Improve logging when username update fails due to registry tags

When a user cannot be renamed due to existing container registry tags, log the
namespace and an example project that has tags.

app/models/concerns/storage/legacy_namespace.rb

@@ -5,8 +5,10 @@ module Storage
     extend ActiveSupport::Concern
     def move_dir
-      if any_project_has_container_registry_tags?
-        raise Gitlab::UpdatePathError.new('Namespace cannot be moved, because at least one project has tags in container registry')
+      proj_with_tags = first_project_with_container_registry_tags
+
+      if proj_with_tags
+        raise Gitlab::UpdatePathError.new("Namespace #{name} (#{id}) cannot be moved because at least one project (e.g. #{proj_with_tags.name} (#{proj_with_tags.id})) has tags in container registry")
       end
       parent_was = if parent_changed? && parent_id_was.present?

app/models/namespace.rb

@@ -135,6 +135,10 @@ class Namespace < ActiveRecord::Base
     all_projects.any?(&:has_container_registry_tags?)
   end
+  def first_project_with_container_registry_tags
+    all_projects.find(&:has_container_registry_tags?)
+  end
+
   def send_update_instructions
     projects.each do |project|
       project.send_move_instructions("#{full_path_was}/#{project.path}")

changelogs/unreleased/sh-improve-container-tags-update-username.yml

+---
+title: Improve logging when username update fails due to registry tags
+merge_request: 22038
+author:
+type: other

spec/models/namespace_spec.rb

@@ -82,6 +82,27 @@ describe Namespace do
     it { expect(namespace.human_name).to eq(namespace.owner_name) }
   end
+  describe '#first_project_with_container_registry_tags' do
+    let(:container_repository) { create(:container_repository) }
+    let!(:project) { create(:project, namespace: namespace, container_repositories: [container_repository]) }
+
+    before do
+      stub_container_registry_config(enabled: true)
+    end
+
+    it 'returns the project' do
+      stub_container_registry_tags(repository: :any, tags: ['tag'])
+
+      expect(namespace.first_project_with_container_registry_tags).to eq(project)
+    end
+
+    it 'returns no project' do
+      stub_container_registry_tags(repository: :any, tags: nil)
+
+      expect(namespace.first_project_with_container_registry_tags).to be_nil
+    end
+  end
+
   describe '.search' do
     let(:namespace) { create(:namespace) }
@@ -184,7 +205,8 @@ describe Namespace do
         end
         it 'raises an error about not movable project' do
-          expect { namespace.move_dir }.to raise_error(/Namespace cannot be moved/)
+          expect { namespace.move_dir }.to raise_error(Gitlab::UpdatePathError,
+                                                       /Namespace .* cannot be moved/)
         end
       end
     end