Recover from errors when a parent is not preloaded

app/models/concerns/group_descendant.rb

@@ -37,7 +37,20 @@ module GroupDescendant
     parent ||= preloaded.detect { |possible_parent| possible_parent.is_a?(Group) && possible_parent.id == child.parent_id }
 
     if parent.nil? && !child.parent_id.nil?
-      raise ArgumentError.new('parent was not preloaded')
+      parent = child.parent
+
+      exception = ArgumentError.new <<~MSG
+        parent: [GroupDescendant: #{parent.inspect}] was not preloaded for [#{child.inspect}]")
+        This error is not user facing, but causes a +1 query.
+      MSG
+      extras = {
+        parent: parent,
+        child: child,
+        preloaded: preloaded.map(&:full_path)
+      }
+      issue_url = 'https://gitlab.com/gitlab-org/gitlab-ce/issues/40785'
+
+      Gitlab::Sentry.track_exception(exception, issue_url: issue_url, extra: extras)
     end
 
     if parent.nil? && hierarchy_top.present?

changelogs/unreleased/bvl-shared-groups-on-group-page.yml

+---
+title: Show shared projects on group list
+  list
+merge_request: 18390
+author:
+type: fixed

spec/models/concerns/group_descendant_spec.rb

@@ -79,9 +79,24 @@ describe GroupDescendant, :nested_groups do
         expect(described_class.build_hierarchy(groups)).to eq(expected_hierarchy)
       end
 
+      it 'tracks the exception when a parent was not preloaded' do
+        expect(Gitlab::Sentry).to receive(:track_exception).and_call_original
+
+        expect { GroupDescendant.build_hierarchy([subsub_group]) }.to raise_error(ArgumentError)
+      end
+
+      it 'recovers if a parent was not reloaded by querying for the parent' do
+        expected_hierarchy = { parent => { subgroup => subsub_group } }
+
+        # this does not raise in production, so stubbing it here.
+        allow(Gitlab::Sentry).to receive(:track_exception)
+
+        expect(GroupDescendant.build_hierarchy([subsub_group])).to eq(expected_hierarchy)
+      end
+
       it 'raises an error if not all elements were preloaded' do
         expect { described_class.build_hierarchy([subsub_group]) }
-          .to raise_error('parent was not preloaded')
+          .to raise_error(/was not preloaded/)
       end
     end
   end