Generate canonical url and remove trailing slash

In this commit we add the logic to generate automatically
the canonical link for urls that end up in `/`.

Only when the url is the host we don't generate it.

app/helpers/page_layout_helper.rb

 # frozen_string_literal: true
 
 module PageLayoutHelper
+  include Gitlab::Utils::StrongMemoize
+
   def page_title(*titles)
     @page_title ||= []
 
@@ -44,7 +46,7 @@ module PageLayoutHelper
     if link
       @page_canonical_link = link
     else
-      @page_canonical_link
+      @page_canonical_link ||= generic_canonical_url
     end
   end
 
@@ -147,4 +149,31 @@ module PageLayoutHelper
 
     css_class.join(' ')
   end
+
+  private
+
+  def generic_canonical_url
+    strong_memoize(:generic_canonical_url) do
+      next unless request.get? || request.head?
+      next unless generate_generic_canonical_url?
+      next unless Feature.enabled?(:generic_canonical, current_user)
+
+      # Request#url builds the url without the trailing slash
+      request.url
+    end
+  end
+
+  def generate_generic_canonical_url?
+    # For the main domain it doesn't matter whether there is
+    # a trailing slash or not, they're not considered different
+    # pages
+    return false if request.path == '/'
+
+    # We only need to generate the canonical url when the request has a trailing
+    # slash. In the request object, only the `original_fullpath` and
+    # `original_url` keep the slash if it's present. Both `path` and
+    # `fullpath` would return the path without the slash.
+    # Therefore, we need to process `original_fullpath`
+    request.original_fullpath.sub(request.path, '')[0] == '/'
+  end
 end

changelogs/unreleased/36820-fj-add-generic-canonical-url.yml

+---
+title: Generate canonical url and remove trailing slash
+merge_request: 46435
+author:
+type: changed

config/feature_flags/development/generic_canonical.yml

+---
+name: generic_canonical
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/46435
+rollout_issue_url:
+type: development
+group: group::editor
+default_enabled: false

spec/features/canonical_link_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe 'Canonical link' do
+  include Spec::Support::Helpers::Features::CanonicalLinkHelpers
+
+  let_it_be(:user) { create(:user) }
+  let_it_be(:project) { create(:project, :public, namespace: user.namespace) }
+  let_it_be(:issue) { create(:issue, project: project) }
+
+  let_it_be(:issue_request) { issue_url(issue) }
+  let_it_be(:project_request) { project_url(project) }
+
+  before do
+    sign_in(user)
+  end
+
+  shared_examples 'shows canonical link' do
+    specify do
+      visit request_url
+
+      expect(page).to have_canonical_link(expected_url)
+    end
+  end
+
+  shared_examples 'does not show canonical link' do
+    specify do
+      visit request_url
+
+      expect(page).not_to have_any_canonical_links
+    end
+  end
+
+  it_behaves_like 'does not show canonical link' do
+    let(:request_url) { issue_request }
+  end
+
+  it_behaves_like 'shows canonical link' do
+    let(:request_url) { issue_request + '/' }
+    let(:expected_url) { issue_request }
+  end
+
+  it_behaves_like 'shows canonical link' do
+    let(:request_url) { project_issues_url(project) + "/?state=opened" }
+    let(:expected_url) { project_issues_url(project, state: 'opened') }
+  end
+
+  it_behaves_like 'does not show canonical link' do
+    let(:request_url) { project_request }
+  end
+
+  it_behaves_like 'shows canonical link' do
+    let(:request_url) { project_request + '/' }
+    let(:expected_url) { project_request }
+  end
+
+  it_behaves_like 'shows canonical link' do
+    let(:query_params) { '?foo=bar' }
+    let(:request_url) { project_request + "/#{query_params}" }
+    let(:expected_url) { project_request + query_params }
+  end
+
+  # Hard-coded canonical links
+
+  it_behaves_like 'shows canonical link' do
+    let(:request_url) { explore_root_path }
+    let(:expected_url) { explore_projects_url }
+  end
+
+  context 'when feature flag generic_canonical is disabled' do
+    before do
+      stub_feature_flags(generic_canonical: false)
+    end
+
+    it_behaves_like 'does not show canonical link' do
+      let(:request_url) { issue_request + '/' }
+    end
+
+    it_behaves_like 'does not show canonical link' do
+      let(:request_url) { project_request + '/' }
+    end
+  end
+end

spec/features/dashboard/root_explore_spec.rb

@@ -30,12 +30,4 @@ RSpec.describe 'Root explore' do
 
     include_examples 'shows public projects'
   end
-
-  it 'includes canonical link to explore projects url' do
-    visit explore_root_path
-
-    canonial_link = page.find("head link[rel='canonical']", visible: false)
-
-    expect(canonial_link[:href]).to eq explore_projects_url
-  end
 end

spec/helpers/page_layout_helper_spec.rb

@@ -137,4 +137,75 @@ RSpec.describe PageLayoutHelper do
       end
     end
   end
+
+  describe '#page_canonical_link' do
+    let(:user) { build(:user) }
+
+    subject { helper.page_canonical_link(link) }
+
+    before do
+      allow(helper).to receive(:current_user).and_return(user)
+    end
+
+    context 'when link is passed' do
+      let(:link) { 'https://gitlab.com' }
+
+      it 'stores and returns the link value' do
+        expect(subject).to eq link
+        expect(helper.page_canonical_link(nil)).to eq link
+      end
+    end
+
+    context 'when no link is provided' do
+      let(:link) { nil }
+      let(:request) { ActionDispatch::Request.new(env) }
+      let(:env) do
+        {
+          'ORIGINAL_FULLPATH' => '/foo/',
+          'PATH_INFO' => '/foo',
+          'HTTP_HOST' => 'test.host',
+          'REQUEST_METHOD' => method,
+          'rack.url_scheme' => 'http'
+        }
+      end
+
+      before do
+        allow(helper).to receive(:request).and_return(request)
+      end
+
+      shared_examples 'generates the canonical url using the params in the context' do
+        specify { expect(subject).to eq 'http://test.host/foo' }
+      end
+
+      shared_examples 'does not return a canonical url' do
+        specify { expect(subject).to be_nil }
+      end
+
+      it_behaves_like 'generates the canonical url using the params in the context' do
+        let(:method) { 'GET' }
+      end
+
+      it_behaves_like 'generates the canonical url using the params in the context' do
+        let(:method) { 'HEAD' }
+      end
+
+      it_behaves_like 'does not return a canonical url' do
+        let(:method) { 'POST' }
+      end
+
+      it_behaves_like 'does not return a canonical url' do
+        let(:method) { 'PUT' }
+      end
+
+      context 'when feature flag generic_canonical is disabled' do
+        let(:method) { 'GET' }
+
+        before do
+          stub_feature_flags(generic_canonical: false)
+        end
+
+        it_behaves_like 'does not return a canonical url'
+      end
+    end
+  end
 end