Add testing support for the private filesystem

One thing I realized: we never tested the private file system in 1.x. The old media module simply allowed choosing, so we had a private file system added so we would trigger that step of the upload process.

Every scenario just had

    # Select the destination (public/private files).
    When I select the radio button "Public local files served by the webserver."

The upload destination is a field storage setting. So field.storage.media.field_media_image controls the uri_scheme.

The upload gets the upload location directly from the field itself:

    // Create a file item to get the upload location.
    $item = $this->createFileItem($media_type);
    $upload_location = $item->getUploadLocation();

The upload location is derived from the settings.

I'm uploading an MR which allows setting the private file system directory. This registers it. But the existing steps are not required since you do not pick the upload destination anymore.

I wonder if we should just delete these items, since they're no longer relevant as part of the upload process.

#    Then I should see "Destination"
    # Select the destination (public/private files).
#    When I select the radio button "Public local files served by the webserver."
#      And I press "Next"

This would be pretty hard. The way URI schemes were changed in Drupal 8 makes it a bit harder.

Or, an additional field on the Media entity for public/private that we'd use in a hook to switch it just before it's saved?

The URI scheme is on the field storage definition itself:

field.storage_settings.file:
  type: base_entity_reference_field_settings
  label: 'File settings'
  mapping:
    display_field:
      type: boolean
      label: 'Enable Display field'
    display_default:
      type: boolean
      label: 'Files displayed by default'
    uri_scheme:
      type: string
      label: 'Upload destination'

The upload location is fetched from \Drupal\file\Plugin\Field\FieldType\FileItem::doGetUploadLocation

  protected static function doGetUploadLocation(array $settings, $data = []) {
    $destination = trim($settings['file_directory'], '/');

    // Replace tokens. As the tokens might contain HTML we convert it to plain
    // text.
    $destination = PlainTextOutput::renderFromHtml(\Drupal::token()->replace($destination, $data));
    return $settings['uri_scheme'] . '://' . $destination;
  }

As far as I can see, there is no way to dynamically choose (hooks or swapping) the URI scheme for a file. You'd need to have a "private" version of each media type.. and that's not fun.

Now, what could be done is altering the source field on the media type.. maybe. So the media types could have a "public" and a "private" URI scheme field. So a "private" boolean to control which field.

\Drupal\media\MediaSourceBase::getSourceFieldValue is what fetches the value. The default configuration is read from $source_field = $this->configuration['source_field'];. But, it does receive the media entity.

public function getSourceFieldValue(MediaInterface $media);

So we'd need to swap the File media source plugin class with a customized one. Then, in \Drupal\media\MediaSourceBase::getSourceFieldValue we can check if the media entity is "private" and switch to the proper source field.

The problem is \Drupal\media\MediaSourceBase::getSourceFieldDefinition. This only has the media type. We'd need to also customize the FileUploadForm in media_library (or shoot, in entity_browser?) for \Drupal\media_library\Form\FileUploadForm::createFileItem. This gets the source field definition from the media type's config.

Basically, I think it may be impossible.

I wonder if this means we'll be missing a feature that users may be depending on?

If it is, it is unfortunately lost due to Drupal core. And, in WYSIWYG, would embedded private files even be filtered out to unprivledged users? Or would the links persist and that user would see a file name and then a dead link?

Created #3228144: Allow users to select upload destination (public, private) when adding media via entity browser / media library. Opening MR to remove commented out steps

MR: https://gitlab.com/panopoly/panopoly/-/merge_requests/9