Consider moving some of the module logic into services or OO code so that it can be easily tested and other modules can make use of it

e.g something like this for the field info

Could do similar for extracting the terms from an entity.

And then another for the db operations.

tests paying for themself - here's the fix

Great work 🙌

I'd be keen to also add something to extract TID values from an entity too. Been running custom code for that on a few projects.

1. +++ b/src/FieldInformation.php
   @@ -0,0 +1,91 @@
   +    $cache_id = 'entity-taxonomy-fields_' . $entity_type_id;
   

   do we need the entity-taxonomy-fields_ prefix? we're only storing one thing in the cache and its per entity-type

2. +++ b/src/FieldInformation.php
   @@ -0,0 +1,91 @@
   +          if ($field_definition->getSetting('target_type') === 'taxonomy_term') {
   

   We can read this from the field storage definition as the target-type is a storage level setting, will likely mean we can avoid the ::getFieldDefinitions call inside the two inner for each loops.

   \Drupal\Core\Entity\EntityFieldManager::getFieldStorageDefinitions can be called outside both loops, and then if a $field_name key exits, we can call

   ::getSetting</code on that.
   
   Should be dramatically more performant.
   
   I realise we're just moving existing code, but we may as well speed it up while we're at it.
   </li>
   
   <li>
   <code>
   +++ b/taxonomy_entity_index.module
   @@ -157,44 +157,14 @@ function taxonomy_entity_index_entity_update(EntityInterface $entity) {
   +  @trigger_error('taxonomy_entity_index_get_taxonomy_field_names() is deprecated in taxonomy_entity_index:8.x-1.13. Use the \Drupal\taxonomy_entity_index\FieldInformation service instead.', E_USER_DEPRECATED);
   

   this will be 8.x-1.15 now (at least)

Oh, right, I wrote the first version of the patch and said the same thing 🤦 three year ago 😂