Sql Migration map returns nonsensical destination ids when looking up against a source id row that failed or was ignored.

In my D9 installation this patch played perfectly! I set it as RTBC.

The only thing failing is the change of the patch from #22 to the /core/modules/migrate_drupal/tests/fixtures/drupal7.php file.
In this MR I opened, I haven't added this change.
Is that actually needed? From what I may understand, I think not.

The change to the test fixture is what surfaces the bug to the core test suite, so the tests fail. This is not ready to commit. There are 14 failed tests because the changes in #22 aren't backwards compatible. Fixing the bug in this way would break existing migrations that have already implemented workarounds. This needs more work to allow opting into the fix and a warning that migrations should be updated to use the fix or they will break in the next major version.

I've had a go at allowing this new behaviour to be optional (disabled by default). So when using the migration_lookup process plugin, a 'skip_unmigrated' option can be passed.

From reading above, it looks as though we might ultimately want the option to be enabled by default, but I'm just being conservative for now - and presumably tests will pass with this as no migrations will actually be using it yet. But sure, the next step could be to invert that, and update existing migrations and add the warning.

I'm also not sure about the BC implications. I've added an optional parameter to some methods to pass the configuration along. I'm not sure if there's a better way. Presumably we could just target 11.x without any backporting, but with a change record?

Thanks @james.williams This looks promising but unfortunately I encountered an issue when using Migrate Tools:

Fatal error: Declaration of Drupal\migrate_tools\IdMapFilter::lookupDestinationIds(array $source_id_values): array must be compatible with Drupal\migrate\Plugin\MigrateIdMapInterface::lookupDestinationIds(array $source_id_values, bool $skip_unmigrated = false)

In @migrate_tools/src/IdMapFilter.php:195@ there is a declaration of

  public function lookupDestinationIds(array $source_id_values): array {
    $map = $this->getInnerIterator();
    \assert($map instanceof MigrateIdMapInterface);
    return $map->lookupDestinationIds($source_id_values);
  }

which would also need to be modified to handle the extra parameter, to work in conjunction with this patch.