Warning when > 26 footnotes on page

Problem/Motivation

If more than 26 auto-numbered footnotes occur on a page, the following warning is thrown:

[warning] Undefined array key 26 FootnotesFilter.php:421

Steps to reproduce

Create a node with more than 26 footnotes. For each footnote, leave the footnote number field blank. Then put the following in a script and execute it using drush php:script:

use Drupal\node\Entity\Node;

ini_set('error_reporting', E_ALL);
$view_builder = Drupal::entityTypeManager()->getViewBuilder('node');
/** @var \Drupal\Core\Render\Renderer $renderer */
$renderer = Drupal::service('renderer');

$query = Drupal::database()->select('node__body', 'b')->fields('b', ['entity_id']);
$query->addExpression('ROUND((CHAR_LENGTH(body_value) - CHAR_LENGTH(REPLACE(body_value, \'<footnotes\', \'\')))/10)', 'fn_count');
$query->having('fn_count > 26');
$nids = $query->execute()->fetchCol();
$nodes = Drupal::entityTypeManager()->getStorage('node')->loadMultiple($nids);

foreach ($nodes as $node) {
  $renderer->renderInIsolation($view_builder->view($node));
}

Proposed resolution

src/Plugin/Filter/FootnotesFilter.php has this code starting at line 416 (in 4.0.0-rc1):

      $alphabet = range('a', 'z');
      $counter = 0;

      // Assign 'a', 'b', 'c' suffixes tentatively.
      foreach (self::$storedFootnotes[$key] as &$stored_footnote) {
        $stored_footnote['backlink_value'] = $stored_footnote['value'] . $alphabet[$counter];
        $counter++;
      }

The $alphabet range can only handle 26 footnotes. We need to have a more robust way of generating backlink values. Maybe instead of using $alphabet[$counter] we do something like '-' . $counter?

Remaining tasks

Commit and release the proposed fix.

User interface changes

None.

API changes

None.

Data model changes

None,