Fix 404 page when viewing TODOs that contain milestones or labels in different projects

A user viewing the TODOs page will see a 404 if there are mentioned labels
in multiple different projects. This is likely a caching bug and only occurs
when Markdown rendering occurs across multiple projects, which is why it's so
tricky to reproduce. This is what I think is happening:

1. LabelReferenceFilter#references_in encounters label ~X for ProjectA and finds the label in the DB as id = 1.
2. LabelReferenceFilter.references_in yields [1, 'X', nil, ...]
3. Since project_ref is nil, AbstractReferenceFilter#project_from_ref_cache caches nil => ProjectA.
4. LabelReferenceFilter#references_in encounters label ~Y for ProjectB and finds the label in the DB as id = 2.
5. LabelReferenceFilter.references_in yields [2, 'Y', nil, ...]
6. AbstractReferenceFilter#project_from_ref_cache lookups nil and returns ProjectA. It was supposed to be ProjectB.
7. A is the wrong project, so the label lookup fails.

This MR caches Markdown references if the key is present.

Closes #17898

1 files changed, 21 insertions, 0 deletions

diff --git a/spec/features/todos/todos_spec.rb b/spec/features/todos/todos_spec.rb
index 3354f529295..4e627753cc7 100644
--- a/spec/features/todos/todos_spec.rb
+++ b/spec/features/todos/todos_spec.rb
@@ -43,6 +43,27 @@ describe 'Dashboard Todos', feature: true do
       end
     end
 
+    context 'User has Todos with labels spanning multiple projects' do
+      before do
+        label1 = create(:label, project: project)
+        note1 = create(:note_on_issue, note: "Hello #{label1.to_reference(format: :name)}", noteable_id: issue.id, noteable_type: 'Issue', project: issue.project)
+        create(:todo, :mentioned, project: project, target: issue, user: user, note_id: note1.id)
+
+        project2 = create(:project)
+        label2 = create(:label, project: project2)
+        issue2 = create(:issue, project: project2)
+        note2 = create(:note_on_issue, note: "Test #{label2.to_reference(format: :name)}", noteable_id: issue2.id, noteable_type: 'Issue', project: project2)
+        create(:todo, :mentioned, project: project2, target: issue2, user: user, note_id: note2.id)
+
+        login_as(user)
+        visit dashboard_todos_path
+      end
+
+      it 'shows page with two Todos' do
+        expect(page).to have_selector('.todos-list .todo', count: 2)
+      end
+    end
+
     context 'User has multiple pages of Todos' do
       before do
         allow(Todo).to receive(:default_per_page).and_return(1)