Avoid using the current's project excess twice

The calculation for the exceeded size for a project used the project's
excess size twice as it was also included in the total repository size
excess. This is corrected by calculating the project's excess size and
substracting it from the total repository size.

ee/lib/ee/gitlab/repository_size_checker.rb

@@ -19,7 +19,7 @@ module EE
       def exceeded_size(change_size = 0)
         exceeded_size = super
         exceeded_size -= remaining_additional_purchased_storage if additional_repo_storage_available?
-        exceeded_size
+        [exceeded_size, 0].max
       end
 
       private
@@ -38,8 +38,16 @@ module EE
         namespace&.additional_purchased_storage_size&.megabytes.to_i
       end
 
+      def current_project_excess
+        [current_size - limit, 0].max
+      end
+
+      def total_excess_without_current_project
+        total_repository_size_excess - current_project_excess
+      end
+
       def remaining_additional_purchased_storage
-        additional_purchased_storage - total_repository_size_excess
+        additional_purchased_storage - total_excess_without_current_project
       end
     end
   end

ee/spec/lib/ee/gitlab/repository_size_checker_spec.rb

@@ -136,26 +136,37 @@ RSpec.describe Gitlab::RepositorySizeChecker do
         stub_feature_flags(namespace_storage_limit: false)
       end
 
-      context 'when current size + total repository size excess are below or equal to the limit + additional purchased storage' do
-        let(:current_size) { 50 }
+      context 'with additional purchased storage' do
         let(:total_repository_size_excess) { 10 }
         let(:additional_purchased_storage) { 10 }
 
+        context 'when current size + total repository size excess are below or equal to the project\'s limit (no need for additional purchase storage)' do
+          let(:current_size) { 50 }
+
           it 'returns zero' do
             expect(subject.exceeded_size).to eq(0)
           end
         end
 
-      context 'when current size + total repository size excess are over the limit + additional purchased storage' do
+        context 'when there is remaining storage (current size + excess for other projects need to use additional purchased storage but not all of it)' do
           let(:current_size) { 51 }
-        let(:total_repository_size_excess) { 10 }
-        let(:additional_purchased_storage) { 10 }
+
+          it 'returns 0' do
+            expect(subject.exceeded_size).to eq(0)
+          end
+        end
+
+        context 'when there storage is exceeded (current size + excess for other projects exceed additional purchased storage' do
+          let(:total_repository_size_excess) { 15 }
+          let(:current_size) { 61 }
 
           it 'returns 1' do
-          expect(subject.exceeded_size).to eq(1.megabytes)
+            expect(subject.exceeded_size).to eq(5.megabytes)
+          end
         end
       end
 
+      context 'without additional purchased storage' do
         context 'when change size will be over the limit' do
           let(:current_size) { 50 }
 
@@ -172,6 +183,7 @@ RSpec.describe Gitlab::RepositorySizeChecker do
           end
         end
       end
+    end
 
     context 'with feature flag :additional_repo_storage_by_namespace disabled' do
       before do