Faster url generation

Modify ActionDispatch::Journey::Formatter#missing_keys method to prevent
rebuilding large regular expressions on each url generation call.

changelogs/unreleased/194144-speed-up-url-helpers.yml

+---
+title: Improve link generation performance
+merge_request: 22426
+author:
+type: performance

config/initializers/action_dispatch_journey_formatter.rb

+# frozen_string_literal: true
+
+# TODO: Eliminate this file when https://github.com/rails/rails/pull/38184 is released.
+# Cleanup issue: https://gitlab.com/gitlab-org/gitlab/issues/195841
+ActionDispatch::Journey::Formatter.prepend(Gitlab::Patch::ActionDispatchJourneyFormatter)
+
+module ActionDispatch
+  module Journey
+    module Path
+      class Pattern
+        def requirements_for_missing_keys_check
+          @requirements_for_missing_keys_check ||= requirements.each_with_object({}) do |(key, regex), hash|
+            hash[key] = /\A#{regex}\Z/
+          end
+        end
+      end
+    end
+  end
+end

lib/gitlab/patch/action_dispatch_journey_formatter.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Patch
+    module ActionDispatchJourneyFormatter
+      def self.prepended(mod)
+        mod.alias_method(:old_missing_keys, :missing_keys)
+        mod.remove_method(:missing_keys)
+      end
+
+      private
+
+      def missing_keys(route, parts)
+        missing_keys = nil
+        tests = route.path.requirements_for_missing_keys_check
+        route.required_parts.each do |key|
+          case tests[key]
+          when nil
+            unless parts[key]
+              missing_keys ||= []
+              missing_keys << key
+            end
+          else
+            unless tests[key].match?(parts[key])
+              missing_keys ||= []
+              missing_keys << key
+            end
+          end
+        end
+        missing_keys
+      end
+    end
+  end
+end

spec/lib/gitlab/patch/action_dispatch_journey_formatter_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::Patch::ActionDispatchJourneyFormatter do
+  let(:group) { create(:group) }
+  let(:project) { create(:project, namespace: group) }
+  let(:pipeline) { create(:ci_empty_pipeline, project: project) }
+  let(:url) { Gitlab::Routing.url_helpers.project_pipeline_url(project, pipeline) }
+  let(:expected_path) { "#{project.full_path}/pipelines/#{pipeline.id}" }
+
+  context 'custom implementation of #missing_keys' do
+    before do
+      expect_any_instance_of(Gitlab::Patch::ActionDispatchJourneyFormatter).to receive(:missing_keys)
+    end
+
+    it 'generates correct url' do
+      expect(url).to end_with(expected_path)
+    end
+  end
+
+  context 'original implementation of #missing_keys' do
+    before do
+      allow_any_instance_of(Gitlab::Patch::ActionDispatchJourneyFormatter).to receive(:missing_keys) do |instance, route, parts|
+        instance.send(:old_missing_keys, route, parts) # test the old implementation
+      end
+    end
+
+    it 'generates correct url' do
+      expect(url).to end_with(expected_path)
+    end
+  end
+end