_spec.rb

@@ -32,6 +32,7 @@ describe Gitlab::StringPath do
     it { is_expected.to be_file }
     it { is_expected.to have_parent }
     it { is_expected.to_not have_descendants }
+    it { is_expected.to exist }
 
     describe '#basename' do
       subject { |example| path(example).basename }
@@ -170,4 +171,16 @@ describe Gitlab::StringPath do
 
     it { is_expected.to eq '/path/file1' }
   end
+
+  describe '#exists?', path: 'another_file' do
+    subject { |example| path(example).exists? }
+    it { is_expected.to be true }
+  end
+
+  describe '#exists?', path: './non_existent/' do
+    let(:universe) { ['./something'] }
+    subject { |example| path(example).exists? }
+
+    it { is_expected.to be false }
+  end
 end