Cache the forks in a namespace in the RequestStore

On the `show` of a project that is part of a fork network. We check if
the user already created a fork of this project in their personal
namespace.

We do this in several places, so caching the result of this query in
the request store prevents us from repeating it.

app/models/namespace.rb

@@ -139,7 +139,17 @@ class Namespace < ActiveRecord::Base
   def find_fork_of(project)
     return nil unless project.fork_network
-    project.fork_network.find_forks_in(projects).first
+    if RequestStore.active?
+      forks_in_namespace = RequestStore.fetch("namespaces:#{id}:forked_projects") do
+        Hash.new do |found_forks, project|
+          found_forks[project] = project.fork_network.find_forks_in(projects).first
+        end
+      end
+
+      forks_in_namespace[project]
+    else
+      project.fork_network.find_forks_in(projects).first
+    end
   end
   def lfs_enabled?

changelogs/unreleased/bvl-limit-fork-queries-on-project-show.yml

+---
+title: Reduce requests for project forks on show page of projects that have forks
+merge_request: 15663
+author:
+type: performance

spec/controllers/projects_controller_spec.rb

@@ -261,6 +261,29 @@ describe ProjectsController do
         expect(response).to redirect_to(namespace_project_path)
       end
     end
+
+    context 'when the project is forked and has a repository', :request_store do
+      let(:public_project) { create(:project, :public, :repository) }
+
+      render_views
+
+      before do
+        # View the project as a user that does not have any rights
+        sign_in(create(:user))
+
+        fork_project(public_project)
+      end
+
+      it 'does not increase the number of queries when the project is forked' do
+        # When a project is part of a fork network, we check if the `current_user`
+        # has a fork in their own namespace. We query this several times. Caching
+        # the result in the RequestStore brings the number of queries for this
+        # request down from 64 to 59.
+
+        expect { get(:show, namespace_id: public_project.namespace, id: public_project) }
+          .not_to exceed_query_limit(59)
+      end
+    end
   end
   describe "#update" do

spec/models/namespace_spec.rb

@@ -531,7 +531,7 @@ describe Namespace do
     end
   end
-  describe '#has_forks_of?' do
+  describe '#find_fork_of?' do
     let(:project) { create(:project, :public) }
     let!(:forked_project) { fork_project(project, namespace.owner, namespace: namespace) }
@@ -550,5 +550,13 @@ describe Namespace do
       expect(other_namespace.find_fork_of(project)).to eq(other_fork)
     end
+
+    context 'with request store enabled', :request_store do
+      it 'only queries once' do
+        expect(project.fork_network).to receive(:find_forks_in).once.and_call_original
+
+        2.times { namespace.find_fork_of(project) }
+      end
+    end
   end
 end