How to validate max post size on custom form

Question

I have a custom module with a multiple managed file upload.

        $form['attachments'] = [
            '#type' => 'managed_file',
            '#multiple' => true,
            '#upload_validators' => [
                'file_validate_extensions' => ['jpg docx pdf xlsx jpeg png gif'],
                'file_validate_size' => [10485760],
            ],
       ];

When uploading one file that is more then 10MB it gives via AJAX the correct error that a file can not be larger then 10MB. That works out of the box.

How can I limit the total MB of all files that can be uploaded via this form?

For example: 3 files of 3MB = fine 4 files of 3MB = error.

I have managed to show a message when that happens

    public function validateForm(array &$form, FormStateInterface $form_state) {
        $max_size = 10485760;
        $total_size = 0;
        $triggered_element = $form_state->getTriggeringElement();
        if($triggered_element['#name'] == 'attachments_upload_button') {
            $fids = (array) $form_state->getValue('attachments', []);
            if(!empty($fids)) {
                $files = File::loadMultiple($fids);
                foreach ($files as $key => $uploadedFile) {
                    $total_size += $uploadedFile->getSize();
                    if($total_size > $max_size) {
                        $form_state->setErrorByName('attachments', $this->t('The total maximum size of your file sizes can not be more than 10MB.'));
                        $form_state->set('attachments',array_pop($fids));
                        return;
                    }
                }
            }
        }
    }

But I can not seem to remove the last uploaded file. It is still there and it is submitted on form submit. A part from the message, the code does not hold the form from submitting.

I want the last submitted file where the total of all files is > 10M to be removed from the form_state and also the tmp server folder. And ideally via AJAX without loss of field input.

I can't find a solution. Thanks in advance.

How can I achieve this via ajax.

Ok, turns out it was not that difficult. With upload validators it is possible to add variables. So I added $form_state as a variable to a custom upload validator.

        $form['attachments'] = [
            '#type' => 'managed_file',
            '#multiple' => true,
            '#upload_validators' => [
                'file_validate_extensions' => ['jpg docx pdf xlsx jpeg png gif'],
                'file_validate_size' => [10485760],
                'size_max_upload' => [$form_state],
            ],
        ];

And wrote a custom validator function

/**
 * Validate maximum size upload.
 *
 * @param \Drupal\file\FileInterface $file
 *   File object.
 *
 * @param \Drupal\Core\Form\FormStateInterface $form_state
 *   Form state object.
 * 
 * @return array
 *   Errors array.
 */
function size_max_upload(File $file, &$form_state) {
  $errors = [];
  $max_size = 10485760;
  $total_size = $file->getSize();
  $triggered_element = $form_state->getTriggeringElement();

  if($triggered_element['#name'] == 'attachments_upload_button') {
    $fids = (array) $form_state->getValue('attachments', []);
  
    if(!empty($fids)) {
      $files = File::loadMultiple($fids);
  
      foreach ($files as $key => $uploadedFile) {
        $total_size += $uploadedFile->getSize();
  
        if($total_size > $max_size) {
          $errors[] = t("The total maximum size of your file sizes can not be more than 10MB.");
          break;
        }
      }
    }
  }

  return $errors;
}