Add query tags to let other modules alter the queries

Thanks aludescher, this looks useful. I have converted your patch into an MR.

Do you have some examples where you have used these tags, to modify the queries?

It will need test coverage too. Let's see if we can re-use one of the existing test modules.

This is working well. I have read through the examples found in api/drupal/core/lib/Drupal/Core/Database/database.api.php/function/hook_query_TAG_alter/10 and can add some tests.

I have a question - you have added four tags:

  $query->addTag('scheduler');
  $query->addTag('scheduler_publish');
  $query->addTag('scheduler_' . $plugin->getPluginId() . '_publish');
  $query->addTag('scheduler_' . $entityTypeId . '_publish');

and these resolve to

[alterTags:protected] => Array (
  [scheduler] => 1
  [scheduler_publish] => 1
  [scheduler_node_scheduler_publish] => 1   << do not need this tag
  [scheduler_node_publish] => 1
)

I'm not sure why we need the third tag, the one using $plugin->getPluginId(), as that does not give any extra flexibility that the $entityTypId does not provide. Or maybe I have missed something?

As expected, the new tests fail (see MR pipeline) in two different places, due to only adding one publish tag, and no unpublish tags

There were 4 failures:
1) Drupal\Tests\scheduler\Functional\SchedulerQueryTagsTest::testPublishingQueryTags with data set "#node" ('node', 'testpage')
After first cron, node 3 should remain unpublished.
2) Drupal\Tests\scheduler\Functional\SchedulerQueryTagsTest::testPublishingQueryTags with data set "#media" ('media', 'test_video')
After first cron, media 3 should remain unpublished.
3) Drupal\Tests\scheduler\Functional\SchedulerQueryTagsTest::testUnpublishingQueryTags with data set "#node" ('node', 'testpage')
After first cron, node 2 should remain published.
4) Drupal\Tests\scheduler\Functional\SchedulerQueryTagsTest::testUnpublishingQueryTags with data set "#media" ('media', 'test_video')
After first cron, media 2 should remain published.

Now adding the remaining tags. Should all pass.

This is a good addition. Thanks aludescher for providing the initial patch.
Committed to 2.x