Error when setting batch on user insert

Problem/Motivation

When upgrading this module from 2.1.1 to 2.1.2 (on Drupal 10.4.6), I can no longer create users without getting the following error:

The website encountered an unexpected error. Try again later.

LogicException: The database connection is not serializable. This probably means you are serializing an object that has an indirect reference to the database connection. Adjust your code so that is not necessary. Alternatively, look at DependencySerializationTrait as a temporary solution. in Drupal\Core\Database\Connection->__sleep() (line 1920 of core/lib/Drupal/Core/Database/Connection.php).

The stack trace that followed showed batch functions, which gave me the clue about where the problem is occurring. I am adding a batch on user insert (this is not new, it's been working for years); if I remove the batch, the error does not occur.

Steps to reproduce

Set a batch on user insert. A simple example (which doesn't do anything) is provided below; and even though I am passing user here (because my actual use case requires it), the error occurs even if user is not passed to the batch process.

use Drupal\user\UserInterface;

/**
 * Implements hook_ENTITY_TYPE_insert().
 */
function my_module_user_insert(UserInterface $user) {
  // Testing for genpass issue.
  $ops = [];
  $ops[] = ['_my_module_batch_test', [$user], []];

  $batch = [
    'title' => t('Batch test on user insert'),
    'operations' => $ops,
    'init_message' => t('Testing.'),
    'progress_message' => t('Step @current of @total'),
    'error_message' => t('An error occurred during processing.'),
    'finished' => '_my_module_batch_test_finished',
  ];

  batch_set($batch);
}

/**
 * Batch operation.
 *
 * @param \Drupal\user\UserInterface $user
 *   The user entity.
 * @param array|object $context
 *   Context passed by reference through the batch process.
 */
function _my_module_batch_test(UserInterface $user, &$context) {
  $context['results']['username'] = $user->getDisplayName();
  $context['message'] = t('Testing...');
  $context['finished'] = 1;
}

/**
 * Batch finished callback.
 *
 * @param bool $success
 *   Whether the batch was successful.
 * @param array $results
 *   The processed results from the batch.
 * @param array $operations
 *   In case of error, the operations that remained unprocessed.
 */
function _my_module_batch_test_finished($success, array $results, array $operations) {
  $messenger = \Drupal::messenger();
  if ($success) {
    $messenger->addStatus(t('Batch test on user insert complete for @user.', [
      '@user' => $results['username'],
    ]));
  }
  else {
    $messenger->addError(t('Error testing batch on user insert.'));
  }
}

Proposed resolution

I took a quick look at the changes introduced in 2.1.2, and although I suspect it has to do with the changes from #3509679: Migrate to OO hooks, unfortunately I'm not sure how to resolve this.

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix
• ✅ Testing to ensure no regression
• ✅ Automated unit testing coverage
• ✅ Automated functional testing coverage
• ➖ UX/UI designer responsibilities
• ➖ Readability
• ➖ Accessibility
• ✅ Performance
• ➖ Security
• ➖ Documentation
• ✅ Code review by maintainers
• ✅ Full testing and approval
• ➖ Credit contributors
• ➖ Review with the product owner
• ✅ Release notes snippet
• ❌ Release

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• #3522473: Error when setting batch on user insert