Node access rebuilds are completely broken when being rebuilt through a batch process?

Coming here from #993186: Node access rebuilds should go newest to oldest I was looking at the code in Drupal 8 for rebuilding nodes via a batch process. I have not tested it but it looks very broken - like someone made an attempt to improve the code along the lines of that issue at some point, but did so incorrectly.

Here's the code:

function _node_access_rebuild_batch_operation(&$context) {
  if (empty($context['sandbox'])) {
    // Initiate multistep processing.
    $context['sandbox']['progress'] = 0;
    $context['sandbox']['current_node'] = 0;
    $context['sandbox']['max'] = \Drupal::entityQuery('node')->count()->execute();
  }

  // Process the next 20 nodes.
  $limit = 20;
  $nids = \Drupal::entityQuery('node')
    ->condition('nid', $context['sandbox']['current_node'], '>')
    ->sort('nid', 'DESC')
    ->range(0, $limit)
    ->execute();
  $nodes = node_load_multiple($nids, TRUE);
  foreach ($nodes as $nid => $node) {
    // To preserve database integrity, only write grants if the node
    // loads successfully.
    if (!empty($node)) {
      \Drupal::entityManager()->getAccessControlHandler('node')->writeGrants($node);
    }
    $context['sandbox']['progress']++;
    $context['sandbox']['current_node'] = $nid;
  }

  // Multistep processing : report progress.
  if ($context['sandbox']['progress'] != $context['sandbox']['max']) {
    $context['finished'] = $context['sandbox']['progress'] / $context['sandbox']['max'];
  }
}

So the first 20 nodes get processed in descending order: 20,19,18....1.

At the end of which, $context['sandbox']['progress'] is 20, and $context['sandbox']['current_node'] is 1.

Therefore, the next time through the batch, it will process these nodes: 21,20,19,18...2.

Oops, each node except the first is double-processed (and most are on their way to being processed 20 times).

But it's actually even worse, since $context['sandbox']['progress'] will be 40 after this second round, even though only 21 nodes have been processed. So it will eventually hit $context['sandbox']['max'] before all nodes have been processed, and the batch will finish with many nodes not rebuilt at all.

Again, I have not tested this so it's possible I'm missing something, but to me it looks like that's what will happen.