Don't use ProjectsFinder in TodosFinder

Using ProjectsFinder in TodosFinder to limit todos to the right projects
leads to overly complicated and slow database queries. This commit
removes the use of ProjectsFinder in favour of using
Project.public_or_visible_to_current_user directly.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/43767

app/finders/todos_finder.rb

@@ -110,10 +110,6 @@ class TodosFinder
     ids
   end
 
-  def projects(items)
-    ProjectsFinder.new(current_user: current_user, project_ids_relation: project_ids(items)).execute
-  end
-
   def type?
     type.present? && %w(Issue MergeRequest).include?(type)
   end
@@ -152,13 +148,14 @@ class TodosFinder
 
   def by_project(items)
     if project?
-      items = items.where(project: project)
+      items.where(project: project)
     else
-      item_projects = projects(items)
-      items = items.merge(item_projects).joins(:project)
-    end
+      projects = Project
+        .public_or_visible_to_user(current_user)
+        .order_id_desc
 
-    items
+      items.joins(:project).merge(projects)
+    end
   end
 
   def by_state(items)

changelogs/unreleased/remove-projects-finder-from-todos-finder.yml

@@ -0,0 +1,5 @@
+---
+title: Don't use ProjectsFinder in TodosFinder
+merge_request:
+author:
+type: performance