[Fix] LegalDocuments #0048204: Avoid MySQL error 1093 when deleting documents

[fhelfer]

Bug

Documents that are added via Terms of Service or Declaration of Data Protection cannot be deleted afterwards. The request fails with an internal error:
ilDatabaseException SQLSTATE[HY000]: General error: 1093 You can't specify target table 'ldoc_documents' for update in FROM clause

DELETE FROM ldoc_documents WHERE id = 9 AND EXISTS ( SELECT 1 FROM ldoc_documents AS t29 WHERE t29.id = ldoc_documents.id AND t29.provider = 'dpro' )
provider = 'dpro' is Data Protection; Terms of Service uses 'tos'. Both hit the same delete path.

Why it is easy to miss

The query is generated in DatabaseDocumentRepository::deleteEntry(). It appends exists(), which always subselects ldoc_documents.

On MariaDB this DELETE is legal. On Oracle MySQL 8 it is not (error 1093). ILIAS 10 still supports MySQL > 8.0.21; the reference system is MariaDB, so local tests often never see the failure.

The same deleteEntry() implementation is still present on release_11 and trunk. Those branches still support MySQL (8.0.x / 8.4.x). Error 1093 is unchanged there, so the fix should be applied to 10, 11, and trunk.

Mantis: https://mantis.ilias.de/view.php?id=48204

Fix

When deleting from ldoc_documents, restrict with provider = ... instead of EXISTS on the same table:

DELETE FROM ldoc_documents WHERE id = 9 AND provider = 'dpro'