Add relative url support to routing contrainers

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

lib/constraints/namespace_url_constrainer.rb

 class NamespaceUrlConstrainer
   def matches?(request)
-    id = request.path.sub(/\A\/+/, '').split('/').first.sub(/.atom\z/, '')
+    id = request.path
+    id = id.sub(/\A#{relative_url_root}/, '') if relative_url_root
+    id = id.sub(/\A\/+/, '').split('/').first
+    id = id.sub(/.atom\z/, '') if id
 
     if id =~ Gitlab::Regex.namespace_regex
       find_resource(id)
@@ -10,4 +13,12 @@ class NamespaceUrlConstrainer
   def find_resource(id)
     Namespace.find_by_path(id)
   end
+
+  private
+
+  def relative_url_root
+    if defined?(Gitlab::Application.config.relative_url_root)
+      Gitlab::Application.config.relative_url_root
+    end
+  end
 end

spec/lib/constraints/namespace_url_constrainer_spec.rb

@@ -17,6 +17,16 @@ describe NamespaceUrlConstrainer, lib: true do
       it { expect(subject.matches?(request '/g/gitlab')).to be_falsey }
       it { expect(subject.matches?(request '/.gitlab')).to be_falsey }
     end
+
+    context 'relative url' do
+      before do
+        allow(Gitlab::Application.config).to receive(:relative_url_root) { '/gitlab' }
+      end
+
+      it { expect(subject.matches?(request '/gitlab/gitlab')).to be_truthy }
+      it { expect(subject.matches?(request '/gitlab/gitlab-ce')).to be_falsey }
+      it { expect(subject.matches?(request '/gitlab/')).to be_falsey }
+    end
   end
 
   def request(path)