Migrate authored todos to GhostUser

Currently, when a user is deleted, we destroy all todos they authored.
It might preferable to change them to the GhostUser, similar to what we
do with Issues, Notes or UserAwards. Just because a user is deleted, it
should not affect my todos.

Hopefully this should fix the problems we are seeing with performance as
well, as the updates are executed in a loop until all todos are
migrated.

Note: In this commit we migrate todos to the GhostUser even if the
hard_delete is set.

See also:
- https://gitlab.com/gitlab-com/gl-infra/production/-/issues/19169
- https://gitlab.com/gitlab-org/gitlab/-/issues/515442

Changelog: changed

app/services/users/migrate_records_to_ghost_user_service.rb

@@ -37,6 +37,7 @@ def migrate_records
 
       return if hard_delete
 
+      migrate_authored_todos
       migrate_issues
       migrate_merge_requests
       migrate_notes
@@ -68,6 +69,10 @@ def delete_snippets
       raise DestroyError, response.message if response.error?
     end
 
+    def migrate_authored_todos
+      batched_migrate(Todo, :author_id)
+    end
+
     def migrate_issues
       batched_migrate(Issue, :author_id)
       batched_migrate(Issue, :last_edited_by_id)

spec/services/users/migrate_records_to_ghost_user_service_spec.rb

@@ -140,6 +140,21 @@
       end
     end
 
+    context 'for todos' do
+      include_examples 'migrating records to the ghost user', Todo, [:author] do
+        let(:issue) { create(:issue, project: project, author: user) }
+        let(:created_record) do
+          create(
+            :todo,
+            project: issue.project,
+            user: create(:user),
+            author: user,
+            target: issue
+          )
+        end
+      end
+    end
+
     context 'for releases' do
       include_examples 'migrating records to the ghost user', Release, [:author] do
         let(:created_record) { create(:release, author: user) }
@@ -208,7 +223,7 @@ def nullify_in_batches_regexp(table, column, user, batch_size: 100)
         created_project = create(:project, creator: user)
 
         # associations to be destroyed
-        todos = create_list(:todo, 2, project: issue.project, user: user, author: user, target: issue)
+        todos = create_list(:todo, 2, project: issue.project, user: user, author: create(:user), target: issue)
         event = create(:event, project: issue.project, author: user)
 
         query_recorder = ActiveRecord::QueryRecorder.new do
@@ -225,7 +240,6 @@ def nullify_in_batches_regexp(table, column, user, batch_size: 100)
         expect(resource_label_event.user_id).to be_nil
         expect(resource_state_event.user_id).to be_nil
         expect(created_project.creator_id).to be_nil
-        expect(user.authored_todos).to be_empty
         expect(user.todos).to be_empty
         expect(user.authored_events).to be_empty
 
@@ -238,7 +252,6 @@ def nullify_in_batches_regexp(table, column, user, batch_size: 100)
         ]
 
         expected_queries += delete_in_batches_regexps(:todos, :user_id, user, todos)
-        expected_queries += delete_in_batches_regexps(:todos, :author_id, user, todos)
         expected_queries += delete_in_batches_regexps(:events, :author_id, user, [event])
 
         expect(query_recorder.log).to include(*expected_queries)