Rename targetEntityType constant in d6_view_mode migration to simplify source plugin

Such a lovely find! And I agree with the solution.

Just to be clear, this could be done without changing the yml file by adding this

    if (isset($this->configuration['constants']['targetEntityType'])) {
      $this->addDependency('module', $this->entityTypeManager->getDefinition($this->configuration['constants']['targetEntityType'])->getProvider());
    }

to DrupalSqlBase::calculateDependencies. That just allows the d6_view_modes.yml to define targetEntityType in the constants in the yml. There is nothing gained by keeping that constant name.

Meant to make this RTBC

What about any custom migrations using this plugin? I think we need to deprecate and tell people to update the migration. I.e do something like:

  /**
   * {@inheritdoc}
   */
  public function calculateDependencies() {
    $this->dependencies = parent::calculateDependencies();
    if (isset($this->configuration['constants']['targetEntityType'])) {
      @trigger_error('The constant targetEntityType is deprecated in drupal:9.2.0 and is removed in drupal:10.0.0. Use entity_type instead. See CR', E_USER_DEPRECARTED);
      $this->addDependency('module', $this->entityTypeManager->getDefinition($this->configuration['constants']['targetEntityType'])->getProvider());
    }
    return $this->dependencies;
  }

@alexpott, thanks. As Matroskeen says better to be safe than sorry.

I reviewed the latest changes to the patch and they restore the method and add the deprecation notice. I read the change record and made a few modifications for readability and shortened the title.

I think this is ready.

I'm not sure that the BC layer is quite right here. See comment on the MR.

The change alexpott suggests is needed for anyone with a configured migration. They won't have a constant 'entity_type' on the row and that is what calculateDependencies() expects.

@Matroskeen, sorry, my comment in #11 isn't right. It is true the parent uses the constant entity_type but in first BC version the dependency was calculated correctly.

Yea, not sure I like overwriting a constant in a configured migration, it could break some custom code. Much better to let any existing constant stay and warn the user, which is what the first BC version did. In that first version a configured migration using entityTargetType would trigger an error but allow both the calculation of dependencies and any custom code to continue to work as is. In the second version bad things could happen.

If entity_type and entityTargetType are different then bad things are going to happen when we remove support for entityTargetType. But I guess the point is you can have that pain when we remove the deprecation and at least you've been warned it is going to happen.

Committed and pushed bd2a1eabe0 to 9.3.x and 577be8956e to 9.2.x. Thanks!