Add helper methods to redirect legacy paths

changelogs/unreleased/bvl-free-paths.yml

+---
+title: Free up some reserved group names
+merge_request: 15052
+author:
+type: other

lib/gitlab/routing.rb

@@ -40,5 +40,14 @@ module Gitlab
     def self.url_helpers
       @url_helpers ||= Gitlab::Application.routes.url_helpers
     end
+
+    def self.redirect_legacy_paths(router, *paths)
+      paths.each do |path|
+        router.match "/#{path}(/*rest)",
+                     via: [:get, :post, :patch, :delete],
+                     to: router.redirect { |_params, request| request.fullpath.gsub(%r{/#{path}/*}, "/-/#{path}/") },
+                     as: "legacy_#{path}_redirect"
+      end
+    end
   end
 end

spec/support/legacy_path_redirect_shared_examples.rb

+shared_examples 'redirecting a legacy path' do |source, target|
+  include RSpec::Rails::RequestExampleGroup
+
+  it "redirects #{source} to #{target} when the resource does not exist" do
+    expect(get(source)).to redirect_to(target)
+  end
+
+  it "does not redirect #{source} to #{target} when the resource exists" do
+    resource
+
+    expect(get(source)).not_to redirect_to(target)
+  end
+end