Respond with 404 Not Found for non-existent tags

Non-existent tags should be handled with 404 Not Found.

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.12.0 (unreleased)
+  - Respond with 404 Not Found for non-existent tags
   - Update the rouge gem to 2.0.6, which adds highlighting support for JSX, Prometheus, and others. !6251
   - Only check :can_resolve permission if the note is resolvable
   - Add ability to fork to a specific namespace using API. (ritave)

app/controllers/projects/tags_controller.rb

@@ -20,6 +20,8 @@ class Projects::TagsController < Projects::ApplicationController
   def show
     @tag = @repository.find_tag(params[:id])
 
+    return render_404 if @tag.nil?
+
     @release = @project.releases.find_or_initialize_by(tag: @tag.name)
     @commit = @repository.commit(@tag.target)
   end

spec/controllers/projects/tags_controller_spec.rb

@@ -17,4 +17,18 @@ describe Projects::TagsController do
       expect(assigns(:releases)).not_to include(invalid_release)
     end
   end
+
+  describe 'GET show' do
+    before { get :show, namespace_id: project.namespace.to_param, project_id: project.to_param, id: id }
+
+    context "valid tag" do
+      let(:id) { 'v1.0.0' }
+      it { is_expected.to respond_with(:success) }
+    end
+
+    context "invalid tag" do
+      let(:id) { 'latest' }
+      it { is_expected.to respond_with(:not_found) }
+    end
+  end
 end