Optimize /api/v4/projects endpoint for visibility level

Previously when a user requested a list of projects,
`Project#public_or_visible_to_user` would search all authorized projects
and public/internal projects as well. However, when a user requests a
specific `visibility_level` (e.g. private), that should reduce the
search space, and we shouldn't need to load public/internal projects.

Improves https://gitlab.com/gitlab-org/gitlab-ce/issues/59329

app/finders/projects_finder.rb

@@ -81,7 +81,7 @@ class ProjectsFinder < UnionFinder
       if private_only?
         current_user.authorized_projects
       else
-        Project.public_or_visible_to_user(current_user)
+        Project.public_or_visible_to_user(current_user, params[:visibility_level])
       end
     end
   end

app/models/project.rb

@@ -459,13 +459,25 @@ class Project < ActiveRecord::Base
   # Returns a collection of projects that is either public or visible to the
   # logged in user.
-  def self.public_or_visible_to_user(user = nil)
-    if user
+  #
+  # requested_visiblity_levels: Normally all projects that are visible
+  # to the user (e.g. internal and public) are queried, but this
+  # parameter allows the caller to narrow the search space to optimize
+  # database queries. For instance, a caller may only want to see
+  # internal projects. Instead of querying for internal and public
+  # projects and throwing away public projects, this parameter allows
+  # the query to be targeted for only internal projects.
+  def self.public_or_visible_to_user(user = nil, requested_visibility_levels = [])
+    return public_to_user unless user
+
+    visible_levels = Gitlab::VisibilityLevel.levels_for_user(user)
+    visible_levels &= Array(requested_visibility_levels) if requested_visibility_levels.present?
+
+    if visible_levels.present?
       where('EXISTS (?) OR projects.visibility_level IN (?)',
-            user.authorizations_for_projects,
-            Gitlab::VisibilityLevel.levels_for_user(user))
+            user.authorizations_for_projects, visible_levels)
     else
-      public_to_user
+      where('EXISTS (?)', user.authorizations_for_projects)
     end
   end

changelogs/unreleased/sh-optimize-projects-api.yml

+---
+title: Optimize /api/v4/projects endpoint for visibility level
+merge_request: 26481
+author:
+type: performance

spec/models/project_spec.rb

@@ -2710,7 +2710,7 @@ describe Project do
   end
   describe '#any_lfs_file_locks?', :request_store do
-    set(:project) { create(:project) }
+    let!(:project) { create(:project) }
     it 'returns false when there are no LFS file locks' do
       expect(project.any_lfs_file_locks?).to be_falsey
@@ -3148,6 +3148,53 @@ describe Project do
         expect(projects).to eq([public_project])
       end
     end
+
+    context 'with requested visibility levels' do
+      set(:internal_project) { create(:project, :internal, :repository) }
+      set(:private_project_2) { create(:project, :private) }
+
+      context 'with admin user' do
+        set(:admin) { create(:admin) }
+
+        it 'returns all projects' do
+          projects = described_class.all.public_or_visible_to_user(admin, [])
+
+          expect(projects).to match_array([public_project, private_project, private_project_2, internal_project])
+        end
+
+        it 'returns all public and private projects' do
+          projects = described_class.all.public_or_visible_to_user(admin, [Gitlab::VisibilityLevel::PUBLIC, Gitlab::VisibilityLevel::PRIVATE])
+
+          expect(projects).to match_array([public_project, private_project, private_project_2])
+        end
+
+        it 'returns all private projects' do
+          projects = described_class.all.public_or_visible_to_user(admin, [Gitlab::VisibilityLevel::PRIVATE])
+
+          expect(projects).to match_array([private_project, private_project_2])
+        end
+      end
+
+      context 'with regular user' do
+        it 'returns authorized projects' do
+          projects = described_class.all.public_or_visible_to_user(user, [])
+
+          expect(projects).to match_array([public_project, private_project, internal_project])
+        end
+
+        it "returns user's public and private projects" do
+          projects = described_class.all.public_or_visible_to_user(user, [Gitlab::VisibilityLevel::PUBLIC, Gitlab::VisibilityLevel::PRIVATE])
+
+          expect(projects).to match_array([public_project, private_project])
+        end
+
+        it 'returns one private project' do
+          projects = described_class.all.public_or_visible_to_user(user, [Gitlab::VisibilityLevel::PRIVATE])
+
+          expect(projects).to eq([private_project])
+        end
+      end
+    end
   end
   describe '.with_feature_available_for_user' do