Media library widget FileUploadForm incorrectly creates multiple media entities if the media file field has multiplicity !=1

Problem/Motivation

When a media entity has a file field with multiplicity >1 or unlimited (-1) then FileUploadForm and multiple files are selected for upload, this results in multiple separate media entities of this type being created, instead of creating one media entity type with the uploaded items as field value!

FileUploadForm already seems to have logic in place to determine an upload field with multiplicity != 1 - It seems to be called $slots here:

protected function buildInputElement(array $form, FormStateInterface $form_state) {
    // [...]

    if (!$state->hasSlotsAvailable()) {
      return $form;
    }

    $slots = $state->getAvailableSlots();

    // [...]
    $form['container']['upload'] = [
      '#type' => 'managed_file',
      '#title' => $this->formatPlural($slots, 'Add file', 'Add files'),
      // @todo Move validation in https://www.drupal.org/node/2988215
      '#process' => array_merge(['::validateUploadElement'], $process, ['::processUploadElement']),
      '#upload_validators' => $item->getUploadValidators(),
      // Set multiple to true only if available slots is not exactly one
      // to ensure correct language (singular or plural) in UI
      '#multiple' => $slots != 1 ? TRUE : FALSE,  // <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
      // Do not limit the number uploaded. There is validation based on the
      // number selected in the media library that prevents overages.
      // @see Drupal\media_library\Form\AddFormBase::updateLibrary()
      '#cardinality' => FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED,
      '#remaining_slots' => $slots,
    ];

But in the later process the submit simply creates one media entity per file, instead of checking if the file field multiplicity is !=1, which means the files should all go into the same media field of the media entity!

  /**
   * Submit handler for the upload button, inside the managed_file element.
   *
   * @param array $form
   *   The form render array.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The form state.
   */
  public function uploadButtonSubmit(array $form, FormStateInterface $form_state) {
    $files = $this->entityTypeManager
      ->getStorage('file')
      ->loadMultiple($form_state->getValue('upload', []));
    $this->processInputValues($files, $form, $form_state);
  }

This later creates the media entities in a loop:

   $media = array_map(function ($source_field_value) use ($media_type, $media_storage, $source_field_name) {
      return $this->createMediaFromValue($media_type, $media_storage, $source_field_name, $source_field_value);
    }, $source_field_values);

Steps to reproduce

1. Create a media item type with a file field (e.g. image field) that has multiplicity !=1, for example a media entity type "Slides" or "360°" or "Gallery"
2. Create an entity, e.g. node with a media entity reference field on this type
3. Select the media library widget
4. Create an entity, e.g. node and click the "Add media" button
5. In the Media library widget modal select multiple image files and submit

Expected:
One media entity is created and the uploaded files all land in the file field of that media entity, like if you'd create the media entity through its form

Actual:
For each uploaded file a separate media entity is created with one file in the file field

Proposed resolution

Only create one media entity, if the multiplicity of the upload field multiplicity of the media entity is !=1 and assign all uploaded values to this one file field!

Alternatively or as a first fix, the upload field and form could be hidden, if $slots !=1 and for example a link to the media types create form could be shown instead to mitigate the problem.

Remaining tasks

• Discuss
• Implement
• Test
• Release

User interface changes

TBD

Introduced terminology

-

API changes

-

Data model changes

-

Release notes snippet

TBD