D6 taxonomy term fields are not migrated with allowed vocabularies

I ran upgrades from the drupal6 fixture through the UI without the patch and confirmed the bug was present.

Then I applied the patch and tried again. This time the target bundle got assigned correctly.

So manual testing was great!

As far as the code itself, I have two comments.

1. I think it would be nice to make it clearer that the new plugin is only used for the d6 migration. Maybe just in the comment above the annotation mentioning d6?

2. What would you think about re-naming the new helper function in the tests? When I saw assertEntityReferenceFields() I was surprised that all it checked was the target bundles. Something like assertTargetBundles() would probably read a little smoother.

Neither of these is a big deal though, and I could be convinced that neither is necessary.

@quietone, I think the changes look great. I also want to point out for the record that all of your changes to the test are additive except for this one line:

+++ b/core/modules/taxonomy/tests/src/Kernel/Migrate/d6/MigrateVocabularyFieldInstanceTest.php
@@ -57,7 +58,7 @@ public function testVocabularyFieldInstance() {
-    $this->assertSame(['field_tags'], $settings['handler_settings']['target_bundles'], 'The target_bundles handler setting is correct.');
+    $this->assertTargetBundles($field_id, ['tags' => 'tags']);

That line was actually asserting that the migration is incorrect because we really want the target bundles to be ['tags' => 'tags']. So we're not losing test coverage there!

But...Last night I started wondering if there was a way to reproduce the effect of the new process plug with existing process plugins. (This is my favorite part of migration!) This morning I was able to hack something together. It's miserable though! Here's the relevant bits.

  _vid:
    -
      plugin: migration_lookup
      migration:  d6_taxonomy_vocabulary
      source: vid
    -
      plugin: skip_on_empty
      method: row
  # Make an array with _vid as the only element with json trick.
  _vid_wrapped:
    -
      plugin: concat
      source:
        - 'constants/json_start' # '["'
        - '@_vid'
        - 'constants/json_end'. # '"]'
    -
      plugin: callback
      callable: json_decode
  'settings/handler_settings/target_bundles':
    plugin: callback
    callable: array_combine
    unpack_source: TRUE
    source:
      - '@_vid_wrapped'
      - '@_vid_wrapped'

It could have been cleaner is php's array() was callable but apparently it's not. That's why I had to do that json thing. I think above is our best option other than creating the new process plugin.

Seeing as the above is absolutely unreadable, I think your patch is the way to go. RTBC. Thanks!

Committed/pushed to 10.0.x and cherry-picked to 9.4.x, thanks!

The new process plugin makes this minor-ish, please re-open if you think it should be backported to 9.3.x.