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.

4 files changed, 48 insertions, 2 deletions

diff --git a/app/models/namespace.rb b/app/models/namespace.rb
index fa76729a702..901dbf2ba69 100644
--- a/app/models/namespace.rb
+++ b/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?
diff --git a/changelogs/unreleased/bvl-limit-fork-queries-on-project-show.yml b/changelogs/unreleased/bvl-limit-fork-queries-on-project-show.yml
new file mode 100644
index 00000000000..299d9bf6b9c
--- /dev/null
+++ b/changelogs/unreleased/bvl-limit-fork-queries-on-project-show.yml
@@ -0,0 +1,5 @@
+---
+title: Reduce requests for project forks on show page of projects that have forks
+merge_request: 15663
+author:
+type: performance
diff --git a/spec/controllers/projects_controller_spec.rb b/spec/controllers/projects_controller_spec.rb
index e7ab714c550..39ee5e8691b 100644
--- a/spec/controllers/projects_controller_spec.rb
+++ b/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
diff --git a/spec/models/namespace_spec.rb b/spec/models/namespace_spec.rb
index 90b768f595e..3817f20bfe7 100644
--- a/spec/models/namespace_spec.rb
+++ b/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