Add method to calculate excess namespace repo size

The method `total_repository_size_excess` calculates the sum of the
repository_size_excess for all the projects in the namespace. It is
calculated by adding the results of two separate queries to increase
query performances.

ee/app/models/ee/namespace.rb

@@ -194,6 +194,26 @@ module EE
         RootStorageSize.new(root_ancestor).above_size_limit?
     end
 
+    def total_repository_size_excess
+      strong_memoize(:total_repository_size_excess) do
+        namespace_size_limit = actual_size_limit
+
+        namespace_limit_arel = Arel::Nodes::SqlLiteral.new(namespace_size_limit.to_s.presence || 'NULL')
+        condition = 'projects.repository_size_limit != 0 AND project_statistics.repository_size > projects.repository_size_limit'
+
+        total_excess = total_repository_size_excess_calculation(condition, ::Project.arel_table[:repository_size_limit])
+
+        if namespace_size_limit.to_i > 0
+          condition = 'projects.repository_size_limit IS NULL AND project_statistics.repository_size > :namespace_size_limit'
+          sanitized_condition = self.class.sanitize_sql_array([condition, namespace_size_limit: namespace_size_limit])
+
+          total_excess += total_repository_size_excess_calculation(sanitized_condition, namespace_limit_arel)
+        end
+
+        total_excess
+      end
+    end
+
     def actual_size_limit
       ::Gitlab::CurrentSettings.repository_size_limit
     end
@@ -410,5 +430,15 @@ module EE
     def shared_runners_remaining_minutes
       [actual_shared_runners_minutes_limit.to_f - shared_runners_minutes.to_f, 0].max
     end
+
+    def total_repository_size_excess_calculation(condition, limit)
+      select_sql = Arel::Nodes::NamedFunction.new('SUM', [::ProjectStatistics.arel_table[:repository_size] - limit]).to_sql
+
+      all_projects
+      .joins(:statistics)
+      .where(condition)
+      .pluck(Arel.sql(select_sql)) # rubocop:disable Rails/Pick
+      .first || 0
+    end
   end
 end

ee/spec/models/ee/namespace_spec.rb

@@ -1431,6 +1431,74 @@ RSpec.describe Namespace do
     end
   end
 
+  describe '#total_repository_size_excess' do
+    let_it_be(:namespace) { create(:namespace) }
+
+    def create_project(repository_size:, repository_size_limit:)
+      create(:project, namespace: namespace, repository_size_limit: repository_size_limit).tap do |project|
+        create(:project_statistics, project: project, repository_size: repository_size)
+      end
+    end
+
+    before do
+      namespace.clear_memoization(:total_repository_size_excess)
+    end
+
+    context 'projects with a variety of repository sizes and limits' do
+      before_all do
+        create_project(repository_size: 100, repository_size_limit: nil)
+        create_project(repository_size: 150, repository_size_limit: nil)
+        create_project(repository_size: 200, repository_size_limit: nil)
+
+        create_project(repository_size: 100, repository_size_limit: 0)
+        create_project(repository_size: 150, repository_size_limit: 0)
+        create_project(repository_size: 200, repository_size_limit: 0)
+
+        create_project(repository_size: 300, repository_size_limit: 400)
+        create_project(repository_size: 400, repository_size_limit: 400)
+        create_project(repository_size: 500, repository_size_limit: 300)
+      end
+
+      context 'when namespace-level repository_size_limit is not set' do
+        it 'returns the total excess size of projects with repositories that exceed the size limit' do
+          allow(namespace).to receive(:actual_size_limit).and_return(nil)
+
+          expect(namespace.total_repository_size_excess).to eq(200)
+        end
+      end
+
+      context 'when namespace-level repository_size_limit is 0 (unlimited)' do
+        it 'returns the total excess size of projects with repositories that exceed the size limit' do
+          allow(namespace).to receive(:actual_size_limit).and_return(0)
+
+          expect(namespace.total_repository_size_excess).to eq(200)
+        end
+      end
+
+      context 'when namespace-level repository_size_limit is a positive number' do
+        it 'returns the total excess size of projects with repositories that exceed the size limit' do
+          allow(namespace).to receive(:actual_size_limit).and_return(150)
+
+          expect(namespace.total_repository_size_excess).to eq(250)
+        end
+      end
+    end
+
+    context 'when all projects have repository_size_limit of 0 (unlimited)' do
+      before do
+        create_project(repository_size: 100, repository_size_limit: 0)
+        create_project(repository_size: 150, repository_size_limit: 0)
+        create_project(repository_size: 200, repository_size_limit: 0)
+
+        allow(namespace).to receive(:actual_size_limit).and_return(150)
+      end
+
+      it 'returns zero regardless of the namespace or instance-level repository_size_limit' do
+        expect(namespace.total_repository_size_excess).to eq(0)
+      end
+    end
+  end
+
   describe '#actual_size_limit' do
     let(:namespace) { build(:namespace) }