Fix Error 500s with anonymous clones for a project that has moved

Closes #41457

changelogs/unreleased/sh-handle-anonymous-clones-project-moved.yml

+---
+title: Fix Error 500s with anonymous clones for a project that has moved
+merge_request:
+author:
+type: fixed

lib/gitlab/checks/project_moved.rb

@@ -2,6 +2,7 @@ module Gitlab
   module Checks
     class ProjectMoved
       REDIRECT_NAMESPACE = "redirect_namespace".freeze
+      ANONYMOUS_ID_KEY = 'anonymous'.freeze
 
       def initialize(project, user, redirected_path, protocol)
         @project = project
@@ -22,7 +23,7 @@ module Gitlab
 
       def add_redirect_message
         Gitlab::Redis::SharedState.with do |redis|
-          key = self.class.redirect_message_key(user.id, project.id)
+          key = self.class.redirect_message_key(user_identifier, project.id)
           redis.setex(key, 5.minutes, redirect_message)
         end
       end
@@ -45,8 +46,14 @@ module Gitlab
 
       attr_reader :project, :redirected_path, :protocol, :user
 
-      def self.redirect_message_key(user_id, project_id)
-        "#{REDIRECT_NAMESPACE}:#{user_id}:#{project_id}"
+      def self.redirect_message_key(user_identifier, project_id)
+        "#{REDIRECT_NAMESPACE}:#{user_identifier}:#{project_id}"
+      end
+
+      def user_identifier
+        return ANONYMOUS_ID_KEY unless user.present?
+
+        user.id
       end
 
       def remote_url_message(rejected)

spec/lib/gitlab/checks/project_moved_spec.rb

@@ -35,6 +35,13 @@ describe Gitlab::Checks::ProjectMoved, :clean_gitlab_redis_shared_state do
       project_moved = described_class.new(project, user, 'foo/bar', 'http')
       expect(project_moved.add_redirect_message).to eq("OK")
     end
+
+    it 'should handle anonymous clones' do
+      project_moved = described_class.new(project, nil, 'foo/bar', 'http')
+
+      expect(project_moved.add_redirect_message).to eq("OK")
+      expect(Gitlab::Redis::SharedState.with { |redis| redis.get("redirect_namespace:anonymous:#{project.id}") }).not_to be_nil
+    end
   end
 
   describe '#redirect_message' do