Add support for computed cardinality

The code is very clear! Found a bug that prevents me from adding fields of any kind, computed fields included (see below). First, some non-blocking nit-picks:

1. generateMissingValues() returns void. I'm not exactly sure where hook_entity_presave() is called (\Drupal\Core\Entity\EntityStorageBase::invokeHook()? \Drupal\Core\Entity\ContentEntityStorageBase::invokeHook()? \Drupal\Core\Config\Entity\ConfigEntityStorage::invokeHook()?) but its docs don't mention it expecting any return values, so the return here is unnecessary and/or a bit confusing...
   

   function computed_field_entity_presave(EntityInterface $entity) {
     return \Drupal::service('computed_field.multiple_values_generator')->setEntity($entity)->generateMissingValues();
   }
   

2. I'd change the first line of the comment to say "Fetches the entity currently in use by the service."...
   

   /**
    * Fetches the site currently in use by the service.
    *
    * @return \Drupal\Core\Entity\EntityInterface|null
    *   The entity currently in use by the service.
    * @throws \Exception
    *   If the entity is missing.
    */
   protected function getEntity() {
     if (is_null($this->entity)) {
       throw new \Exception('This operation requires that the service be set with an entity.');
     }
     return $this->entity;
   }
   

When I go to test it...

1. Download Drupal 8.9.0 and clone Computed Field branch 8.x-2.x (when I wrote this, the latest commit was 4b8a10b)
2. Install Drupal using the Standard install profile. When that is done, install only the Computed Field (computed_field) module. Apply the patch in #6.
3. Go to /admin/structure/types/manage/page/fields, add a "Computed (Integer)" field named "Number of assignees" (Machine name: field_number_of_assignees). Click "Save and continue".
   • Actual behavior: I see an error message "There was a problem creating field Number of assignees: Getting the base fields is not supported for entity type Field storage." Further testing shows this error occurs for any field I try to install. This error only happens while the patch is applied.
   • Expected behavior: I should be able to choose the field settings next, including the field cardinality.

Grepping for "Getting the base fields is not supported for entity type", it is thrown in \Drupal\Core\Entity\EntityFieldManager::buildBaseFieldDefinitions().

Notably, computed_field_entity_presave() runs for all entities, (including configuration entities like Field storage), so in \Drupal\computed_field\MultipleValuesGenerator::generateMissingValues()...

public function generateMissingValues() {
  $entity = $this->getEntity();
  $field_definitions = $this->entityFieldManager->getFieldDefinitions($entity->getEntityTypeId(), $entity->bundle());

  foreach ($field_definitions as $field_id => $field_definition) {
    if ($this->fieldIsNotAComputedField($field_definition)) {
      continue;
    }

    $this->generateMissingValuesForField($field_id, $field_definition);
  }
}

... $this->entityFieldManager->getFieldDefinitions() calls \Drupal\Core\Entity\EntityFieldManager::getBaseFieldDefinitions() which calls ::buildBaseFieldDefinitions(), which throws the error.

A quick smoke test suggests...

function computed_field_entity_presave(EntityInterface $entity) {
  if ($entity instanceof \Drupal\Core\Entity\FieldableEntityInterface) {
    return \Drupal::service('computed_field.multiple_values_generator')->setEntity($entity)->generateMissingValues();
  }
}

... fixes the problem; but since that conditional was based on my gut instinct about the problem, you should probably check whether that leaves out any important cases.

If it turns out that instanceof \Drupal\Core\Entity\FieldableEntityInterface is too broad of a condition, then I'd see if \Drupal\Core\Entity\ContentEntityInterface is better.

Note also that In particular, Path Aliases are now revisionable entities, and they seem to have a number of Entity API special cases and implement both FieldableEntityInterface and ContentEntityInterface, so I'd check that case (and as a maintainer, you can make the call not to support attaching computed fields to them at this time).